UK Accounting · Corporation Tax
UK Corporation Tax and the CT600
Corporation Tax is the tax a UK limited company pays on its taxable profits, reported to HMRC on a Company Tax Return (CT600). A key quirk catches directors out: the tax is normally payable within nine months and one day of your year-end, while the return itself is not due until twelve months after — so you usually pay before you file. The rate is set by the government and changes between Budgets. For US-owned UK companies, Corporation Tax interacts with US rules such as GILTI and Form 5471.
Corporation Tax is the tax your UK limited company pays on its taxable profits — trading profits, investment income and gains. You report it to HMRC on a Company Tax Return (CT600), supported by your company accounts. The rate is set by the government and can move between Budgets, so the calculation is done on current rules each year.
The single most important thing to understand is the timing, because it is counter-intuitive: the tax is due before the return is. For most companies, payment falls nine months and one day after the year-end, while the CT600 itself is not due until twelve months after. Plenty of directors discover too late that they owed the tax months before they expected to file.
Getting it right is partly accuracy and partly planning — claiming every relief and allowance you are entitled to, and knowing the number early enough to set the cash aside. That is the difference between Corporation Tax being a shock and being managed.
Corporation Tax at a glance
- Charged on
- Taxable company profits
- Return
- CT600 to HMRC
- Payment due
- ~9 months + 1 day after year-end
- Return due
- ~12 months after year-end
- Quirk
- Pay before you file
- Rate
- Set by government; can change
Who pays Corporation Tax
- All UK limited companies with taxable profits
- Contractors and consultants operating through a company
- SMEs and owner-managed businesses
- US citizens and green card holders owning a UK company
- Startups that have moved into profit
- Companies with investment income or chargeable gains
How the tax is worked out
Corporation Tax starts from your accounting profit and is then adjusted for tax: adding back costs that are not tax-deductible, and applying capital allowances on qualifying assets instead of accounting depreciation. The applicable rate is then applied to the resulting taxable profit. Because both the rate and the available reliefs can change, this is done on current rules each year rather than from memory.
Pay first, file later
It bears repeating because it is where companies slip up. Your payment is normally due nine months and one day after the year-end; your return twelve months after. HMRC charges interest on late payment and penalties on late filing, both escalating. We work the figure out early so the payment is planned, not panicked.
Common mistakes we see
The recurring ones: assuming the payment deadline is the same as the filing deadline (it is earlier); not setting cash aside for the bill; missing legitimate capital allowances and reliefs; relying on last year’s rate when it has changed; and — for US owners — paying UK Corporation Tax while overlooking the US side entirely.
For US owners: GILTI and Form 5471
Paying UK Corporation Tax does not settle your US position. Under rules such as GILTI, the US can tax your UK company’s profits in your hands even before they are paid out to you, and Form 5471 reporting applies. The two systems have to be planned together — exactly the cross-border work we specialise in.
How we handle it
Corporation Tax, planned and filed
Calculate early
We work out the taxable profit and the tax due well before the payment deadline, so there are no surprises.
Claim reliefs
We capture every capital allowance and relief you are entitled to, on current rules.
File the CT600
We prepare and file an accurate Company Tax Return with HMRC, consistent with your accounts.
Coordinate US side
For US owners, we align the UK position with GILTI and Form 5471 so nothing is double-counted or missed.
Frequently asked questions
Corporation Tax is the tax a UK limited company pays on its taxable profits — broadly its trading profits, investment income and chargeable gains. It is reported to HMRC on a Company Tax Return (CT600), supported by the company’s accounts. The rate is set by the government and can change between Budgets.
This catches many directors out: the payment deadline falls before the filing deadline. For most companies, Corporation Tax is payable within nine months and one day of the accounting year-end, while the CT600 return itself is not due until twelve months after the year-end. So you often have to pay the tax before the return is even filed.
It is charged on taxable profits, which start from your accounting profit and are then adjusted for tax purposes — for example adding back disallowable costs and applying capital allowances on qualifying assets. The applicable rate is then applied. Because the rate and the reliefs can change, the calculation should be done on current rules each year.
HMRC charges penalties for filing the CT600 late and interest on tax paid late, both of which escalate over time. Because the payment deadline comes first, it is possible to be late on payment even while still preparing the return — which is exactly why we plan the numbers early.
Legitimately, yes — through claiming all available reliefs and allowances, such as capital allowances on qualifying purchases, allowable expenses, and reliefs relevant to your sector. Effective planning is about claiming what you are entitled to and timing decisions well, not aggressive schemes. We focus on the former.
Yes. Paying UK Corporation Tax does not remove US obligations. Rules such as GILTI can tax your UK company’s profits in your hands on the US side, even before they are distributed, and Form 5471 reporting applies. The UK and US sides must be coordinated, which is the core of what we do.
Get your Corporation Tax right — and on time
From the CT600 to planning the bill in advance, book a consultation and we'll make sure you pay what you owe, claim what you can, and never miss the payment deadline.