Accounting period
An accounting period is the span of time a company's accounts and corporation tax return cover, normally twelve months ending on its accounting reference date. Every filing and payment deadline a limited company has is counted from that date.
Also known as: accounting reference date, company year end
How it works
Companies House sets your first accounting reference date as the last day of the month in which the company was incorporated, one year later. A company formed on 14 May 2026 therefore has a first period running to 31 May 2027 — slightly longer than a year, which is why a first corporation tax period is often split in two: twelve months, then the odd few days.
The date drives everything else. Accounts are due at Companies House nine months after the period end for a private company, and twenty-one months after incorporation for a first set. Corporation tax is payable nine months and one day after the period end. The CT600 return is due twelve months after it. Miss the accounts deadline and Companies House issues an automatic penalty starting at £150 and doubling if you were late the previous year.
You can change your year end, which many companies do to align with a parent, a season or 31 March for simplicity. You may shorten a period as often as you like, but you can normally only lengthen it once every five years, and never beyond eighteen months. A change also moves the filing deadline, and shortening one after the original deadline has passed is not permitted.
A period that is not twelve months long affects more than deadlines: the corporation tax profit limits, the Annual Investment Allowance limit and the small profits rate thresholds are all proportioned to the length of the period.
Who this affects
- New companies whose first period is longer than twelve months and splits for tax
- Directors changing year end to line up with a group or a trading season
- Seasonal businesses wanting the year end after the busy period so figures settle
- Any company near the £50,000 profit limit, which is proportioned in short periods
Common mistakes
- Treating the Companies House accounts deadline and the corporation tax payment date as the same thing
- Lengthening the period twice inside five years, which Companies House rejects
- Forgetting that a short period reduces allowance and profit limits proportionately
Frequently asked questions
What is my company's accounting period?
Normally the twelve months ending on your accounting reference date, which is shown on your Companies House record. Your first period runs from incorporation to the end of that month a year later.
Can I change my company year end?
Yes. You can shorten a period as often as you like and extend it once in five years, to a maximum of eighteen months, provided the filing deadline has not already passed.
Why does my first corporation tax return cover two periods?
A corporation tax accounting period cannot exceed twelve months. A first Companies House period of, say, twelve months and seventeen days splits into a twelve-month period and a seventeen-day one, each needing a return.
When are my accounts due?
Nine months after the period end for a private company, or twenty-one months after incorporation for a first set, whichever gives the later date for that first filing.
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Reviewed by Waqas Sagar ACA FCCA FMAAT · Last reviewed 13 September 2026 · Figures for 2026/27 · About our practice
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