Corporation Tax Accounting Periods Explained for UK Companies

Every UK limited company has a Corporation Tax accounting period, and understanding exactly what it is — and how it differs from your company’s financial year — is essential to avoiding late filing penalties and interest charges. It’s a surprisingly common source of confusion, particularly for new directors, because the accounting period isn’t always the same length as the company’s financial year shown at Companies House. This guide sets out clearly what a Corporation Tax accounting period is, how it’s set, and what deadlines follow from it.

What Is a Corporation Tax Accounting Period?

A Corporation Tax accounting period is the period HMRC uses to calculate how much Corporation Tax your company owes. It’s usually 12 months long and generally matches your company’s financial year, which runs to your accounting reference date set at Companies House. However, the two aren’t automatically identical — HMRC’s rules cap a single Corporation Tax accounting period at 12 months, so if your company’s financial year is longer than 12 months (which can happen in a company’s first year of trading), HMRC splits it into two separate accounting periods for tax purposes.

Financial Year vs Accounting Period: What’s the Difference?

These two terms are often used interchangeably, but they serve different purposes:

• Financial year (accounting reference period): the period covered by your statutory accounts, filed at Companies House. This can be up to 18 months for a company’s first set of accounts.
• Corporation Tax accounting period: the period HMRC uses to assess Corporation Tax. This is capped at 12 months, so a longer financial year is split accordingly.

For example, a company incorporated on 1 July 2025 with an accounting reference date of 31 March 2026 would have a first financial year of nine months (1 July 2025 to 31 March 2026), which in this case fits within the 12-month cap, so financial year and accounting period would match. But a company with a longer first period — say, 15 months from incorporation to its first accounting reference date — would need to split that into a 12-month Corporation Tax accounting period followed by a shorter second period covering the remaining months, each requiring its own calculation and, where applicable, its own CT600 return.

How Your First Accounting Period Is Set

Your first Corporation Tax accounting period begins on the date your company starts trading, not necessarily the date it was incorporated. Many companies are incorporated some time before they actually begin trading — for example, a property investment company might be set up months before it acquires its first property. Corporation Tax obligations, including the requirement to register with HMRC, generally begin once trading activity actually starts, so it’s important to notify HMRC within three months of trading commencing.

If you’re setting up a company to hold or invest in property, our guide on property investment through a limited company covers some of the wider considerations around structuring, alongside the accounting period rules covered here.

The Three Key Deadlines That Follow From Your Accounting Period

Once your accounting period is established, three separate deadlines follow from its end date:

• Corporation Tax payment: due 9 months and 1 day after the end of the accounting period
• Company Tax Return (CT600): due 12 months after the end of the accounting period
• Companies House annual accounts: due 9 months after the end of the financial year for most private companies (21 months from incorporation for a first set of accounts)

Note that the tax payment deadline actually falls before the CT600 filing deadline — a common point of confusion. For example, for an accounting period ending 31 March 2026, Corporation Tax would be due by 1 January 2027, while the CT600 return wouldn’t be due until 31 March 2027. Directors sometimes assume that because the return isn’t due yet, the payment isn’t either, which can lead to interest charges on genuinely late tax.

Why Splitting an Accounting Period Matters in Practice

When a financial year longer than 12 months is split into two Corporation Tax accounting periods, each period requires its own profit calculation, its own set of allowances and reliefs applied proportionately, and potentially its own CT600 submission. This most commonly affects newly incorporated companies with an unusually long first trading period, and it’s an area where getting the split wrong can lead to an incorrect tax calculation or a missed filing obligation for the second, shorter period. This is particularly relevant for companies formed to hold buy-to-let or investment property, where the gap between incorporation and the first accounting reference date can sometimes exceed 12 months.

Can You Change Your Accounting Period?

Yes, a company can shorten or extend its accounting reference date at Companies House, which in turn affects the Corporation Tax accounting periods that follow. This might be done to align the company’s year-end with a parent company, a calendar year, or simply for administrative convenience. Any change should be considered carefully, since it can affect both the Companies House filing deadline and how HMRC treats the corresponding tax periods, and once shortened, some restrictions apply to changing it again too frequently.

What Happens if You Miss an Accounting Period Deadline?

Missing the Companies House accounts deadline triggers an automatic penalty, starting from £150 for filings up to one month late and rising with the length of the delay, doubling if you were also late the previous year. Separately, missing the CT600 filing deadline triggers its own fixed penalties from HMRC, and missing the tax payment deadline results in daily interest accruing on the outstanding amount. Because these are three distinct obligations with three distinct deadlines, it’s entirely possible to file your accounts on time but still incur penalties for a late Corporation Tax payment, or vice versa.

Dormant Companies Aren’t Exempt

Even if your company hasn’t started trading, it still has an accounting reference period for Companies House purposes and must file dormant accounts on the same schedule as a trading company. Once trading does begin, the Corporation Tax accounting period clock starts, and HMRC registration becomes necessary within three months.

How Felix Accountants Can Help

We help directors of trading companies and property investment companies alike get their accounting periods, deadlines and filings right from day one, whether that’s registering a newly trading company with HMRC, managing a split accounting period in year one, or simply confirming when your next Corporation Tax payment is actually due. See our wider business tax services for how we support companies through the full filing cycle.

Frequently Asked Questions

Is my Corporation Tax accounting period the same as my financial year?

Usually, yes, but not always. A Corporation Tax accounting period can never exceed 12 months, so if your company’s financial year is longer than 12 months, it will be split into two separate accounting periods for Corporation Tax purposes.

When does my first accounting period start?

Your first Corporation Tax accounting period begins when your company starts trading, which is not necessarily the same as its incorporation date.

What’s the deadline to pay Corporation Tax?

Corporation Tax is generally due 9 months and 1 day after the end of your accounting period, which is earlier than the deadline to file your CT600 return.

Do I need to file a CT600 if my company hasn’t traded yet?

If your company is genuinely dormant, you’ll still need to file dormant accounts at Companies House, but a CT600 is only required once the company has started trading and been registered with HMRC for Corporation Tax.

Can I change my company’s accounting reference date?

Yes, you can shorten or extend your accounting reference date via Companies House, though this affects your future Corporation Tax accounting periods and filing deadlines, so it’s worth getting advice before making the change.