CT1 Return and Corporation Tax Deadlines in Ireland: When to File, When to Pay and What Late Filing Costs

Corporation tax deadlines are the most common source of avoidable cost for Irish companies. The tax itself is usually paid in full. The problem is timing: preliminary tax paid late or underpaid attracts interest, and a CT1 filed late attracts a surcharge and, more painfully, a restriction on loss relief that can cost far more than the surcharge.

This guide sets out the three dates every company has to manage, the rules for small and large companies, a worked calendar for the most common year end, and exactly what happens when a date is missed. Every rule below comes from Revenue's Tax and Duty Manuals, linked at the end.

The three dates every company has

For each accounting period, a company has:

1. A preliminary tax payment date, before the period ends.
2. A CT1 filing date, nine months after the period ends.
3. A balance payment date, which is the same as the filing date.

Companies that are part of a group or that pay income tax on certain payments have additional obligations, but those three dates cover the vast majority of Irish companies.

The 23rd of the month rule

Most of the statutory dates are expressed as a day in a month, for example 31 days before the period ends or nine months after it ends. Revenue then brings the date forward to the 21st of that month, or the 23rd where the company files and pays through ROS. Since almost every company is required to file electronically, the 23rd is the date that matters in practice.

The rule works as "the earlier of" two dates. Where the statutory day falls on or after the 23rd, the deadline moves to the 23rd. Where it falls before the 23rd, the statutory day itself is the deadline. A company with a 4 January year end, for example, has a statutory balance date of 4 October, which is earlier than 23 October, so 4 October applies.

Preliminary tax for small companies

A small company for this purpose is one whose corporation tax liability for the previous accounting period was €200,000 or less. Close company surcharges count towards that figure. Where the previous period was shorter than twelve months, the limit is reduced in proportion.

A small company makes one preliminary tax payment. It can choose to pay either:

• 100% of its corporation tax liability for the previous accounting period, or
• 90% of its corporation tax liability for the current accounting period.

The payment is due on the earlier of 31 days before the end of the accounting period or the 23rd of that month. For a twelve-month period ending on 31 December, that means 23 November.

The 100% prior-year option is the safe harbour. If the company pays 100% of last year's liability on time, it has met its obligation even if this year's profits turn out to be much higher. The 90% current-year option saves cash in a declining year but requires an accurate forecast with more than a month of the year still to run. If the final liability comes in higher than expected and the payment turns out to be less than 90%, interest runs from the preliminary tax date on the shortfall.

Preliminary tax for large companies

A large company is one whose corporation tax liability for the previous accounting period exceeded €200,000. It pays preliminary tax in two instalments.

The first instalment is due on the earlier of the last day of the sixth month of the accounting period or the 23rd of that month. The amount is either 50% of the previous period's liability or 45% of the current period's liability.

The second instalment is due on the earlier of 31 days before the end of the period or the 23rd of that month. It must bring the total paid up to 90% of the current period's liability.

A company whose accounting period is shorter than seven months pays 90% in a single instalment.

New companies

A new company does not have to pay preliminary tax for its first accounting period, provided its liability for that period is under €200,000. It simply pays the full liability when it files its first CT1. This is a genuine cash-flow advantage in year one, but it also means the first balance payment and the second year's preliminary tax can land close together, so it is worth planning for.

Worked calendar: year end 31 December 2026

For a small company with a 31 December 2026 year end that files and pays through ROS:

• 23 November 2026: preliminary tax due. Pay either 100% of the 2025 liability or 90% of the estimated 2026 liability.
• 23 September 2027: CT1 for 2026 due, together with the balance of corporation tax for 2026.
• 23 December 2027: latest date for iXBRL financial statements, where required, under Revenue's practice of allowing them up to three months after the CT1 due date. Filing them with the CT1 is safer, for the reasons below.

For a large company with the same year end:

• 23 June 2026: first instalment, being 50% of the 2025 liability or 45% of the estimated 2026 liability.
• 23 November 2026: second instalment, bringing the total to 90% of the 2026 liability.
• 23 September 2027: CT1 and balance.

If the year end is not 31 December, the same pattern applies to whichever months are 31 days before the period end and nine months after it. The dates for every month end are listed on our tax calendar.

What happens if preliminary tax is underpaid or late

Where preliminary tax is insufficient or paid late, the whole liability for the period is treated as having been due on the preliminary tax date, and interest runs from that date until the tax is paid. Revenue's worked examples use a daily rate of 0.0219%, which is roughly 8% a year.

In Revenue's example, a small company with a €125,000 liability pays €100,000 on 23 November and the remaining €25,000 the following September. Because €100,000 is less than both 90% of the current year and 100% of the prior year, interest runs on the €25,000 for the full ten months. Even where the right amount is paid but a few weeks late, interest runs on the full amount for those weeks.

Interest also arises retrospectively. If a Revenue audit or a voluntary disclosure increases the liability for a past period, the company has to go back and test whether its preliminary tax for that period still met the 90% or 100% test. Often it does not, and interest is recalculated from the original preliminary tax date.

What happens if the CT1 is late

Two separate consequences follow a late CT1.

The first is the late filing surcharge under section 1084. The tax liability for the period is increased by 5% where the return is filed within two months of the deadline, capped at €12,695, or by 10% where it is more than two months late, capped at €63,485. The surcharge applies even if the tax was paid in full and on time. A deliberately or carelessly incorrect return is treated as a late return unless it is corrected before the filing date.

The second is the restriction of reliefs under section 1085, and this is the one that catches companies with losses. Where the return is less than two months late, claims for loss relief, excess capital allowances and group relief are cut by 25%, capped at €31,740 for the period. Where it is two months or more late, the cut is 50%, capped at €158,715. A loss-making company that thinks there is nothing to lose by filing late can find that half of its carried-forward loss has disappeared.

Revenue's manual also makes the point that a CT1 which omits a close company surcharge or the income tax due on a loan to a shareholder is an incomplete return, which opens the door to interest and a surcharge for failing to submit a correct return.

iXBRL accounts

Most companies must also file their financial statements in iXBRL format. A company is excluded only if it meets all three of these criteria: a balance sheet total of no more than €4.4 million, turnover of no more than €8.8 million, and an average of no more than 50 employees. Excluded companies complete the extracts from accounts section of the CT1 instead.

Revenue's current practice allows iXBRL accounts to be filed with the CT1 or up to three months after the CT1 due date. Until they are filed, the CT1 is treated as incomplete, which can hold up refunds and block a tax clearance application, so filing them with the CT1 is the safer course. Where the ROS upload sticks at "processing" past the deadline, Revenue will consider waiving any surcharge if the company contacts the ROS helpdesk and raises the issue through MyEnquiries with evidence.

How the CT1 is filed

The CT1 is filed on ROS. In outline:

1. Finalise the accounts and the corporation tax computation, including capital allowances, loss claims, any close company surcharge and any start-up relief or R&D credit claim.
2. Log in to ROS, select the company, and open the CT1 for the accounting period.
3. Complete the panels for trading results, other income, chargeable gains, capital allowances, losses, close company details and reliefs claimed. The figures must match the computation.
4. Upload the iXBRL financial statements where required.
5. Review the calculated liability against the computation, then sign and submit.
6. Pay the balance through ROS by the same date, by debit instruction or card.

The ROS form is unforgiving of inconsistencies between panels. The most common errors we see on returns prepared in-house are rental income entered as trading income, which understates tax at 25%, capital allowances claimed on assets that were expensed in the accounts, and close company surcharge panels left blank.

Getting help with your CT1 and corporation tax

We prepare CT1 returns and iXBRL accounts for Irish companies, calculate preliminary tax under both the prior-year and current-year methods so you can choose the better one, and diarise every date so nothing is late. Contact us today if you need help with CT filings, or get in touch here and we will get back to you.

Sources

The rules and figures in this guide are taken from Tax and Duty Manual Part 41A-07-02, Payment of Preliminary Corporation Tax; Part 47-06-08, Surcharge for late submission of returns; Part 47-06-04, Late submission of corporation tax returns and restriction of reliefs; and Part 47-06-01, Return filing dates and iXBRL; and Part 41A-03-01, Submission of iXBRL financial statements. Information provided courtesy of the Revenue Commissioners under a Creative Commons Attribution 4.0 International (CC BY 4.0) licence.