Your company has one year end and three deadlines hanging off it — and they arrive in an order nobody expects. You pay your Corporation Tax before the return is due. Your first year may need two tax returns. And the CT600 never goes to Companies House at all. Five minutes here saves an expensive afternoon later.
The single most-searched confusion in company filing is “Companies House CT600” — and the answer is that there is no such thing. Companies House gets your annual accounts, for the public record. HMRC gets your Company Tax Return — the CT600 — with a copy of the accounts and the tax computations, both tagged in iXBRL, for tax.
Until 31 March 2026 a free government service let small companies do both jobs in one sitting, which is why so many directors reasonably believed it was one filing. That service has closed. They are now two separate jobs, done through commercial software, on two separate deadlines.
Filing one does not file the other. Every year, companies tick the Companies House box and genuinely believe they're done — then meet HMRC's penalty ladder.
Take a company with a 31 March 2026 year end. Its deadlines land in this order:
9 months after the year end — though a company's very first accounts are due 21 months after the date it was registered, which can fall earlier. This one comes first, and it's the one with the automatic penalty the moment it's missed.
Usually 9 months and one day after the year end — that is the rule for taxable profits up to £1.5 million; larger companies pay by quarterly instalments that start before the year end. Yes: the money is due before the return. Interest runs from this date whether or not the CT600 is in.
12 months after the end of the accounting period. The most relaxed date of the three — which is why treating it as the deadline for doing the work is the classic mistake.
The practical consequence of the ordering: you cannot know what to pay on 1 January without having effectively preparedthe return three months before it's due. Companies that plan around the 12-month date discover this in the most expensive way available.
Your first accounts run from the day of incorporation to your first year end — almost always a stretch of more than 12 months, because Companies House sets the year end at the month-end a year after incorporation. But a Corporation Tax accounting period cannot be longer than 12 months.
So HMRC's own guidance says you may have to file two returns to cover your first accounts — one for the first 12 months, a second for the leftover days. Whether you do depends on when the company actually started trading: the accounting period only begins once the company starts trading, so one that stayed dormant and then began trading may file just one. Where two are due, that means two CT600s, two payment dates, one set of accounts. Almost nobody warns new directors about this.
Your first accounts deadline is different too: 21 months after the date you registered with Companies House, rather than the usual 9 months after year end.
Miss Companies House and the ladder reads £150 → £375 → £750 → £1,500as the months pass — doubled if you're late two years running. Miss HMRC and it's £200 on day one, another £200 at 3 months, then 10% of the unpaid tax at 6 months and 10% again at 12. File late three times in a row and those £200s become £1,000 each.
Note the £200. Most articles — and most accountants' websites — still say the CT600 penalty starts at £100. gov.uk says £200. The two regimes also stack: one forgotten year end can produce penalties from both regulators at once, plus interest on the tax.
Put three dates in the calendar the day your year end passes — accounts at 9 months (21 months after incorporation for your very first accounts), payment at 9 months and a day (unless your taxable profits top £1.5 million, when it's quarterly instalments instead), return at 12 — and treat the first one as the deadline for doing all the work, because the payment date needs the numbers anyway. If your company is in its first year, you may face two CT600s — check the dates HMRC sends you rather than assuming.
And if you used to do all this through HMRC's free service, that route is gone — the filing itself now needs commercial software, which is its own decision. We've written up what DIY CT600 filing actually costs now — or take the 60-second check below.
No — the CT600 goes to HMRC, never to Companies House. They are two different regulators wanting two different things: Companies House gets your annual accounts for the public record, and HMRC gets your Company Tax Return (the CT600) plus a copy of the accounts and your tax computations, both tagged in iXBRL. Filing one does not file the other.
12 months after the end of the accounting period it covers. So a company with a 31 March 2026 year end must file its CT600 by 31 March 2027. Note that this is the latest of all your company deadlines — your accounts and your tax payment are both due earlier.
Usually 9 months and one day after the end of your accounting period — which is before the return itself is due. That timing holds for companies with taxable profits up to £1.5 million; companies above that pay by quarterly instalments, which start before the year end. A 31 March year end means paying by 1 January but filing by the following 31 March. In practice you prepare the return early to know what to pay; interest runs on late payment regardless of the filing deadline.
A Corporation Tax accounting period cannot be longer than 12 months, but your first accounts usually run from incorporation to a year end more than 12 months away. So HMRC says you may have to file 2 returns to cover the period of those first accounts: one for the first 12 months, one for the remainder. Whether you do depends on when the company actually started trading — a Corporation Tax accounting period only begins when the company starts trading, so a company that stayed dormant and then began trading may file just one.
They are separate for each regulator, and they stack. Companies House: £150 up to 1 month late, £375 to 3 months, £750 to 6 months, £1,500 beyond — doubled if your accounts are late 2 years in a row. HMRC: £200 the day the CT600 is late and another £200 at 3 months (these rise to £1,000 each if you're late 3 times in a row), then 10% of the unpaid tax at 6 months and another 10% at 12 months.
No. From 1 April 2028, accounts filed with Companies House must go through commercial software in iXBRL format, and small companies and micro-entities must also file a profit and loss account — though they'll be able to opt out of having it published on the public register (Companies House, HMRC and law enforcement still see it; the opt-out mechanics are still to be confirmed). Abridged accounts are abolished at the same time. But the deadlines themselves don't move — the route changes; the dates don't.
We track every deadline — accounts, payment, return, confirmation statement — and file the lot through our software. Book a free call and we'll map your company's dates while you're on it.
Book a free call →General guidance, not advice for your specific situation — accounting periods can be changed, shortened and extended, and yours may differ. Last reviewed 14-07-2026.
We checked these rather than relying on memory. Every figure and deadline above comes from HMRC directly — go and read them yourself if you'd like to.
Last reviewed 14-07-2026. Tax rules change — if you're reading this long after that date, check the source.
