Corporation tax in Northern Ireland runs on the same rules that apply everywhere else in the United Kingdom. Companies with taxable profits above £250,000 pay the main rate of 25%, those with profits of £50,000 or less pay the small profits rate of 19%, and companies in between get marginal relief that tapers the effective rate upward. Legislation exists that would let the Northern Ireland Assembly set its own rate, but that power has never been commenced, so every Northern Ireland company registers, files, and pays through HMRC in the standard way.
Who Owes Corporation Tax
Three kinds of organisation are liable:
- Limited companies incorporated in the UK, including those formed in Northern Ireland, on their profits.
- Foreign companies that trade through a UK branch or office, on the profits generated by that local operation.
- Unincorporated associations such as community groups, sports clubs, and co-operatives, if they turn a profit from trading or investments.
The trigger is profit, not legal structure. A local football club earning money from memberships and a bar carries the same obligation as a Belfast tech company, and an unincorporated association that makes a profit files a Company Tax Return in the same way a limited company does.1GOV.UK. Corporation Tax2GOV.UK. Unincorporated Associations – Section: If You Make a Profit
The Rate Bands and Marginal Relief
Two rates, one taper. Profits of £50,000 or less are taxed at 19%. Profits above £250,000 are taxed at 25%. Profits between those two figures get marginal relief, which uses a standard fraction of 3/200 to gradually raise the effective rate from 19% toward 25%. A company earning £150,000, for example, pays an effective rate somewhere around 23% rather than the full 25%.
How Associated Companies Change the Thresholds
If your company has associated companies, the £50,000 and £250,000 thresholds are divided by the total number of associates including yours. A company with three associates splits the thresholds four ways: the small profits limit drops to £12,500 and the main rate threshold to £62,500. The rule, set out in the Corporation Tax Act 2010, prevents a business from fragmenting into several entities to keep each one in the lower band.
What Gets Taxed: Residency and Local Operations
A company is UK-resident for tax purposes if it was incorporated in the UK or if its central management and control are exercised from within the UK. Resident companies owe corporation tax on their worldwide profits, so a Belfast company that earns money from a contract in Germany pays UK corporation tax on those earnings.
Non-resident companies face a narrower charge. A foreign company that trades through a permanent establishment in Northern Ireland — a factory, workshop, office, or any fixed place where it carries out its core business — owes corporation tax only on the profits attributable to that local operation. Profits earned elsewhere by the same foreign company fall outside the UK’s reach.
Registering Within Three Months of Becoming Active
You must register for corporation tax within three months of your company becoming active. Active does not mean the date of incorporation; it means the date the company first starts trading, earns income, or buys assets. Many directors confuse the two and miss the deadline.
To add corporation tax services to your HMRC business tax account, you need your company registration number from Companies House, your 10-digit Unique Taxpayer Reference (posted by HMRC to your registered address after incorporation, and available online if it has not arrived within 15 working days), the date you started doing business, and the date your first accounts are made up to.3GOV.UK. Add Corporation Tax Services to Your Business Tax Account
Two Deadlines: Pay First, File Later
This catches first-time directors out often enough to be worth stating plainly: the deadline to pay is earlier than the deadline to file.
For companies with taxable profits up to £1.5 million, payment is due nine months and one day after the end of the accounting period.4GOV.UK. Pay Your Corporation Tax Bill The Company Tax Return (form CT600) is due 12 months after the end of the same accounting period.5GOV.UK. Company Tax Returns: Overview If your accounting period runs from 1 April 2025 to 31 March 2026, you must pay by 1 January 2027 and file by 31 March 2027.
The CT600 is filed through HMRC’s online service and reports the company’s income, deductions, and tax owed.6GOV.UK. File Your Accounts and Company Tax Return Most businesses use compatible accounting software to submit the figures.
Quarterly Installments for Large Companies
Companies with annual profits above £1.5 million cannot wait until the normal deadline. They pay in four quarterly installments spread across the accounting period and just after it. For a 12-month period running January to December 2026, the installments fall on 14 July 2026, 14 October 2026, 14 January 2027, and 14 April 2027.7GOV.UK. Pay Corporation Tax if You’re a Large Company
The £1.5 million threshold is divided by the number of associated companies in the same way the rate thresholds are divided, so a company with one associate hits the installment requirement at £750,000. Companies whose total tax liability is under £10,000 are exempt from installments even if their profits cross the threshold.7GOV.UK. Pay Corporation Tax if You’re a Large Company
Reliefs That Reduce the Bill
Northern Ireland companies can claim the same reliefs available across the UK. Three are worth knowing about even before you plan to use them.
Annual Investment Allowance
You can deduct up to £1 million per year on qualifying plant and machinery purchases in the year you buy them, rather than writing the cost off over several years. This covers equipment, vehicles, and fixtures used in the business.8GOV.UK. Claim Capital Allowances: Overview
Research and Development Tax Relief
The merged R&D scheme, which replaced the separate SME and large company schemes for accounting periods starting on or after 1 April 2024, gives a 20% above-the-line expenditure credit on qualifying R&D costs. Companies that spend at least 30% of their total expenditure on R&D qualify as R&D-intensive and can access enhanced support with more generous terms.9GOV.UK. Research and Development Tax Relief: The Merged Scheme and Enhanced R&D Intensive Support
Patent Box
Companies that earn profits from patented inventions can elect into the Patent Box, which taxes those profits at an effective rate of 10%. The relief is delivered as an additional deduction from taxable profits and applies only to income from exploiting the patented innovation; the rest of the company’s profits are taxed normally.10GOV.UK. Patent Box: Corporation Tax Main Rate Consequential Amendment
Penalties for Missing the Filing Deadline
From 1 April 2026, HMRC doubled the flat-rate penalties for late Company Tax Returns. The escalation runs:
- One day late: £200 automatic penalty.
- Three months late: a further £200, taking the total to £400.
- Six months late: HMRC estimates your tax liability and adds a penalty of 10% of the unpaid tax on top of the flat-rate fines.
- Twelve months late: a further 10% of any unpaid tax.
If your company files late for three or more consecutive accounting periods, the flat-rate penalties rise again: £1,000 for the first missed deadline and £2,000 if the return is still outstanding after three months.11HM Revenue & Customs. Corporation Tax: Penalty Determinations CT211 Notes – Section: Flat-Rate Penalties These figures are a real step up from the previous £100 and £500 penalties, and they make even a short delay expensive for a small company.
Interest on Late Payment
HMRC charges interest on unpaid corporation tax from the date payment was originally due. As of early 2026, the late payment interest rate is 7.75%.12GOV.UK. HMRC Interest Rates for Late and Early Payments The rate moves with the Bank of England base rate, so check the current figure before budgeting.
Penalties for Getting the Numbers Wrong
Filing on time but reporting the wrong figures carries its own penalties, scaled to the seriousness of the error:
- Lack of reasonable care: 0% to 30% of the additional tax owed.
- Deliberate error: 20% to 70% of the additional tax owed.
- Deliberate and concealed error: 30% to 100% of the additional tax owed.
HMRC can reduce these penalties where you disclose the error voluntarily, help calculate the correct amount, and give access to your records. The line between a careless mistake and a deliberate one often comes down to record-keeping: if the books are disorganised and that leads to an underpayment, HMRC is unlikely to accept reasonable care as a defence.13HM Revenue & Customs. Penalties: An Overview for Agents and Advisers
The Devolved Northern Ireland Rate Has Not Started
The Corporation Tax (Northern Ireland) Act 2015 created a legal route for the Northern Ireland Assembly to set its own rate on certain trading profits. The Northern Ireland Executive publicly indicated an intention to start the regime in April 2018 at 12.5%, matching the Republic of Ireland.14UK Parliament. Corporation Tax in Northern Ireland It never happened. The UK government said it would commence the Act only once the Executive demonstrated its finances were on a sustainable footing, and that condition has not been met.15GOV.UK. Northern Ireland Corporation Tax Regime: Draft Guidance
The CT600 already contains Northern Ireland boxes for when devolution eventually happens, but HMRC’s instructions are explicit: do not fill them in until a Northern Ireland rate has actually been introduced.16GOV.UK. Corporation Tax for Company Tax Return CT600 2025 Version 3 Until that changes, a company in Belfast pays the same rates as a company in London or Manchester.