How to File a CT1 Return in Ireland (2026): Step-by-Step for SME Directors

The most common CT1 mistake isn’t a calculation error. It’s a director assuming the return has already been filed when it hasn’t.

Every September, accountants across Ireland field the same call: a director who thought their CT1 was “with the accountant” weeks ago, only to discover the file is still missing a final document, a signed approval, or a tagged set of accounts. By the time the gap is found, there are days left, not weeks. Revenue does not send a reminder letter before the deadline passes. The surcharge applies automatically and is calculated based on your tax liability, regardless of whether you knew the deadline had arrived.

This guide walks through the full CT1 filing process: what you need to prepare, how the filing actually works inside ROS, the iXBRL requirement that almost no guide explains from a director’s point of view, how preliminary tax reconciles against your final bill, and the five errors that account for most CT1 problems among Irish SMEs. 

If you need a refresher on corporation tax rates, reliefs, and the salary-versus-dividend decision first, read our full corporation tax guide before coming back to this one. This post assumes you already know what corporation tax is and want to know how the filing itself gets done.

What Is the CT1 Return and Who Must File?

The CT1 is the annual corporation tax return that every Irish-resident company files through the Revenue Online Service. It applies regardless of whether the company traded during the period or made a profit — a company that did nothing all year still has a filing obligation, not just a company that turned a profit.

Non-resident companies operating an Irish branch or agency also file a CT1 for the Irish-source activity. Sole traders do not file a CT1 at all—they file a Form 11, the self-assessed income tax return. This trips up a specific group of directors: people who previously traded as sole traders and have since incorporated, who instinctively reach for the Form 11 process they already know, even though the obligation has shifted to a different return entirely.

One distinction most guides skip: a dormant company must still file a CT1 every single year, declaring €0 profit. Directors of dormant or shelf companies frequently assume that no trading activity means no filing obligation. It doesn’t. The CT1 obligation is tied to the company’s existence and tax registration, not whether it generated income under the Taxes Consolidation Act 1997.

CT1 vs Form 11: Comparison

Factors	CT1	Form 11
Who files it	Irish-resident companies; non-resident companies with an Irish branch	Sole traders, proprietary directors (in respect of non-PAYE income), partners, landlords, anyone with significant non-PAYE income
What it taxes	The company’s profits, separately from the individuals who own it	The individual’s personal income, including salary, dividends received, and other untaxed income
Filed via	ROS, by the company or its tax agent	ROS, by the individual or their tax agent
Standard deadline	23rd of the 9th month after the company’s accounting period end	31 October following the tax year (ROS extension typically to mid-November)
Common confusion point	Directors assume a personal Form 11 filing covers the company — it doesn’t	Proprietary directors must still file a Form 11 personally, even though the company also files a CT1

The two returns aren’t a choice — if you run a limited company, the company files a CT1, and you, as a director with non-PAYE income (including dividends and director’s fees), generally still file a Form 11 personally. 

Before You File — What to Prepare

A CT1 filing only moves quickly when the underlying information is ready before anyone logs into ROS. If your accountant is waiting on you for any of the items below, the filing stalls — and the deadline doesn’t move to accommodate that.

Have the following ready:

• Finalised profit and loss account and balance sheet for the accounting period. Draft figures aren’t sufficient — the CT1 computation and the iXBRL tagging both need to be built from the same, final version.
• A breakdown of trading income versus passive income (rental income, deposit interest, foreign dividends). These are taxed at different rates – 12.5% for trading income and 25% for passive income – so the split has to be explicit, not estimated.
• Capital allowances schedule. Plant and machinery typically qualify for wear-and-tear allowances at 12.5% per year over eight years; certain energy-efficient equipment may qualify for accelerated capital allowances at 100% in the year of purchase.
• R&D expenditure records, if you’re claiming the R&D tax credit. This needs supporting documentation for the qualifying activity, not just the total spend.
• Director’s loan account balance. This feeds into the close company surcharge calculation, covered in Section 7 below.
• Your preliminary tax payment receipt from ROS, so the balance due on the CT1 can be reconciled against what’s already been paid.
• iXBRL-tagged financial statements — explained fully in the next section.
• Form 46G (Company), which reports payments to third parties (contractors, consultants, and professional fees) where the aggregate to any one party exceeds €6,000 in the period. It’s filed separately from the CT1 but is due at the same time, and it’s the document most commonly forgotten.

Most CT1 guides leave Form 46G out entirely. Skipping it doesn’t trigger a financial surcharge the way a late CT1 does – but it does tend to trigger a revenue compliance intervention letter, which costs far more adviser time to resolve than simply filing the form on time would have.

If your bookkeeping records aren’t reconciled going into year-end, none of the above can be finalised on schedule. Clean-up-to-date bookkeeping is the actual starting point for an on-time CT1, well before the return itself is touched.

The CT1 Filing Deadline — Exact Dates

The rule is simple to state and easy to miscalculate in practice: the CT1 is due by the 23rd day of the 9th month after your accounting period ends and is filed via ROS.

For the most common scenario — a company with a 31 December year-end — the accounting period ending 31 December 2025 has a CT1 deadline of 23 September 2026.

Preliminary tax is a separate, earlier date: the 23rd day of the 11th month of the same accounting period, before the period has even ended. For that same December year-end, preliminary tax for the 2025 period was due 23 November 2025. The CT1 is filed in September 2026 and then reconciles what was paid in November 2025 against the company’s actual final liability.

Both dates are fixed with reference to your company’s accounting period, not the calendar year, and Revenue does not send advance reminders for either. If your company doesn’t use a 31 December year-end, the full deadline reference table covering every common year-end is in Section 3 of our corporation tax guide — it’s worth bookmarking rather than recalculating each year.

How This Differs from Your CRO Annual Return

It’s worth separating two obligations directors regularly conflate: the CT1 goes to Revenue, and your company’s annual return (Form B1) goes to the Companies Registration Office (CRO). They’re governed by different legislation — tax law for the CT1, company law for the B1 — run on different clocks, and missing one doesn’t excuse the other.

Your B1 is due within 56 days of your company’s Annual Return Date (ARD), a date set independently by the CRO and not tied to your accounting period end the same way the CT1 is. Depending on when your ARD falls, your B1 deadline can land months before or after your CT1 deadline. Missing the B1 window risks losing your audit exemption for two years — a separate consequence from anything Revenue is imposed for a late CT1. Treat them as two calendars, not one.

The iXBRL Requirement — What SME Directors Actually Need to Know

This is the part of the CT1 filing that almost every other guide either skips or writes for an accountant audience. Here it is from the director’s side.

What is iXBRL?

iXBRL — Inline eXtensible Business Reporting Language — is a tagging format that Revenue requires for the financial statements submitted alongside the CT1. The file appears to be a normal set of accounts when opened. Still, underneath, every figure is tagged with a machine-readable label that Revenue’s systems can read automatically, rather than a person having to retype figures from a PDF.

Who has to file it?

Every company filing a CT1 has an iXBRL obligation. No turnover or size threshold exempts a company outright — a two-person Irish Ltd with €50,000 in annual turnover has the same iXBRL requirement as a business with €10 million in annual turnover.

Revenue introduced iXBRL in phases between 2014 and 2017, starting with larger companies first. That phased rollout left behind a cohort of directors — typically people who set up their company before the requirement existed for smaller entities — who still believe, incorrectly, that they’re exempt because “it didn’t apply when we started”. It does apply now, regardless of when the company was incorporated.

What does this mean for an SME director in practice?

In practice, most directors never touch the iXBRL file directly. Your accountant or outsourced provider produces the tagged accounts using dedicated software – Relate Accounts, Sage, or a specialist tagging tool – once your year-end accounts are finalised. Your job as director isn’t to do the tagging; it’s to make sure the right version gets tagged.

Three things to actually do:

1. Sign off on the finalised profit & loss and balance sheet before tagging starts. Tagging a draft in which later changes occur — see Section 7, Error 1.
2. Confirm explicitly with your accountant that iXBRL-tagged accounts have been prepared, not just that the CT1 tax computation is done. These are two different deliverables, and a computation can be finished while the tagging is still outstanding.
3. Don’t confuse the iXBRL accounts filed with Revenue with the annual return accounts filed with the CRO. They cover similar financial information but are separate statutory obligations, filed with different bodies, on different timelines.

What causes iXBRL rejections?

Revenue’s system automatically validates the iXBRL file before accepting the submission. Common rejection triggers include:

• The iXBRL file isn’t correctly linked to the CT1 submission inside ROS
• Mandatory tags are missing — company name, accounting period end date, or accounting basis are frequent gaps
• The figures in the tagged accounts don’t reconcile with the figures entered in the CT1 tax computation

This last point matters more than it sounds. A rejected submission is treated as if no return was filed at all until it’s corrected. The surcharge clock — if you’re close to the deadline — runs from the original due date, not from when the corrected version is returned. Filing three days before the deadline and getting rejected can still count as late, even though something was technically submitted on time.

Stuck on iXBRL tagging or worried about the 23 September deadline? Aone’s CTA-qualified team handles the tagging, the ROS submission, and the reconciliation end-to-end. Get a corporate tax quote

Step-by-Step — Filing the CT1 via ROS

This is the filing sequence as it runs inside ROS. You don’t need to operate every screen yourself if an accountant or agent is filing on your behalf, but knowing the sequence means you know what to check before you sign off.

1. Log in to ROS at ros.ie using the company’s ROS digital certificate. Only the company’s registered tax agent or an officer with ROS access can file; if neither has current access, that needs to be sorted well before the deadline, not the week of it. Director check: confirm your agent’s ROS access is current well before the deadline. 
2. Navigate to File a Return → Corporation Tax → CT1, and select the correct accounting period. If the period you need doesn’t appear as an option, the company’s CT registration may not be active or correctly dated — this needs to be resolved with Revenue before filing can proceed. Director check: if the period doesn’t appear, the CT registration may not be active — flag immediately. 
3. Enter the CT1 computation: taxable trading income, passive income, capital allowances, any loss relief being claimed, and credits such as the R&D credit, start-up relief, or the Knowledge Development Box. Every field here should map back to a figure in the finalised accounts — nothing should be a fresh estimate entered for the first time on this screen. Director check: ask to see the computation mapped line-by-line against your finalised accounts. 
4. Attach the iXBRL-tagged financial statements. ROS validates the file format on upload. If validation fails at this point, stop and go back to whoever prepared the tagging rather than trying to push the submission through. Director check: ask your accountant to confirm the iXBRL validation passed before they hit submit. 
5. Complete the supplementary panels: director details, the close company declaration if it applies, the R&D credit claim if you’re making one, and the Form 46G data. Director check: confirm explicitly whether the close company declaration applies — don’t assume it doesn’t. 
6. Reconcile preliminary tax. ROS calculates the balance of CT due by netting your computed liability against the preliminary tax already paid. Check this figure against your accountant’s own calculation before going further — see the note below on what this figure does and doesn’t include. Director check: ask whether the figure includes interest on any preliminary tax shortfall (it usually doesn’t). 
7. Review and submit. Once submitted, ROS issues a submission reference number. Keep it as your filing evidence. Revenue typically issues a Notice of Assessment within four to six weeks of a clean submission. Director check: get the submission reference number yourself, not just via your accountant’s file. 

The point at which most errors occur is Step 6. Directors often approve the filing based on the “balance due” figure ROS shows, without checking whether that figure includes interest on any preliminary tax shortfall. It usually doesn’t. The balance due shown on the CT1 screen reflects only the tax liability; if preliminary tax was underpaid, Revenue calculates and charges daily interest on that shortfall as a separate item outside the CT1 filing screen entirely. A director who signs off on the ROS balance, assuming it’s the full amount owed, can be caught off guard by a follow-up interest charge weeks later.

Preliminary Tax — Reconciling What You Already Paid

Preliminary tax isn’t a separate tax in its own right — it’s an advance payment toward the CT liability the CT1 later settles.

• Small companies (where prior-year CT liability was €200,000 or below) can base preliminary tax on either 100% of the prior year’s CT bill or 90% of the current year’s estimated liability, whichever the director prefers to plan around. This is due by the 23rd day of the 11th month of the accounting period.
• Large companies (prior-year CT liability above €200,000) pay in two instalments — at months 6 and 11 of the accounting period — with the two instalments together totalling 90% of the current year’s estimated liability.
• Underpaid preliminary tax accrues daily interest at a rate of 0.0219% per day, calculated from the original due date. This interest is charged separately from anything shown on the CT1 itself.
• Overpaid preliminary tax is refunded once the CT1 has been processed, generally within four to six weeks of submission.

For the complete breakdown of small versus large company thresholds and worked instalment examples, see the preliminary tax section of our corporation tax guide.

What Late Filing Actually Costs

Most guides state that a surcharge applies without putting a number on it. Here’s the actual cost structure, set out under sections 1084 and 1085 of the Taxes Consolidation Act 1997:

Delay	Surcharge	Cap
Filed within 2 months of the deadline	5% of the tax due for the period	€ 12,695
Filed more than 2 months after the deadline	10% of the tax due for the period	€ 63,485

On top of the surcharge, two further consequences apply:

• Daily interest of 0.0219% accrues on any unpaid tax from the original due date — separately from the filing surcharge and regardless of whether the surcharge itself has been paid.
• Restriction of reliefs. A late CT1 can result in restrictions on reliefs that would otherwise be available, including loss relief and group relief, with the severity of the restriction generally linked to how late the filing is. This is often the more expensive consequence in practice — a surcharge is a fixed percentage, but losing the ability to offset a loss can cost far more than the surcharge itself.

These figures aren’t something Revenue negotiates case by case, and they apply automatically once the deadline has passed — there’s no warning notice issued first.

The 5 Most Common CT1 Errors Irish SMEs Make

These are the errors that recur in practice — not a list copied from a revenue guidance page, but the patterns that actually cause rejections, surcharges, or unnecessary compliance correspondence.

Error 1: iXBRL figures don’t match the CT1 computation. 

This is the most common cause of rejection. It happens when the CT1 computation is finalised first, and the accounts get tagged from an earlier draft that doesn’t match it exactly. The fix is procedural: always tag from the final, agreed version of the accounts — never from a version still subject to change.

Error 2: The R&D credit goes unclaimed. 

The credit has to be claimed on the CT1’s supplementary panel — it isn’t enough for qualifying R&D spend to simply sit in the accounts. Plenty of SMEs incur genuinely qualifying expenditure (software development, product testing, process experimentation) but never claim it because no one flagged the activity as eligible. Worth knowing: the claim window is 12 months from the end of the accounting period, which is a separate and later deadline than the CT1 filing date itself — so a missed claim on this year’s CT1 isn’t necessarily lost, but it shouldn’t be left to chance either.

Error 3: Treating the close company surcharge as optional. 

Under section 440 of the Taxes Consolidation Act 1997, a 20% surcharge applies automatically to undistributed rental or investment income held inside a close company once an 18-month distribution window has passed. It cannot be waived retroactively once that window closes — there’s no appeal based on oversight. Directors of property-holding companies are the group most commonly caught by this, often because the surcharge isn’t reviewed until well after the relevant accounting period has closed.

A pattern we see in practice: A director sets up a company to hold a rental property; the rent accumulates in the company without a dividend ever being declared, and eighteen months later the surcharge has already crystallised before anyone reviews the position. By the time it’s spotted at the CT1 filing stage, there’s no longer a way to undo it for that period — only to make sure the next period doesn’t repeat it.

Error 4: Preliminary tax shortfalls go unaddressed. 

A preliminary tax underpayment accrues daily interest from the original due date, independent of whether the CT1 itself is filed on time. This is a distinct exposure from the late-filing surcharge — both can apply at once, on the same return, for different reasons.

Error 5: Form 46G gets forgotten. 

Missing the Form 46G deadline doesn’t incur a financial penalty, unlike a late CT1. Still, it routinely triggers a revenue compliance intervention letter — and resolving that correspondence usually takes more adviser time than simply filing the form alongside the CT1 would.

When to Outsource Your CT1 Filing

Outsourcing the CT1 filing tends to pay for itself in a few specific situations rather than being a blanket recommendation for every company:

• Your company has both trading and passive income streams, where the blended rate calculation introduces more room for error than a single-rate company
• You have an R&D tax credit claim to prepare, which involves a separate technical submission beyond the standard computation.
• Your close company position hasn’t been reviewed in the last twelve months
• You’re not fully confident that your preliminary tax payment was calculated correctly the first time

Aone’s CT1 service covers the full filing cycle — iXBRL tagging, ROS submission, R&D credit claims, and preliminary tax reconciliation — with every filing handled by CTA-qualified staff and covered by professional indemnity insurance. 

Don’t want to manage iXBRL tagging, the close company review, and the preliminary tax reconciliation yourself?  See what our CT1 filing service includes

Conclusion

Filing a CT1 correctly is less about the tax rate itself and more about sequencing: finalised accounts before tagging, tagging before submission, preliminary tax reconciled rather than assumed, and Form 46G filed alongside rather than forgotten. Most of the errors covered above aren’t caused by directors not knowing the rules — they’re caused by a document or a sign-off arriving one step too late in the process. Build in that lead time, and the filing itself becomes routine rather than a deadline-week scramble.

If any part of this process — the iXBRL tagging, the R&D claim, the close company review, or the preliminary tax reconciliation — feels like more than you want to manage directly, our CT1 filing service is built to take it off your plate entirely.

FAQs

What documents do I need to file a CT1 return in Ireland? 

At minimum: finalised profit and loss account and balance sheet, a trading-versus-passive income breakdown, your capital allowances schedule, iXBRL-tagged financial statements, your preliminary tax payment receipt, and Form 46G if you’ve made payments over €6,000 to any third party in the period. If you’re claiming the R&D credit, you’ll also need supporting documentation of the qualifying activity.

Can I file a CT1 return myself without an accountant? 

Yes, if you or another company officer has ROS access and a digital certificate. In practice, most directors use an accountant or an outsourced provider because iXBRL tagging requires dedicated software that most companies don’t otherwise have and because the computation itself draws on accounting judgement (capital allowances, loss relief, and the trading/passive split) that benefits from professional review.

What is iXBRL, and do all Irish companies need it? 

iXBRL is a machine-readable tagging format that Revenue requires for the financial statements filed with the CT1. Every company filing a CT1 has this obligation — there’s no turnover or size exemption. A small two-person company has the same iXBRL requirement as a large multinational subsidiary.

What happens if my iXBRL file is rejected by Revenue? 

A rejected iXBRL file means the entire CT1 submission is treated as not filed until the issue is corrected. If the rejection occurs close to your deadline, the surcharge clock continues to run from the original due date — not from when the corrected version is resubmitted. This is why it’s worth submitting with some margin before the deadline rather than at the last possible moment.

Does a dormant company have to file a CT1? 

Yes. A dormant company still files a CT1 every year, declaring €0 profit, as long as it remains registered for corporation tax. No trading activity doesn’t remove the filing obligation.

How long does Revenue take to process a CT1 return? 

Revenue typically issues a Notice of Assessment within four to six weeks of a clean submission. If preliminary tax was overpaid, the refund is generally processed on a similar four-to-six-week timeline once the CT1 has been processed.

Can I get an extension on the CT1 filing deadline? 

Generally, no — not as a matter of routine. Revenue extensions are limited to specific circumstances such as confirmed ROS system downtime around the deadline or a formal application in genuinely exceptional circumstances. 

What happens if I make a mistake on a filed CT1? 

You can file an amended CT1 through ROS. Filing the correction doesn’t undo interest that has already accrued on any underpaid tax from the original due date — the amendment fixes the return going forward, but it doesn’t reset the clock on tax that was outstanding in the meantime.

Do I need to file a CT1 if my Irish company made a loss? 

Yes. Filing is mandatory regardless of profit or loss, and for a loss-making period, filing is also what formally establishes the loss so it can be carried forward to offset profits in future periods. Skipping the filing doesn’t just risk a surcharge — it risks the loss itself not being available to use later.