CT1 Corporation Tax Return: Step-by-Step Guide for 2026

Every Irish-resident company has to file a CT1 corporation tax return each year. It sounds straightforward — and once you've done it five times, it is. The first few iterations are where mistakes are made, deadlines missed and penalties incurred. This guide takes you through the full cycle in plain English, with the specific Irish rules that apply (note: the UK form is called "CT600"; the Irish equivalent is the CT1 via ROS).

Key dates for a December year-end company

• Preliminary corporation tax: due 23 November (the 23rd of the month before year-end)
• CT1 return + iXBRL accounts: due 23 September of the following year (9 months + 23 days after 31 December)
• Balance of tax owed: due on the same date as the CT1 return
• CRO annual return (B1) + financial statements: due based on your Annual Return Date, not your accounting period — easy to confuse

Step 1 — Calculate your preliminary corporation tax

Preliminary tax is a payment on account. Pay it correctly or face statutory interest at 8% per annum on the underpayment. You have three calculation options:

1. 100% of prior-year tax — simplest and safe if the prior year is representative
2. 90% of current-year actual liability — requires good in-year forecasting
3. 105% of the year-before-prior — direct debit only, useful when income is volatile

"Small" companies (prior-year corporation tax under €200,000) get a concession: they can use option 1 with no interest exposure. Most Dublin SMEs fall into this category.

Step 2 — Finalise the statutory accounts

You can't file CT1 without finalised accounts. The accounts need to be in iXBRL format — a digital reporting standard that tags every figure with a machine-readable label. Most SME accounting software produces iXBRL on demand; if yours doesn't, your accountant should be able to convert manually.

Step 3 — Reconcile profit to taxable trading income

Accounting profit isn't tax profit. The CT1 walks you through the adjustments:

• Add back disallowable expenses (entertainment, fines, depreciation, most car expenses)
• Subtract capital allowances (the tax equivalent of depreciation, often more generous)
• Add back provisions until they crystallise into real expenditure
• Adjust for stock movements where tax treatment differs
• Treat trading vs non-trading income separately (12.5% vs 25%)

Step 4 — Apply available reliefs

Don't file without checking:

• Research & Development (R&D) tax credit — 30% credit on qualifying R&D spend, including software development for many SMEs
• Knowledge Development Box (KDB) — 6.25% effective rate on qualifying IP profits
• Section 481 — film and TV tax credit (specialised)
• Capital allowances — full review of all qualifying purchases, especially Energy-Efficient capital allowances
• Loss relief — current-year, carry-back to the prior year, or carry-forward

Step 5 — File on ROS

The CT1 is submitted through Revenue Online Service (ROS) with the iXBRL accounts attached. Make sure the figures on the CT1 match the figures in the accounts exactly — discrepancies are flagged by Revenue's automated checks and frequently lead to enquiries.

Step 6 — Pay the balance

The balance owed (or refund due) is settled at the same time as filing. Direct debit, Single Euro Payments Area (SEPA) transfer or ROS Single Debit Authority are the typical methods. Plan cash flow accordingly: SMEs often forget that they're paying preliminary corporation tax on the current year alongside the prior year's balance in the same 23-month window.

The five most common CT1 mistakes

1. Late preliminary tax — accrues 8% interest from day one of the underpayment
2. Disallowable expenses not added back — entertainment and depreciation are the usual suspects
3. Capital allowances not claimed — leaving free tax relief on the table
4. R&D credit not even considered — eligibility is broader than most directors realise
5. iXBRL tagging errors — software warnings ignored at submission

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