How to File Your Corporation Tax Return (CT600)
Complete guide to preparing and filing your CT600 Corporation Tax return with HMRC. Includes what to include and how to calculate your tax.
What You'll Need
- Complete accounting records for the period
- Bank statements (reconciled)
- Purchase invoices for capital allowances
- Prior year accounts
- HMRC Government Gateway login
Step-by-Step Guide
Gather your financial records
You need your complete accounts for the period: income, expenses, assets, liabilities. Ensure everything is reconciled.
- •Reconcile bank accounts first
- •Categorize all expenses correctly
- •Identify capital vs revenue expenditure
Prepare your accounts
Create Profit & Loss account and Balance Sheet for the accounting period. These form the basis of your tax calculation.
- •Use accounting software for accuracy
- •Follow UK GAAP or FRS 102/105
- •Check prior year comparatives
Calculate adjustments for tax
Accounting profit isn't the same as taxable profit. Add back disallowable expenses and subtract capital allowances.
- •Entertainment is disallowable
- •Depreciation replaced by capital allowances
- •Fines and penalties are disallowable
Claim capital allowances
Calculate AIA (Annual Investment Allowance) on equipment purchases and Writing Down Allowance on existing assets.
- •AIA is 100% up to £1 million
- •Cars have special rules
- •Full expensing available for companies
Calculate Corporation Tax liability
Apply the correct tax rate: 19% for profits under £50,000, 25% for profits over £250,000, or marginal relief in between.
- •Check for associated companies (share thresholds)
- •Marginal relief formula is complex
- •Use HMRC calculator or AccountsOS
Complete the CT600 form
Fill in the CT600 form with your company details, income, deductions, and calculated tax. Attach iXBRL accounts.
- •CT600 has many boxes but most are optional
- •Online filing generates iXBRL automatically
- •Check company details are correct
Submit online to HMRC
File through HMRC's online service or commercial software. You must file online - paper returns are not accepted.
- •Deadline is 12 months after period end
- •You'll receive confirmation immediately
- •Keep a copy for your records
Pay your Corporation Tax
Payment is due 9 months and 1 day after your accounting period ends. Pay online, by bank transfer, or Direct Debit.
- •Payment deadline is before filing deadline
- •Interest charged from day after due date
- •Large companies pay in instalments
Common Mistakes to Avoid
Missing the payment deadline (it's earlier than filing)
Not claiming all available capital allowances
Forgetting to add back disallowable expenses
Not checking for marginal relief eligibility
Frequently Asked Questions
When is my Corporation Tax return due?
Your CT600 must be filed within 12 months of the end of your accounting period. However, the Corporation Tax payment itself is due 9 months and 1 day after your accounting period ends. Late filing incurs automatic penalties starting at £100.
Can I file my Corporation Tax return myself?
Yes, you can file your CT600 yourself using HMRC-recognised software. You will need your company accounts, details of all income and expenses, and any capital allowances. Many small company directors use accounting software to generate and file the return.
What expenses can I deduct on my Corporation Tax return?
You can deduct all expenses incurred wholly and exclusively for business purposes. This includes staff costs, office rent, equipment, travel, professional fees, and marketing. Capital items like computers are claimed through capital allowances rather than as direct expenses.
What is the Corporation Tax rate for small companies?
Companies with profits under £50,000 pay the small profits rate of 19%. Profits between £50,000 and £250,000 benefit from marginal relief. Companies with profits over £250,000 pay the main rate of 25%.
What happens if I file my Corporation Tax return late?
Late filing penalties are automatic: £100 if one day late, another £100 if three months late, and HMRC estimates your tax bill after six months. After 12 months, an additional penalty of 10% of the unpaid tax is applied. Interest also accrues on any late payment.
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