How to File Corporation Tax in Ireland via ROS: CT1 Step-by-Step Guide 2026

Corporation Tax is one of those compliance areas where the Irish system looks simple on the surface (a 12.5% rate, file once a year) but the mechanics of how and when you pay catch founders out every year. The preliminary tax system requires you to pay an estimate of your tax bill before your accounting year has even closed. The CT1 filing deadline is in September, which doesn't feel like an obvious "year-end" to anyone running a December financial year. And the platform you use, ROS, requires a digital certificate that lives in your browser and needs backing up.

This guide covers all of it: how to register for ROS, what the CT1 contains, the exact dates you need to know for common year-ends, who counts as a small versus large company for preliminary tax purposes, the R&D tax credit that can turn a CT bill into a cash refund, and the penalties you want to avoid. At the end, there's a worked example for a typical Irish Ltd with a December year-end so you can see every payment and deadline in one place.

For context on the headline rates and how trading versus non-trading income works, read the companion article: Corporation Tax in Ireland: 2026 Guide for Founders. This article focuses specifically on the mechanics of registration, preliminary tax, and filing.

What is ROS?

ROS stands for Revenue Online Service. It is the Irish Revenue Commissioners' secure online platform for all business tax filings and payments. If you run an Irish company, almost every interaction with Revenue happens through ROS. There is no paper option for most filings, and there is no separate platform: one login handles Corporation Tax, VAT, PAYE, and everything else.

ROS is accessible at ros.ie. It is separate from myAccount, which is the equivalent portal for PAYE employees and private individuals managing personal tax credits. If you are both a company director and a PAYE employee elsewhere, you may need both.

What ROS handles

• Corporation Tax: Form CT1 filings and payments for all accounting periods
• VAT: Bi-monthly VAT3 returns, due the 23rd of the month following each period
• Employer PAYE/PRSI: Monthly payroll submissions and P30 remittances, due the 23rd of the following month
• Form 11: Self-assessment for directors with non-PAYE income
• RCT: Relevant Contracts Tax for principal contractors in construction and forestry
• Capital Gains Tax: Two payment instalment dates per year (31 January and 31 October)
• Dividend Withholding Tax: Returns for companies paying dividends

ROS filers get a deadline extension that paper filers do not. VAT returns and payments otherwise due by the 19th of the month extend to the 23rd for ROS filers. The CT1 extension works the same way: paper filers must meet the 21st of the ninth month; ROS filers have until the 23rd.

The ROS digital certificate

This is where many directors get caught out. ROS uses a digital certificate system rather than a simple username and password. The certificate is a file that lives in your browser. Every time you log in to ROS, you select this certificate and enter your ROS password.

The key risk: if you lose the certificate and have no backup, you have to restart the registration process from scratch, including waiting for a new access code to arrive by post. This has delayed CT1 filings for companies whose accountant changed, whose employee left, or whose computer was replaced without exporting the certificate first.

We will cover registration in detail below. The one thing to take away now: back up your ROS digital certificate as a .p12 file the moment you create it.

See the full ROS glossary entry for additional detail on browser compatibility and certificate renewal.

What is Form CT1?

Form CT1 is Ireland's annual Corporation Tax return. Every Irish-resident company must file one for each accounting period, covering all taxable income and gains arising during that period.

The CT1 is not a simple form. It is a full tax computation covering trading income, non-trading income, chargeable gains, capital allowances, available credits, group relief, and any surcharges for close companies. For most owner-managed Irish Ltds, the computation involves:

• Adjusting the accounting profit from the profit and loss account to arrive at taxable trading profit
• Claiming capital allowances in place of accounting depreciation
• Splitting any passive income (bank interest, rental income) and applying the 25% rate
• Claiming the R&D tax credit if qualifying expenditure was incurred
• Disclosing any chargeable gains from asset disposals

The company does not need to file separate accounts with Revenue as a standalone document: medium and large companies attach iXBRL-tagged financial statements as part of the CT1 submission itself. Small companies are exempt from iXBRL but must still include the required schedules.

CT1 rates in 2026

For the vast majority of owner-managed Irish Ltds, every euro of profit is trading income taxed at 12.5%. The 25% rate is relevant if the company holds a rental property, earns significant bank interest on retained profits, or holds shares paying foreign dividends.

See the Form CT1 glossary entry for the full list of schedules and a worked example of a small company's CT1 computation.

Key dates: preliminary tax vs CT1 filing

This is the most important section of this guide, because the Irish system has two separate obligations that many directors confuse or conflate.

Obligation 1: Preliminary tax: paid before your accounting year closes. This is not a filing. There is no form to submit. You simply log in to ROS and make a payment.

Obligation 2: CT1 filing: submitted after your year-end, along with any balance of tax due. This is the return itself.

Many UK founders moving to Ireland are surprised that they must pay a significant chunk of their tax bill before the year has even ended. In the UK, corporation tax is paid nine months and one day after year-end. In Ireland, you pay an estimate one month before year-end, then the balance when you file the return nine months later.

Dates for common year-ends (small companies)

The pattern is consistent: preliminary tax on the 23rd of the month before year-end, CT1 on the 23rd of the ninth month after year-end. Both dates apply only to ROS filers; paper filers face earlier deadlines (the 21st), but virtually no Irish companies file on paper.

Why September trips up December year-end companies

A company with a 31 December year-end naturally associates its filing obligations with "end of year" activity. But the CT1 is not due at year-end, and it is not even due in the calendar year in which the accounting period falls. It is due in September of the following year.

A company with a December 2025 year-end needs to file its CT1 by 23 September 2026. Directors who do not work with an accountant, or who do not set calendar reminders in January for the following September, often miss this. It is the single most common reason for CT1 surcharges in Ireland.

Who qualifies as a small versus large company?

The preliminary tax rules differ significantly depending on whether your company is classified as "small" or "large" for CT purposes. The dividing line is the prior year CT liability.

Small company: prior year CT liability under €200,000

Small companies have two options for calculating preliminary tax:

• Option A: Pay 100% of the prior year's final CT liability. This is the safe harbour: as long as you pay this amount by the deadline, Revenue cannot charge interest on the preliminary tax, regardless of what your actual current year liability turns out to be.
• Option B: Pay 90% of the current year's estimated CT liability. Useful if your profits have dropped significantly from the prior year and paying 100% of the prior year would mean overpaying by a large amount.

Most owner-managed Irish Ltds use Option A because the prior year figure is known and there is no risk of underpaying.

The preliminary tax for a small company is due on the 23rd of the month immediately before the accounting period ends. For a December year-end company, that means 23 November of the same year the period is running.

Large company: prior year CT liability of €200,000 or more

Large companies cannot use the simple prior-year safe harbour. They must pay preliminary tax in two instalments:

First instalment (month 6 of the accounting period):

• 50% of the prior year's CT liability, OR
• 45% of the current year's estimated liability

Second instalment (month 11 of the accounting period):

• Enough to bring total preliminary tax paid to 90% of the estimated current year liability

For a company with a 31 December year-end and a prior year CT liability above €200,000:

• First instalment: 23 June of the current year
• Second instalment: 23 November of the current year
• CT1 filing and final balance: 23 September of the following year

The two-instalment regime is more complex and cash-flow-intensive. Getting the estimates right at month 6, when much of the year is still to come, requires careful forecasting.

First year companies

A company in its first accounting period has no prior year liability. The small-company safe harbour (100% of prior year) effectively means zero preliminary tax is required in year one. Revenue does not penalise a first-year company for paying no preliminary tax, provided the company genuinely had no prior year liability.

This is one of the genuine benefits of the Irish system for startups: no cash-flow pressure from preliminary tax in year one. The CT1 is still due nine months after the first year-end, and the full liability is paid at that point.

Step-by-step: registering for ROS

If your company is not yet registered for ROS, here is the process. Allow ten working days from start to first login because part of the registration requires waiting for a letter in the post.

Step 1: get your company's tax reference number

Your company's Corporation Tax reference number is assigned by Revenue when the company is registered for tax. It typically consists of 7 digits followed by a letter, for example 1234567A. If you do not have this, check your company's certificate of incorporation from the CRO and then contact Revenue's business helpline to confirm the tax registration number.

Step 2: apply for ROS at ros.ie

Go to ros.ie and click "Register for ROS." You will need:

• Your company's tax reference number
• The company's registered address (to receive the access code letter)
• An email address and mobile number

Revenue will send a one-time access code to the company's registered address by post. Allow 3 to 5 working days for the letter to arrive. This is the step that prevents same-day registration: there is no way to skip the postal access code.

Step 3: create your digital certificate

Once you have the access code from the letter, return to ros.ie and use it to:

• Create your ROS password
• Generate your ROS digital certificate
• Download the certificate to your browser

The certificate generation creates a file that your browser stores. You will be prompted to set a password for the certificate itself (separate from your ROS login password).

Step 4: back up the certificate immediately

This is not optional. Before doing anything else, export your ROS digital certificate as a .p12 file and save it somewhere secure, such as an encrypted cloud storage location or a password manager that supports file attachments. If you run the company with a company secretary or accountant, make sure they have the backup too.

The .p12 file is protected by the certificate password you set in Step 3. Without both the file and the password, the backup is useless.

When you move to a new computer, import the .p12 file before decommissioning the old machine. The import option is in your browser's certificate manager.

Step 5: test your login

Log in to ROS using your certificate. Navigate to the "My Services" section. You should see the company's tax registrations: Corporation Tax, VAT (if registered), PAYE (if registered as an employer).

If you do not see CT listed, the company may not be registered for Corporation Tax yet. Contact Revenue to register for CT before attempting to file the CT1.

Agent access

Most companies do not file their CT1 themselves: they instruct an accountant or tax agent who has their own agent ROS login. In this model, the agent is linked to your company's Revenue record and files on your behalf. You do not need to log in to ROS yourself for the CT1, but you should still have your own ROS access for transparency and to handle tasks the agent does not manage (such as VAT payments if you handle those in-house).

Step-by-step: filing the CT1

The CT1 itself is typically filed by an accountant. The following describes what information needs to be gathered, what the return contains, and what the submission process involves, so you know what to expect and what to provide.

What your accountant needs

Before they can prepare the CT1, your accountant will need:

1. Final signed financial statements: profit and loss account and balance sheet for the accounting period. These need to be prepared under FRS 105 (micro entities) or FRS 102 (small companies) and signed by the directors.
2. A full list of fixed assets: purchases, disposals, and year-end written-down values, to compute capital allowances.
3. Details of any R&D activity: if the company incurred qualifying R&D expenditure, a description of the projects and the eligible costs (staff time, contractors, materials).
4. Dividend payments made during the year: amounts, dates, and recipients.
5. Any capital asset disposals: sale proceeds and original cost, to calculate any chargeable gain.
6. Details of any losses from prior years: carried forward trading losses that can be used against the current year profit.

If you use accounting software that produces a trial balance, providing this to the accountant speeds up the process significantly.

What the CT1 form contains

The CT1 is a long form with multiple sections. The key ones are:

Trading income computation: starts with accounting profit, then adds back non-deductible items (depreciation, entertaining, personal expenses claimed through the company, capital expenditure wrongly treated as revenue) and deducts capital allowances (wear and tear at 12.5% per year straight-line on plant and machinery).

Non-trading income: any rental income, bank interest, foreign dividends, or other passive income. This is taxed at 25%, not 12.5%.

Chargeable gains: net gains from asset disposals during the period. CGT in a company context is charged at 33%.

Capital allowances schedule: itemises qualifying assets, original cost, accumulated allowances to date, and the current year's claim. Accelerated allowances for energy-efficient equipment are claimed here.

R&D tax credit schedule: if claiming, the form requests details of the qualifying projects, expenditure breakdown by category, and the credit calculation. The credit is 30% of qualifying spend. If it exceeds the CT liability, the excess can be refunded in three annual instalments, or in full in year one if the credit is under €50,000.

Section 486C Start-Up Relief: available to companies in the first five years of trading where employer PRSI paid does not exceed €40,000. Maximum relief is €40,000 of CT per year. This must be claimed on the CT1; it is not automatic.

Close company surcharge: if the company is a close company (broadly, controlled by five or fewer directors/shareholders or by its directors) and has undistributed investment or estate income above a threshold, an additional 20% surcharge applies. Most trading owner-managed Ltds are close companies but rarely have non-distributed passive income that triggers this.

iXBRL financial statements: medium and large companies must attach iXBRL-tagged accounts. Small companies (turnover under €12 million, balance sheet under €6 million, fewer than 50 employees in two consecutive years) are exempt, though the accounts still need to be prepared and retained.

Submitting on ROS

Once the CT1 is prepared:

1. The filer (accountant or company) logs in to ROS using the digital certificate
2. Navigates to "File a Return" and selects Corporation Tax for the relevant accounting period
3. Completes the online form sections, attaching iXBRL accounts if required
4. Reviews the computed tax liability
5. Submits the return
6. Pays the balance of CT due via ROS (direct debit, debit card, or bank transfer)

A filing receipt is generated automatically. Keep this: Revenue retains an electronic record but the receipt provides immediate confirmation of submission.

When to file early

There is no benefit to filing the CT1 before the balance payment is due, except that filing early removes the risk of last-minute technical issues with ROS. If you are expecting a tax refund (for example, because preliminary tax was overpaid against a lower-than-expected actual liability), filing early also accelerates the refund.

Common mistakes

Mistake 1: treating preliminary tax as optional

Preliminary tax is not a filing, so some founders do not notice they have missed it until the interest notice arrives. Revenue charges interest at approximately 0.0219% per day (around 8% per year) on any underpayment of preliminary tax. On a €50,000 CT liability, missing the preliminary tax entirely means accumulating about €4,000 of interest before you file the CT1 in September.

The fix: set a calendar reminder in mid-October (for December year-end companies) to check what last year's final CT liability was, calculate 100% of that figure, and pay it via ROS by 23 November.

Mistake 2: confusing the September filing date

Companies with December year-ends expect their tax filing to land in December or the following January. It does not. It lands in September of the following year. A company with a 31 December 2025 year-end owes the CT1 by 23 September 2026. The mismatch between the year-end and the filing date is a known trap.

Mistake 3: losing the ROS certificate

If the person who created the ROS certificate leaves the company, is unreachable, or their computer was wiped without exporting the .p12 file, the company loses its ROS access. Re-registering takes two to three weeks (waiting for the postal access code). If this happens close to a filing deadline, a surcharge becomes very hard to avoid.

Mistake 4: not claiming Start-Up Relief

Section 486C Start-Up Relief can eliminate CT entirely for the first five years of a trading company's life, subject to a link to employer PRSI paid. Maximum relief is €40,000 of CT per year. It must be explicitly claimed on the CT1. Revenue does not alert you to it. Many eligible founders simply do not know it exists.

Mistake 5: skipping iXBRL when required

If a company has grown past the small-company thresholds in two consecutive accounting periods, it loses the iXBRL exemption. An accountant following the thresholds will catch this; companies trying to file without an accountant often do not. ROS will auto-reject a CT1 submission if iXBRL accounts are required but not attached.

Mistake 6: underpaying preliminary tax because the year went better than expected

The 100% of prior year safe harbour protects you regardless of how the current year turns out. But if profits grew significantly and you chose to pay 90% of your own estimate (Option B), and the estimate was too low, Revenue will charge interest on the shortfall. Companies in growth years should either default to the prior year safe harbour or get a careful current-year estimate from their accountant.

R&D tax credits and the CT1

Ireland's R&D tax credit is one of the most generous in Europe and is directly relevant to CT1 filing for any company carrying out qualifying research and development.

The credit rate

From 1 January 2024, the R&D tax credit rate is 30% of qualifying expenditure. This was increased from 25% in the Finance Act 2023. The credit is calculated on qualifying R&D costs incurred during the accounting period.

What qualifies

Qualifying R&D means systematic, investigative, or experimental activity in the field of science or technology aimed at making an advance in knowledge or capability, or creating new products or processes. It is not enough to be working on technology: there needs to be genuine uncertainty that the work is trying to resolve.

For Irish software companies and tech startups, qualifying costs typically include:

• Salaries and employer PRSI for staff working on qualifying projects (apportioned to R&D time)
• Contractor costs for external R&D support
• Materials and consumables used directly in experiments or testing
• A portion of overhead costs attributable to R&D activity

Revenue audits R&D credit claims and focuses on the documentation of the scientific or technological uncertainty. Claims should be supported by contemporaneous project records, not retrospective descriptions.

Cash refund when CT liability is zero or low

This is the most useful aspect of the Irish R&D credit for early-stage companies. If the credit exceeds the company's CT liability for the period, the excess is refundable in cash. For credits under €50,000, the full amount is refunded in the first year. Larger credits are spread over three years.

A startup with minimal profits but significant R&D spending can receive a meaningful cash payment from Revenue. This effectively turns R&D expenditure into a direct cash subsidy.

How to claim on the CT1

The R&D credit is claimed on a separate schedule within the CT1, sometimes accompanied by a Form RDT (Research and Development Tax Credit Claim). The accountant will prepare the claim and attach it to the CT1 submission. The key things the company needs to provide: a project-by-project description of the qualifying activities, the eligible expenditure per project, and the staff time attribution.

Penalties for late filing and late payment

Late filing surcharge

If the CT1 is filed after the deadline:

• Within 2 months late: 5% surcharge on the CT liability, capped at €12,695
• More than 2 months late: 10% surcharge on the CT liability, capped at €63,485

A company with a €100,000 CT bill that files two weeks late owes an extra €5,000. A company that files four months late owes an extra €10,000 (capped). On top of the surcharge, interest accrues on any unpaid tax.

Interest on late payment

Interest on unpaid CT runs at approximately 0.0219% per day, which is roughly 8% per year. This applies from the date the tax was due, not from the date the return was filed.

Interest applies separately to:

• Preliminary tax underpayments (from the preliminary tax due date)
• Final CT balance (from the CT1 filing deadline)

No reminder from Revenue

Revenue does not send CT1 filing reminders. The deadline is the company's and the accountant's responsibility. This is different from the CRO, which sends B1 reminder letters. Treat your CT1 deadlines as self-managed obligations that need calendar reminders.

Corporation Tax and the CRO: two separate bodies, two separate filings

A common source of confusion for Irish founders, especially those coming from the UK, is the relationship between the CRO and Revenue. These are entirely separate organisations with entirely separate filing obligations.

Revenue (Revenue Commissioners): collects taxes. The CT1 goes here. Revenue also collects VAT and PAYE. Filings go through ROS.

CRO (Companies Registration Office): maintains the public register of companies. The Form B1 Annual Return goes here, along with the company's financial statements. Filed via CORE (core.cro.ie).

You file the CT1 with Revenue by 23 September (for December year-ends). You file the B1 with the CRO within 56 days of your Annual Return Date. These are different dates, different portals, and different documents.

Filing one does not satisfy the other. A company can be fully up to date with Revenue and still face CRO penalties for a missed B1. Two consecutive late B1 filings with the CRO result in the permanent loss of audit exemption, which is an expensive outcome for any small company that qualifies for it.

If you searched for "CRO company" filings looking for guidance on this, see the Ireland companies section for a full overview of both CRO and Revenue obligations, including the B1 Annual Return deadlines and the Register of Beneficial Ownership (RBO) requirements.

AccountsOS and Ireland: how Finn tracks your CT1 deadlines

AccountsOS is live in Ireland. Finn, the AI accountant built into the platform, knows the Irish Corporation Tax system in detail: the preliminary tax rules, the CT1 deadlines for any year-end, the CRO B1 cycle, the R&D credit, and Start-Up Relief.

Here is what Finn does automatically for Irish companies:

Deadline tracking: Finn calculates your preliminary tax date and CT1 date from your accounting period end and adds them to your deadlines dashboard. You see both dates when you log in, with a countdown. Revenue doesn't send reminders; Finn does.

Preliminary tax calculation: based on your previous year's CT liability (which Finn knows from your transactions and accounts), Finn can show you the safe harbour amount for small-company preliminary tax. If you ask "how much preliminary tax do I owe this November?", Finn can answer that from your data.

Running CT estimate: as transactions come in through the year, Finn maintains a running estimate of your Corporation Tax liability. By October or November, you have a reasonable view of the current year's CT before the preliminary tax date, which helps you decide whether to pay the prior year safe harbour or a 90% current year estimate.

R&D credit flagging: Finn knows about the R&D tax credit. If you are running a software or tech business and have staff on development projects, Finn can flag that qualifying R&D expenditure may be claimable and prompt you to discuss it with your accountant before the CT1 is filed.

Multi-entity support: if you run a UK Ltd and an Irish Ltd, the same AccountsOS login switches between entities. Finn loads the correct tax authority, currency, and deadline calendar per entity.

Start a free trial to see Finn track your Irish CT1 and preliminary tax deadlines automatically. Or read more about AccountsOS in Ireland.

Worked example: a December year-end Irish Ltd in 2026

Company: Hazel Digital Ltd, software consultancy, Irish-resident Ltd

Accounting period: 1 January 2025 to 31 December 2025

Prior year CT liability (for 2024): €22,000

Step 1: Preliminary tax payment

Hazel Digital is a small company (prior year CT under €200,000). The safe harbour is 100% of the prior year liability: €22,000.

Due date: 23 November 2025 (the 23rd of the month before year-end).

Hazel Digital's director logs in to ROS on 20 November 2025 and pays €22,000. No return to file, just the payment.

Step 2: Year-end accounts preparation

The accounting period ends on 31 December 2025. The director instructs the company accountant in January 2026 to prepare the accounts.

Final figures:

• Trading profit: €210,000
• Bank interest: €3,000
• Total taxable income: €213,000

CT calculation:

• €210,000 × 12.5% = €26,250 (trading)
• €3,000 × 25% = €750 (passive)
• Total CT: €27,000

Step 3: CT1 filing

Preliminary tax already paid: €22,000

Balance due: €27,000 - €22,000 = €5,000

Due date: 23 September 2026 (the 23rd of the ninth month after year-end)

The accountant prepares the CT1, Hazel Digital reviews it, and the CT1 is submitted via ROS with the €5,000 balance payment on 18 September 2026.

Total cash flow timeline:

• 23 November 2025: €22,000 preliminary tax
• 23 September 2026: €5,000 balance
• Total CT for 2025 year: €27,000

Note: the prior year preliminary tax (for 2025, paid November 2025) used 100% of the 2024 liability. That was €22,000, even though the actual 2025 liability turned out to be €27,000. The underpayment (€5,000) is paid with the CT1, with no interest, because the safe harbour was met.

FAQ

What is ROS and how do I access it?

ROS (Revenue Online Service) is the Irish Revenue Commissioners' online platform for all business tax filings and payments. It is at ros.ie. Access requires a digital certificate installed in your browser. You apply for the certificate by registering at ros.ie with your company's tax reference number. Revenue then posts an access code to your registered address, which you use to generate the certificate. Registration takes around 5 to 10 working days.

When is Form CT1 due for a company with a December year-end?

A company with a 31 December year-end must file the CT1 via ROS by 23 September of the following year. So a company with a 31 December 2025 year-end files by 23 September 2026. Paper filers have until the 21st, but virtually all Irish companies file electronically via ROS.

What is preliminary tax in Ireland and when must I pay it?

Preliminary tax is an advance payment of Corporation Tax due before your accounting year closes. Small companies (prior year CT liability under €200,000) must pay 100% of the prior year's final CT liability by the 23rd of the month before year-end. For a December year-end company, that is 23 November. There is no form to file: you log in to ROS and make the payment. Failure to pay on time results in daily interest at approximately 8% per year on the shortfall.

What happens if I file the CT1 late?

Late filing of the CT1 attracts a surcharge: 5% of the CT liability (capped at €12,695) if filed within 2 months of the deadline, or 10% (capped at €63,485) if more than 2 months late. Interest on unpaid tax also accrues at approximately 0.0219% per day. Revenue does not send filing reminders, so missed deadlines are usually the result of poor diary management rather than any notification failure on Revenue's part.

What is the difference between the CRO and Revenue?

The CRO (Companies Registration Office) and Revenue are separate bodies with different obligations. Revenue collects taxes: the CT1 goes to Revenue via ROS. The CRO maintains the company register: the Form B1 Annual Return and financial statements go to the CRO via CORE (core.cro.ie). Filing one does not satisfy the other, and they have different deadlines and different penalties for late filing.

What is the R&D tax credit in Ireland and how do I claim it?

The R&D tax credit is 30% of qualifying research and development expenditure. If the credit exceeds the company's CT liability, the excess can be refunded in cash. Credits under €50,000 are refunded in full in year one. The credit is claimed on a schedule within the CT1. Qualifying activities must involve genuine scientific or technological uncertainty, and claims are supported by project documentation.

What is Section 486C Start-Up Relief?

Section 486C Start-Up Relief reduces or eliminates Corporation Tax for new trading companies in their first five years. Maximum relief is €40,000 of CT per year, tapered between €40,000 and €60,000 of CT liability. The relief is linked to employer PRSI paid during the period (capped at €5,000 per employee, €40,000 total). It must be claimed on the CT1; it is not automatic.

What is iXBRL and which companies need it for their CT1?

iXBRL (Inline eXtensible Business Reporting Language) is a format for tagging financial statement data in a machine-readable way. Revenue requires iXBRL-tagged accounts to be attached to the CT1 for medium and large companies. Small companies, broadly those meeting two of three criteria (turnover under €12 million, balance sheet under €6 million, fewer than 50 employees) in two consecutive years, are exempt. If a CT1 is submitted without iXBRL when it is required, ROS will auto-reject the filing.