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 when they mean two tax returns rather than one.

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.

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

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 some first-year companies 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 needed when the company traded for more than 12 months inside one period of account. That is the usual case for a company that started trading in its first few weeks: incorporated on 14 March 2025 with a year end of 31 March 2026, trading from day one, it has a first return for 14 March 2025 to 13 March 2026 and a second for the last 18 days, 14 to 31 March 2026. It also 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. Each CT600 is due 12 months after the end of the accounts, so when there are two, both are due on the same day. The corporation tax is not: each return's tax is due 9 months and a day after its own tax period ends, so the first year's tax is due weeks before the rest.

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. If it comes out at 12 months or less, that is one return. If it is longer, the wizard splits it into the two returns HMRC expects, shares the profit between them by the day, and sends both with the one set of accounts attached. If the first return was already filed elsewhere, say so and we send only the second. Your Companies page then shows each return with its own deadline and tax payment date.

Illustration 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.