Guide

Your year end starts three different clocks, not one

The single most common year-end confusion: treating the accounts, the tax payment and the tax return as one deadline. They are three, they belong to two different government bodies, and the money is due before the paperwork.

The three clocks

For a company with a 31 March 2026 year end:

9 months after year end

Accounts to Companies House

The micro-entity balance sheet goes on the public register. For a 31 March year end that means 31 December.

Late: £150 rising to £1,500, doubled if late two years running

9 months + 1 day

Corporation tax PAYMENT to HMRC

The money is due before the return is. If you file early you know the number; if you leave the return late you must estimate and pay anyway.

Late: interest charged daily from the due date

12 months after year end

CT600 return to HMRC

The return itself, with accounts and computations attached in iXBRL, is due three months after the money. It can then be amended for a further 12 months.

Late: £100, another £100 at 3 months, then tax-geared penalties

The trap inside the trap

Filing your accounts at Companies House does NOT file them with HMRC. They are separate bodies with separate systems, and HMRC requires its own copy of the accounts attached to the CT600, tagged in iXBRL, even though the public register already holds them. Plenty of first-time directors file at Companies House, relax, and get an HMRC penalty three months later.

First-year quirks

Your first accounts run from incorporation to your accounting reference date, usually longer than 12 months. First accounts are due at Companies House 21 months after incorporation, not 9 months after year end.
A corporation tax accounting period cannot exceed 12 months, so a long first period needs TWO CT600s: one for the first 12 months, one for the remainder.
The sensible fix most companies choose: keep the first period at whatever Companies House assigned, then everything settles into the one-year rhythm from year two.

Why filing early is the cheat code

Nothing stops you filing everything the week after year end. The tax is still not due until month nine, but you know the exact number, the refund clock starts earlier if one is due (see loss carry-back), and the deadlines stop being deadlines.

Know your dates in one lookup

Type your company number and the wizard pulls your accounting dates straight from Companies House, so the right period and deadlines are worked out before you enter a single figure.

Start your return

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.