How to Do a Self Assessment Tax Return: UK Step-by-Step Guide 2025/26

To do a Self Assessment tax return in the UK, register with HMRC for Self Assessment, set up a Government Gateway account, collect all your income records, complete the SA100 form online, and submit it by 31 January following the end of the tax year. You pay any tax owed by the same date. Late filing triggers an automatic £100 penalty, and late payment incurs interest and surcharges.

Around 12 million people file Self Assessment returns each year in the UK. Whether you are a sole trader, a company director taking dividends, a landlord with rental income, or someone with multiple income streams, this guide walks you through every step of the process for the 2025/26 tax year (6 April 2025 to 5 April 2026).

Last updated: March 2026

Who Needs to File a Self Assessment Tax Return?

Not everyone needs to file. HMRC requires Self Assessment from anyone with income that is not fully taxed at source through PAYE. Specifically, you must register and file if any of the following apply to you:

Situation	Must File?	Notes
Self-employed with income over £1,000	Yes	Trading allowance covers the first £1,000
Company director taking dividends	Yes	Dividends are not taxed through PAYE
Salary above £100,000	Yes	Personal allowance tapering requires a return
Rental income above £1,000	Yes	Property allowance covers the first £1,000
Untaxed savings or investment income	Yes	Bank interest above your Personal Savings Allowance
Capital gains above the Annual Exempt Amount	Yes	£3,000 for 2025/26
High Income Child Benefit Charge	Yes	Adjusted net income above £60,000
Foreign income	Yes	Even if taxed abroad, you must declare it
Partner in a business partnership	Yes	Your share of partnership profits
Income from trusts or estates	Yes	Untaxed trust income
Claiming tax relief not given through PAYE	Yes	Professional subscriptions, working from home, etc.
Tips or commission not taxed through PAYE	Yes	Casual earnings above certain thresholds

If your only income is a PAYE salary under £100,000 with no dividends, no rental income, no capital gains, and no other untaxed income, you probably do not need to file. Everyone else should assume they need to.

Company directors deserve special mention. If you run a limited company and take even a single pound in dividends, you must file Self Assessment. See our detailed guide on Self Assessment for directors for director-specific scenarios.

How to Register for Self Assessment

If you have never filed a Self Assessment return before, you must register with HMRC. The process depends on your circumstances.

Step 1: Register Online

Go to HMRC's website and choose the correct registration form:

• Self-employed: Register using form CWF1 (online) or call the Self Assessment helpline
• Not self-employed (e.g., company director, landlord, investor): Register using form SA1
• Partnerships: The nominated partner registers the partnership using form SA400, and each partner registers individually using form SA401

You will need your National Insurance number, personal details, and information about your income sources.

Step 2: Receive Your Unique Taxpayer Reference (UTR)

After registering, HMRC will post your 10-digit Unique Taxpayer Reference (UTR) to your registered address. This typically arrives within 10 working days, though it can take up to 4 weeks during busy periods.

Your UTR is permanent. It stays the same for life and across tax years. Keep it somewhere safe. You will need it every time you file and every time you contact HMRC about Self Assessment.

Step 3: Set Up Your Government Gateway Account

If you do not already have one, create a Government Gateway account at gateway.tax.service.gov.uk. You will need:

• Your UTR number
• Your National Insurance number
• A valid UK address
• An email address

After creating the account, you must request an activation code to enrol for Self Assessment online. This code is posted to your address and typically arrives within 7 working days. Enter the activation code into your Government Gateway account to complete enrolment.

Timeline: How Long Does Registration Take?

Step	Typical Timeframe
Submit registration form online	Same day
Receive UTR by post	10-20 working days
Create Government Gateway account	Same day
Request activation code	Same day
Receive activation code by post	7-10 working days
Enter code and enrol	Same day
Total from registration to filing-ready	3-6 weeks

Start early. If you leave registration until December, you risk not being set up in time for the 31 January deadline. HMRC recommends registering by 5 October following the end of the tax year in which you first need to file. For the 2025/26 tax year, that means registering by 5 October 2026.

Paper vs Online Filing: Which Should You Choose?

You have two options for submitting your Self Assessment return: paper or online.

Paper Filing

• Deadline: 31 October following the end of the tax year
• Form: SA100 plus any supplementary pages, printed and posted
• HMRC calculates your tax: They work out what you owe and send you a calculation
• No software needed: Just a pen and the official forms

Online Filing

• Deadline: 31 January following the end of the tax year (3 extra months)
• Platform: HMRC's free online service or commercial tax software
• Instant calculation: You see your tax bill immediately
• Faster processing: Refunds processed more quickly
• Error checking: Built-in validation catches common mistakes

There is no good reason to file on paper. Online filing gives you three extra months, instant calculations, error checking, and faster refunds. The only scenario where paper might be necessary is if HMRC's online system genuinely cannot handle your situation, which is exceedingly rare.

For the 2025/26 tax year:

• Paper deadline: 31 October 2026
• Online deadline: 31 January 2027

The SA100 Form: Main Return Explained

The SA100 is the core Self Assessment form. Everyone who files completes this. It covers your personal details, total income, and tax reliefs. Here is what each section contains:

Personal Information

Your name, address, UTR, National Insurance number, date of birth, and contact details. HMRC pre-populates most of this if you file online.

Income Sections on the SA100

The main return captures:

• Employment income already taxed through PAYE (entered from your P60)
• State Pension and other pensions
• Taxable state benefits (Jobseeker's Allowance, Employment and Support Allowance)
• Other UK income not covered by supplementary pages (occasional freelance work under a certain level, for example)
• Interest and dividends from UK banks and building societies

Tax Reliefs

You can claim certain reliefs directly on the SA100:

• Gift Aid donations
• Pension contributions not made through your employer
• Blind Person's Allowance
• Married Couple's Allowance (for those born before 6 April 1935)
• Venture capital scheme reliefs (SEIS/EIS)
• Maintenance payments

Student Loan Repayments

If you have a student loan and any of your income was not taxed through PAYE, you may owe student loan repayments through Self Assessment. The SA100 captures your student loan plan type and calculates the amount.

Finishing the Return

The final sections cover:

• Any underpaid tax from previous years
• How you want HMRC to collect any tax owed (adjust your tax code or pay directly)
• A declaration that the information is correct and complete

Supplementary Pages: Which Do You Need?

Not everyone completes just the SA100. Depending on your income sources, you may need one or more supplementary pages. Here is the full list:

Form	Name	Who Needs It
SA102	Employment	Income from employment (one form per employer)
SA103S	Self-Employment (Short)	Self-employed with turnover under £85,000 and simple affairs
SA103F	Self-Employment (Full)	Self-employed with turnover of £85,000 or above
SA104S	Partnership (Short)	Partner in a partnership with simple affairs
SA104F	Partnership (Full)	Partner in a partnership with complex affairs
SA105	UK Property	Rental income from UK land and property
SA106	Foreign	Income from outside the UK
SA107	Trusts	Income from trusts and settlements
SA108	Capital Gains	Capital gains above the Annual Exempt Amount
SA109	Residence, Remittance Basis	Non-domiciled individuals or those claiming remittance basis
SA110	Tax Calculation	HMRC calculates this for paper filers (auto for online)

SA102: Employment Income

If you are employed (including as a company director paying yourself through PAYE), you complete SA102 for each employment. You will need:

• Your P60 from each employer, showing total pay and tax deducted
• Your P11D if you received benefits in kind (company car, private medical, etc.)
• Details of any expenses you want to claim against your employment income

For directors with a single company, this is straightforward. You enter your gross salary and the tax already deducted through PAYE. For guidance on the optimal director salary, see our guide on PAYE for directors.

SA103: Self-Employment

Sole traders and freelancers complete SA103. The short version (SA103S) is for those with turnover below £85,000 and straightforward accounts. Everyone else uses the full version (SA103F).

You will report:

• Total business income (turnover)
• Allowable business expenses (broken down by category)
• Capital allowances
• Net profit or loss
• Whether you use cash basis or traditional accruals accounting

SA105: UK Property

Landlords complete SA105 for rental income. You declare:

• Total rental income received
• Allowable expenses (letting agent fees, insurance, repairs, ground rent)
• Finance costs (mortgage interest is now a tax credit at the basic rate, not a deduction)
• Net rental profit or loss

SA106: Foreign Income

If you received any income from outside the UK, you need SA106. This covers:

• Foreign employment income
• Foreign self-employment income
• Foreign dividends and interest
• Foreign property income
• Foreign pensions

You must report all foreign income in GBP, converted at either the exchange rate at the time of receipt or the average rate for the year. If you paid foreign tax, you can usually claim Foreign Tax Credit Relief to avoid double taxation.

SA108: Capital Gains

You only need SA108 if your total gains exceed the Annual Exempt Amount (£3,000 for 2025/26) or if you disposed of assets worth more than four times the exempt amount (£12,000), even if you made a loss overall.

Capital gains arise from selling:

• Shares and investments
• Property (other than your main home)
• Business assets
• Cryptocurrency
• Valuable personal possessions worth over £6,000

What Income Must You Declare?

You must declare all income on your Self Assessment return, even income where tax has already been deducted. Here is a comprehensive list:

Employment Income

• Salary, wages, bonuses, commission
• Benefits in kind (shown on P11D)
• Tips not included in your pay
• Redundancy pay above £30,000

Self-Employment Income

• All trading income from sole trader or freelance work
• Income from side hustles above the £1,000 trading allowance

Investment Income

• Dividends: All UK and foreign dividends. The £500 dividend allowance means you will not pay tax on the first £500, but you still declare the total
• Bank interest: All interest received. The Personal Savings Allowance means basic rate taxpayers do not pay tax on the first £1,000 of interest and higher rate taxpayers on the first £500, but again, declare the total
• Interest from peer-to-peer lending
• Unit trust and OEIC distributions

Note: Interest and dividends within ISAs are completely tax-free and do not need to be declared.

Property Income

• Rental income from buy-to-let properties
• Income from renting a room (above the £7,500 Rent a Room Scheme threshold)
• Holiday letting income

Pension Income

• Private pension payments (if not taxed through PAYE)
• State Pension (always taxable but not taxed at source)

Other Income

• Capital gains (from selling assets)
• Foreign income of any type
• Trust income
• Casual earnings
• Cryptocurrency gains

What Expenses Can You Claim?

Allowable expenses reduce your taxable profit. What you can claim depends on your income type.

Self-Employment Expenses

If you are self-employed, you can deduct:

• Office costs: Stationery, phone bills, postage, printing
• Travel: Business mileage (45p/mile for first 10,000 miles, 25p after), public transport, parking
• Clothing: Uniforms and protective clothing (not everyday clothing)
• Staff costs: Salaries, subcontractor costs, employer's NI
• Stock and materials: Goods for resale, raw materials
• Financial costs: Insurance, bank charges, accountancy fees
• Premises: Rent, business rates, utilities, repairs
• Marketing: Advertising, website costs, business cards
• Professional subscriptions: Membership of professional bodies
• Training: Courses directly related to your existing business
• Use of home: If you work from home, a proportion of household costs

If you use simplified expenses, you can claim flat rates instead of actual costs for vehicle expenses (45p/mile), working from home (£6/week without evidence, or actual apportioned costs), and living at your business premises.

Property Expenses

Landlords can claim:

• Letting agent and management fees
• Buildings insurance
• Maintenance and repairs (not improvements)
• Legal fees for lets of a year or less
• Ground rent and service charges
• Council tax and utility bills (if paid by you)
• Accountancy fees
• Direct costs of finding tenants

Mortgage interest is no longer a direct deduction for residential property. Instead, you receive a tax credit at the basic rate (20%) on your finance costs.

Employment Expenses

Employees (including directors) can claim:

• Professional subscriptions and fees
• Tools and specialist equipment
• Working from home expenses
• Travel for work (not regular commuting)
• Uniforms and protective clothing
• Mileage at approved rates

Step-by-Step: Completing Your Return Online

Here is the exact process for filing online through HMRC's free service.

Step 1: Log In

Go to gov.uk/log-in-self-assessment and sign in with your Government Gateway user ID and password. HMRC may ask you to verify your identity using a code sent to your phone or email.

Step 2: Select the Tax Year

Choose the correct tax year. For the year ending 5 April 2026, you are filing the 2025/26 return.

Step 3: Tailor Your Return

HMRC asks a series of yes/no questions to determine which sections and supplementary pages you need. Answer honestly. Common questions include:

• Were you employed?
• Were you self-employed?
• Did you receive income from UK property?
• Did you have foreign income?
• Did you dispose of chargeable assets?
• Did you receive dividends?

Based on your answers, HMRC generates the correct pages.

Step 4: Complete Each Section

Work through each section in order. HMRC's online system saves your progress, so you do not need to complete everything in one sitting.

For each income source, enter the figures from your records:

• P60 and P11D for employment income
• Your business accounts for self-employment
• Bank and building society statements for interest
• Dividend vouchers or statements for dividends
• Completion statements and solicitor's calculations for capital gains

Step 5: Claim Your Reliefs and Allowances

Enter any tax reliefs:

• Gift Aid donations
• Pension contributions
• Marriage Allowance transfer
• EIS/SEIS investments

Step 6: Review the Calculation

HMRC's system automatically calculates your tax. Review the figures carefully. The calculation shows:

• Your total income
• Your taxable income (after allowances)
• Income tax due at each rate
• National Insurance contributions
• Student loan repayments
• Tax already paid through PAYE
• Your remaining tax bill or refund

Step 7: Submit

Once you are satisfied the figures are correct, submit the return. You will receive an acknowledgement with a reference number. Save this.

Step 8: Pay Any Tax Owed

If you owe tax, pay it by 31 January. See the section on payment methods below.

Worked Example 1: Company Director with Salary and Dividends

Sarah is the sole director of her consultancy limited company. In 2025/26 she takes:

• Salary: £12,570 (the optimal tax-free amount via PAYE)
• Dividends: £40,000
• Bank interest: £800

Employment income (SA102):

• Gross salary: £12,570
• Tax deducted through PAYE: £0 (covered by Personal Allowance)

Dividends:

• Total dividends: £40,000
• Dividend allowance: £500 (tax-free)
• Taxable dividends: £39,500
• Tax at 8.75% (basic rate): £39,500 falls within the basic rate band (£12,571 to £50,270)
• However, Sarah's employment income of £12,570 uses up her personal allowance
• Dividends fill the basic rate band: £50,270 - £12,570 = £37,700 taxed at 8.75% = £3,298.75
• Remaining £1,800 taxed at 33.75% (higher rate) = £607.50

Bank interest:

• Total interest: £800
• Personal Savings Allowance (basic rate): £1,000
• Taxable interest: £0

Total tax bill: £3,298.75 + £607.50 = £3,906.25

Sarah has no tax to offset from PAYE (her entire salary was covered by the Personal Allowance), so she owes £3,906.25 through Self Assessment.

Since Sarah's bill exceeds £1,000, she may also face payments on account for the following year (see below).

Worked Example 2: Sole Trader

James runs a freelance web design business. In 2025/26:

• Turnover: £65,000
• Allowable expenses: £12,000 (software subscriptions £3,000, home office £2,000, equipment £4,000, travel £1,500, accountancy £1,500)
• Net profit: £53,000
• Bank interest: £200

Self-employment income (SA103S):

• Turnover: £65,000
• Expenses: £12,000
• Net profit: £53,000

Tax calculation:

• Personal Allowance: £12,570
• Taxable income: £53,000 + £200 - £12,570 = £40,630
• Basic rate (20%) on £37,700: £7,540.00
• Higher rate (40%) on £2,930: £1,172.00
• Total income tax: £8,712.00

Class 4 National Insurance:

• Profits between £12,570 and £50,270 at 6%: £2,262.00
• Profits above £50,270 at 2%: £54.60
• Total Class 4 NI: £2,316.60

Class 2 National Insurance:

• £3.45 per week x 52 weeks = £179.40 (payable if profits above £12,570)

Total Self Assessment bill: £8,712.00 + £2,316.60 + £179.40 = £11,208.00

Worked Example 3: Employed Person with Rental Income

Priya works full-time earning £55,000 (taxed through PAYE). She also has a buy-to-let property:

• Rental income: £14,400 (£1,200/month)
• Allowable property expenses: £3,200 (letting agent £1,800, insurance £600, repairs £800)
• Mortgage interest: £6,000

Employment income (SA102):

• Gross salary: £55,000
• Tax deducted through PAYE: approximately £8,486 (standard tax code)

UK Property (SA105):

• Rental income: £14,400
• Allowable expenses: £3,200
• Net rental profit: £11,200

Tax on rental profit:

• Priya's salary already uses her Personal Allowance and fills the basic rate band to £55,000
• The basic rate band ends at £50,270, so she is already a higher rate taxpayer
• Rental profit of £11,200 is taxed entirely at 40%: £4,480.00

Finance cost tax credit (mortgage interest):

• Mortgage interest: £6,000
• Tax credit at 20%: -£1,200.00

Additional tax through Self Assessment: £4,480 - £1,200 = £3,280.00

This is in addition to the PAYE tax already deducted from her salary.

Worked Example 4: Multiple Income Streams

Tom is a company director who also does freelance consulting and has investments:

• Director's salary: £12,570 (via PAYE)
• Dividends from his company: £30,000
• Freelance consulting (self-employed): Turnover £18,000, expenses £3,000, profit £15,000
• Capital gains from selling shares: £8,000 gain
• Bank interest: £1,500

Taxable income summary:

Source	Gross	Allowance/Deduction	Taxable
Salary	£12,570	Personal Allowance £12,570	£0
Self-employment profit	£15,000	—	£15,000
Dividends	£30,000	Dividend allowance £500	£29,500
Bank interest	£1,500	PSA (basic rate) £1,000	£500
Capital gains	£8,000	AEA £3,000	£5,000

Income tax:

• Non-savings, non-dividend income: £15,000
• This fills basic rate band from £12,571 to £27,570
• Savings income: £500 at 20% = £100
• Dividend income: £29,500
  • Basic rate band remaining: £50,270 - £28,070 = £22,200 at 8.75% = £1,942.50
  • Higher rate: £7,300 at 33.75% = £2,463.75
• Total income tax: £3,000 + £100 + £1,942.50 + £2,463.75 = £7,506.25

Class 4 NI on self-employment:

• £15,000 - £12,570 = £2,430 at 6% = £145.80

Class 2 NI: £179.40

Capital Gains Tax:

• Tom is a higher rate taxpayer (total income above £50,270)
• Shares taxed at 20%: £5,000 x 20% = £1,000.00

Total Self Assessment bill: £7,506.25 + £145.80 + £179.40 + £1,000 = £8,831.45 (less any PAYE already deducted)

Payments on Account: What Are They and How Do They Work?

Payments on account are advance payments towards next year's tax bill. HMRC requires them if your Self Assessment bill is more than £1,000 and more than 80% of your total tax liability was not collected at source (through PAYE, for example).

How Payments on Account Are Calculated

Each payment on account is 50% of your previous year's Self Assessment bill. You make two payments:

• First payment on account: 31 January (same day as filing and paying the current year's tax)
• Second payment on account: 31 July

Then, on the following 31 January, you pay any balancing payment (the difference between what you actually owe and what you have already paid through payments on account).

Example: Payments on Account

If your 2025/26 Self Assessment bill is £6,000:

Payment	Date	Amount
2025/26 tax bill	31 January 2027	£6,000
First payment on account for 2026/27	31 January 2027	£3,000
Second payment on account for 2026/27	31 July 2027	£3,000
Total due 31 January 2027		£9,000

This catches many people off guard. Your first Self Assessment payment can be 150% of your actual tax bill because it includes the first payment on account for the following year.

Can You Reduce Payments on Account?

Yes. If you know your income will be lower next year, you can apply to reduce your payments on account through your Government Gateway account. Be cautious though. If you reduce them too much and your actual tax bill turns out to be higher, you will owe interest on the underpayment.

When Payments on Account Do Not Apply

You do not have to make payments on account if:

• Your Self Assessment bill is £1,000 or less
• More than 80% of your tax was deducted at source (e.g., through PAYE)

How to Pay Your Self Assessment Tax Bill

HMRC accepts several payment methods. Processing times vary, so plan ahead.

Payment Method	Processing Time	Notes
Online banking (Faster Payments)	Same or next day	Use HMRC's bank details + your UTR as reference
Direct Debit	3 working days	Set up through your Government Gateway account
Debit card (online)	Same day	Via HMRC's online payment portal
Credit card	Not accepted	HMRC stopped accepting credit cards in January 2018
CHAPS	Same day	Usually a fee from your bank (£20-30)
Cheque by post	Allow 5 working days	Send to HMRC's address with your payment slip
At your bank or building society	3 working days	Take your payslip from HMRC
Budget Payment Plan	Spread payments	Set up regular weekly or monthly payments in advance

Recommended: Pay by Bank Transfer

The simplest method is online banking (Faster Payments). Log into your bank, set up a new payee with HMRC's bank details:

• Sort code: 08 32 10
• Account number: 12001039
• Account name: HMRC Cumbernauld
• Reference: Your UTR followed by K (e.g., 1234567890K)

The K at the end of your UTR tells HMRC this is a Self Assessment payment. The money usually clears within hours.

Setting Up a Budget Payment Plan

If you want to spread the cost, you can set up a Budget Payment Plan through your Government Gateway account. This lets you make regular payments towards your bill throughout the year. It is not a Time to Pay arrangement (that is for when you cannot afford to pay on time). It is simply a way to budget by paying in instalments rather than a lump sum.

Key Deadlines for Self Assessment 2025/26

Deadline	Date	What
Tax year starts	6 April 2025	Start of the 2025/26 tax year
Tax year ends	5 April 2026	End of the 2025/26 tax year
Register for Self Assessment	5 October 2026	Deadline to register if you are new to Self Assessment
Paper return deadline	31 October 2026	Last day to file a paper return
Online return deadline	31 January 2027	Last day to file online and pay your tax
First payment on account	31 January 2027	If applicable (see above)
Second payment on account	31 July 2027	If applicable
Balancing payment	31 January 2028	Any remaining tax for 2026/27

The 31 January 2027 deadline is the critical one. This is when your completed return must be submitted online and any tax owed must be paid in full. For more detail on this specific deadline and what happens if you miss it, see our guide on the Self Assessment deadline 2026.

HMRC Time to Pay

If you genuinely cannot pay your full tax bill by the deadline, contact HMRC before the deadline to arrange a Time to Pay agreement. This lets you spread your payments over up to 12 months (sometimes longer). You will still pay interest on the outstanding amount, but you will avoid the late payment penalties. You can set this up online if your bill is £30,000 or less and your return is filed.

Penalties for Late Filing and Late Payment

HMRC's penalty regime has two components: penalties for filing late and penalties for paying late.

Late Filing Penalties

How Late	Penalty
1 day late	£100 fixed penalty (applies even if you owe no tax)
3 months late	£10 per day for up to 90 days (maximum £900)
6 months late	5% of the tax due or £300 (whichever is greater)
12 months late	5% of the tax due or £300 (whichever is greater) — can be up to 100% of the tax due in serious cases

The penalties are cumulative. If you file 12 months late, you could face the £100 initial penalty, plus £900 daily penalties, plus two 5% penalties. On a £5,000 tax bill, that is £100 + £900 + £300 + £300 = £1,600 in penalties (in addition to the tax itself and interest).

Late Payment Penalties

How Late	Penalty
30 days late	5% of the tax unpaid at that date
6 months late	Additional 5% of the tax still unpaid
12 months late	Further 5% of the tax still unpaid

Interest on Late Payment

In addition to penalties, HMRC charges interest on late payments. The interest rate is the Bank of England base rate plus 2.5%. As of early 2026, this is approximately 7.25% per annum. Interest runs from the original due date until the date of payment. For more on current rates, see our article on HMRC interest rates for late payment.

Reasonable Excuse

You can appeal a penalty if you have a "reasonable excuse." HMRC accepts:

• Serious illness preventing you from filing
• Death of a partner or close relative shortly before the deadline
• Fire, flood, or theft preventing you from completing your return
• HMRC system failure (documented outages)
• Postal delays (for paper returns, if posted in good time)

HMRC does not accept as reasonable excuses:

• You did not know you needed to file
• You did not receive a reminder from HMRC
• Your accountant was late
• You found the process difficult
• You did not have the money to pay

Amending Your Tax Return

Mistakes happen. You can amend your Self Assessment return within 12 months of the original filing deadline.

For a 2025/26 return filed online:

• Original deadline: 31 January 2027
• Amendment deadline: 31 January 2028

To amend, log into your Government Gateway account, find the relevant return, and make changes. HMRC recalculates your tax automatically. If you owe more, you should pay the difference promptly to avoid interest. If you are due a refund, HMRC will process it.

After the 12-month amendment window closes, you can no longer amend online. You would need to write to HMRC and make an "overpayment relief claim" (within 4 years) or notify them of an error.

Record Keeping: What to Keep and for How Long

HMRC requires you to keep records that support the figures in your tax return. If HMRC opens an enquiry, you must be able to produce these records.

What Records to Keep

• Employment: P60s, P11Ds, payslips
• Self-employment: All invoices issued, all receipts for expenses, bank statements, mileage logs
• Property: Tenancy agreements, rental statements, receipts for repairs and expenses, mortgage statements
• Investments: Contract notes for share purchases and sales, dividend vouchers, bank interest certificates
• Capital gains: Purchase and sale records, valuations, improvement costs
• General: Bank statements for all accounts, gift aid receipts, pension contribution certificates

How Long to Keep Records

Situation	Keep Records For
Employed or pensioner with simple affairs	22 months from the end of the tax year
Self-employed	5 years from the 31 January filing deadline
Company director	5 years from the 31 January filing deadline
Property landlord	5 years from the 31 January filing deadline
If HMRC opens an enquiry	Until the enquiry is settled

For a 2025/26 return, the 31 January filing deadline is 31 January 2027. Self-employed individuals, directors, and landlords should keep their records until at least 31 January 2032.

Digital records are acceptable. Photographs of receipts, PDFs of bank statements, and digital copies of invoices are all fine, provided they are legible and complete. AccountsOS lets you upload and store all your receipts and documents digitally, so you always have them when you need them.

Using an Agent or Accountant

You can appoint an accountant or tax agent to file your Self Assessment on your behalf. They file using their own agent credentials and can access your tax record with your authorisation.

How to Authorise an Agent

1. Your agent requests authorisation through HMRC's online agent services
2. HMRC sends you a notification (letter or online) to approve
3. You confirm the authorisation
4. Your agent can then view your records and file on your behalf

What an Agent Does

An agent typically:

• Prepares your tax return from records you provide
• Calculates your tax liability
• Files the return with HMRC
• Advises on tax planning and reliefs
• Handles correspondence with HMRC on your behalf
• Represents you if HMRC opens an enquiry

What an Agent Does Not Do

Even with an agent:

• You remain legally responsible for the accuracy of your return
• You still need to keep your own records
• Penalties are charged to you, not your agent
• The filing deadline is the same whether you use an agent or not

Typical costs for an accountant to prepare and file a straightforward Self Assessment return range from £150 to £500, depending on complexity and location. If you are a company director with a limited company, the fee is usually part of your annual accountancy package (£750 to £2,000 per year for a micro business).

Common Mistakes to Avoid

1. Missing the Deadline

This is the most expensive mistake. An automatic £100 penalty applies from day one, regardless of whether you owe any tax. Set a reminder for early January at the latest.

2. Forgetting to Declare All Income

HMRC receives data from employers, banks, and investment platforms. If you forget to include bank interest or a small freelance job, HMRC's systems will likely flag it. Always declare everything.

3. Not Claiming All Allowable Expenses

Many people under-claim. If you work from home, claim the home office proportion of your bills. If you drive for business, claim mileage. If you pay professional subscriptions, claim them. Keep receipts for everything.

4. Confusing Turnover with Profit

Self-employed individuals declare turnover (total income) and expenses separately. Your tax is on the profit, not the turnover. Make sure you list all allowable expenses.

5. Not Registering in Time

If you need to file for the first time and leave registration until January, you may not receive your UTR and activation code in time. Register by October at the latest.

6. Ignoring Payments on Account

Your first Self Assessment bill might be far higher than expected because it includes the first payment on account. Budget for this.

7. Using the Wrong Tax Year Figures

The tax year runs 6 April to 5 April, not the calendar year. Make sure you are entering figures for the correct period.

8. Not Keeping Records

If HMRC asks for evidence and you cannot produce it, your expense claims may be disallowed and you could face additional penalties.

9. Entering Gross Instead of Net (or Vice Versa)

Employment income on your P60 is gross (before tax). Enter the gross figure on your return. HMRC separately accounts for the tax already deducted.

10. Forgetting to Include Your State Pension

The State Pension is taxable income. It is not taxed at source, so it must go on your tax return. Many retirees forget this.

Budgeting for Your Tax Bill

The biggest complaint about Self Assessment is the shock of a large tax bill in January. Here is how to avoid that.

The 30% Rule

A simple rule of thumb for self-employed individuals: set aside 30% of your net profit each month into a separate savings account. This covers income tax and National Insurance for most basic rate taxpayers. Higher rate taxpayers should set aside 40-45%.

Monthly Budgeting Example

If your monthly self-employment profit is £4,000:

Approach	Monthly Saving	Annual Saving
30% rule (basic rate)	£1,200	£14,400
40% rule (higher rate)	£1,600	£19,200

You can also use HMRC's Budget Payment Plan to make regular payments throughout the year, so the January bill is already partially or fully covered.

Use a Tax Calculator

Rather than guessing, use a tax calculator to estimate your actual liability based on your specific income mix. This gives a far more accurate figure to budget from.

Making Tax Digital and Self Assessment

From April 2026, Making Tax Digital for Income Tax Self Assessment (MTD for ITSA) begins to affect some Self Assessment filers. Initially, this applies to self-employed individuals and landlords with qualifying income above £50,000.

Under MTD for ITSA, you must:

• Use MTD-compatible software to keep digital records
• Submit quarterly updates to HMRC (not just an annual return)
• Submit a final declaration (replacing the traditional Self Assessment return)

If your qualifying income is below £50,000 (or you are a company director filing only for dividends), you are not affected by MTD for ITSA initially. The threshold drops to £30,000 from April 2027.

For full details, see our guide on Making Tax Digital 2026.

Self Assessment for Company Directors: Key Differences

Company directors have a specific Self Assessment profile. If you run a limited company, here is what applies to you:

• Salary through PAYE: Reported on SA102. Your company operates PAYE, deducts tax and NI, and gives you a P60
• Dividends: Reported in the dividends section of the main SA100. You do not use a supplementary page for dividends
• Director's Loan Account (DLA): If your DLA is overdrawn at the company's year end, the company pays a Section 455 charge. This does not appear on your personal Self Assessment, but if the loan is written off, it becomes a benefit in kind that does
• Benefits in kind: Company car, private medical insurance, etc. are reported on P11D and declared on your return

For a deeper dive into Self Assessment specifically for directors, read our guide on Self Assessment for directors.

What Happens After You File?

Once your return is submitted:

1. Acknowledgement: HMRC sends an electronic acknowledgement with a reference number
2. Tax calculation: Available immediately if filed online (or posted to you for paper returns)
3. Payment: Pay any tax owed by 31 January
4. Refund: If you have overpaid, HMRC processes a refund (usually within 5-12 working days for online filers)
5. Tax code adjustment: HMRC may adjust your PAYE tax code to collect underpayments through your salary
6. Enquiry window: HMRC can open an enquiry into your return within 12 months of the filing date (or longer if the return was filed late)

HMRC Enquiries

HMRC selects returns for enquiry based on:

• Random selection
• Risk profiling (unusual patterns, large changes year-on-year)
• Information from third parties that does not match your return
• Specific compliance campaigns targeting certain industries or reliefs

If selected, HMRC will write to you requesting specific records and explanations. Having complete, well-organised records makes this process far less painful.

Checklist: Before You Submit

Before hitting "Submit," verify:

• All income sources are included (employment, self-employment, dividends, interest, rental, pension, capital gains, foreign)
• Figures match your P60, P11D, bank statements, and other source documents
• All allowable expenses are claimed with supporting records
• Tax reliefs are correctly entered (Gift Aid, pension, EIS/SEIS)
• Student loan plan type is correct (if applicable)
• Marriage Allowance transfer is claimed or received (if applicable)
• High Income Child Benefit Charge is included (if income exceeds £60,000)
• The correct tax year is selected (2025/26 for filing by 31 January 2027)
• Your bank details are correct if you are expecting a refund
• You understand payments on account and the total amount due on 31 January

Frequently Asked Questions

What is the Self Assessment deadline for 2025/26?

The online filing deadline for the 2025/26 tax year is 31 January 2027. This is also the deadline for paying any tax owed and the first payment on account for 2026/27. Paper returns must be filed by 31 October 2026. Missing the online deadline triggers an automatic £100 penalty.

How do I find my UTR number?

Your 10-digit Unique Taxpayer Reference (UTR) appears on previous Self Assessment correspondence from HMRC, on your SA302 tax calculation, and in your Government Gateway account. If you have never registered, you need to register for Self Assessment and HMRC will post your UTR to you within 10-20 working days.

Can I file my Self Assessment for free?

Yes. HMRC's online Self Assessment service is completely free. You file through your Government Gateway account at no cost. Commercial software (such as tax return apps) may charge fees, but HMRC's own service handles the majority of situations and is free to use.

What happens if I miss the Self Assessment deadline?

You receive an automatic £100 penalty even if you owe no tax. After 3 months, daily penalties of £10 per day begin (up to 90 days, maximum £900). At 6 months and 12 months, further penalties of 5% of the tax due (or £300, whichever is greater) are charged. Late payment also triggers separate 5% surcharges at 30 days, 6 months, and 12 months, plus interest at the Bank of England base rate plus 2.5%.

Do I need to file Self Assessment if I earn under £1,000 from a side hustle?

No. The trading allowance gives you £1,000 of tax-free income from self-employment or casual trading. If your gross income from all trading activities is £1,000 or less, you do not need to declare it or register for Self Assessment (unless you have other reasons to file). If your trading income exceeds £1,000, you must register and file, but you can deduct the £1,000 trading allowance instead of actual expenses.

How long does a Self Assessment refund take?

If you file online, HMRC typically processes refunds within 5 to 12 working days. If you file a paper return, it takes longer (up to 12 weeks). Refunds are paid directly into the bank account you specified on your return, or by cheque if no bank details were provided.

Can I amend my Self Assessment after submitting?

Yes. You can amend your return online within 12 months of the original filing deadline. For a 2025/26 return, the amendment deadline is 31 January 2028. Log into your Government Gateway account, find the return, and make changes. After the 12-month window, you need to write to HMRC with details of the error.

Do I need to declare dividends on my Self Assessment?

Yes. All dividends must be declared on your Self Assessment return, including dividends from your own limited company and dividends from shares you hold in other companies. The first £500 of dividends is covered by the dividend allowance and taxed at 0%, but you must still declare the total amount. Dividends within ISAs are tax-free and do not need to be declared.

What is the difference between payments on account and my tax bill?

Your tax bill is the actual tax you owe for the year you are filing. Payments on account are advance payments towards next year's tax bill, each equal to 50% of the current year's bill. If your Self Assessment bill exceeds £1,000 and more than 80% of your tax was not collected at source, you must make payments on account. This means your first Self Assessment payment includes your actual tax bill plus the first payment on account for the following year.

Should I use an accountant for Self Assessment?

If your affairs are simple (one PAYE job, perhaps some bank interest), you can easily file yourself using HMRC's free online service. If you are self-employed, a company director with dividends, a landlord, or have multiple income streams, an accountant adds value through tax planning advice and ensuring you claim all available reliefs. The cost (£150-£500 for a straightforward return) often pays for itself in tax savings you would not have identified alone.