Guide
Your first accounts cover more than a year, and that is normal
Almost every new company's first period of account runs 12 months plus a few weeks. It surprises directors, and it makes some filing software refuse the return outright. Here is where the extra weeks come from, and why they usually do not mean two tax returns.
Where the extra weeks come from
When a company is incorporated, Companies House sets its accounting reference date automatically: the last day of the month of the first anniversary. Your first period of account then runs from the day you incorporated to that date. Nobody chooses it, and the overshoot is simply the days left in the month you incorporated in.
| Incorporated | Default year end | First period |
|---|---|---|
| 12 May 2025 | 31 May 2026 | 385 days |
| 1 March 2025 | 31 March 2026 | 396 days |
| 28 February 2025 | 28 February 2026 | 366 days |
So the default is 12 months plus anything from nothing to about 30 days. Longer than that only happens if you deliberately change your accounting reference date.
The 18-month ceiling
The law (section 391, Companies Act 2006) says a first accounting reference period must be more than 6 months and no more than 18 months from incorporation. To use that headroom you change your year end on form AA01, usually to line up with a parent company or a seasonal trading cycle, or to put off the first filing while the company is quiet. Shortening your year end can be done as often as you like; extending it is normally limited to once every five years, and is not allowed once the accounts are already overdue.
Companies House is relaxed. Corporation tax is not
Takes ONE set of accounts covering the whole period, however long, up to that 18-month limit. Nothing special is needed.
A corporation tax accounting period can never exceed 12 months. A longer period of account is split into two: the first 12 months, then the remainder, each with its own CT600.
The part almost nobody is told
A corporation tax accounting period does not start when the company is incorporated. It starts when the company comes within the charge to corporation tax: when it starts trading, or first has income. Before that the company is dormant for tax, and those weeks simply are not part of any tax period.
That is why most first-year companies still file only one CT600. Take a company incorporated on 12 May 2025 with a year end of 31 May 2026: a 385-day period of account. If it did not start trading until 1 June 2025, its tax period runs 1 June 2025 to 31 May 2026, which is 365 days, and a single return covers it. The accounts still cover all 385 days, because the accounts are the company's, and the company existed for all of them.
Two returns are only genuinely needed when the company actually traded for more than 12 months inside one period of account, which is what happens when a year end has been extended towards the 18-month ceiling.
What to have to hand
- Your incorporation date and year end. Both are on the public register, and the wizard reads them for you.
- The date you started trading, or first had any income. This is the one only you know, and it is what decides whether your first year is one return or two. Be honest about it: it sets a legal filing period, so a convenient date is not a safe guess.
- First accounts are due 21 months after incorporation at Companies House, not 9 months after your year end. The CT600 is still due 12 months after the end of the tax period.
What we do with it
When your accounts cover more than 12 months, the wizard asks when you started trading and shows you the tax period that results, so you can see it fall under a year. If it still comes out over 12 months, we say so plainly rather than filing something wrong: we can still prepare and file your accounts to Companies House, and the corporation tax side is one for an accountant until we support split returns.
First year? Start with your company number
The wizard pulls your incorporation date and year end from Companies House, works out your tax period once you say when trading began, and tells you straight away whether it is one return or two.
Start your returnIllustration only, simplified for clarity. Real returns can differ. This page explains the mechanism; it is not tax or accountancy advice, and the figures on any return remain the company's responsibility.