Ask five business owners when their Corporation Tax is due and you’ll likely get five different answers — not because anyone’s wrong, but because the payment deadline, the CT600 filing deadline, and the Companies House accounts deadline are three separate dates that all depend on your company’s year-end. Mixing them up is one of the most common reasons directors end up with a penalty they didn’t see coming.
This guide breaks down exactly when each deadline falls, how to work out yours from your accounting period, and what happens if you miss one.
The three deadlines you need to know
Every UK limited company faces three separate dates tied to the end of its accounting period:
- Corporation Tax payment: 9 months and 1 day after your accounting period ends
- CT600 (Company Tax Return) filing: 12 months after your accounting period ends
- Companies House annual accounts: 9 months after your accounting period ends (for private companies)
The detail that catches people out: you have to pay your Corporation Tax bill roughly three months before you’re required to file the return that calculates it. HMRC expects payment based on your own estimate, not a bill they send you — there is no physical demand letter waiting in the post.
Working out your deadline from your year-end
Your accounting period is usually the same as your company’s financial year, and it’s set when you incorporate — though it can be changed later if needed. Once you know your year-end, the maths is simple:
Example: year-end of 31 March
- Corporation Tax payment due: 1 January the following year (9 months + 1 day)
- Companies House accounts due: 31 December the following year (9 months)
- CT600 filing due: 31 March the year after that (12 months)
Example: year-end of 31 December
- Corporation Tax payment due: 1 October the following year
- Companies House accounts due: 30 September the following year
- CT600 filing due: 31 December the following year
If your first accounting period runs longer than 12 months — common in a company’s first year of trading — HMRC splits it into two separate accounting periods, each with its own set of deadlines. Statutory accounts can cover up to 18 months, but Corporation Tax periods are capped at 12, so a longer first year usually means two href = “https://smartfiling.co.uk/ct600-corporation-tax-explained/”> CT600 returns rather than one.
What if you’re a micro-entity?
The 9-month Companies House deadline applies regardless of company size, but how much detail you need to include in your accounts differs. If you qualify as a micro-entity under FRS 105, your annual accounts are considerably simpler than a standard company’s — we’ve covered the full filing requirements in our guide to micro-entity accounts, including exactly what you’re exempt from including.
What happens if you miss a deadline
Each of the three deadlines carries its own consequence, and they’re not interchangeable:
- Late payment triggers daily interest from the day after the deadline until HMRC receives the money — there’s no fixed late-payment fine, but interest compounds the longer it’s left unpaid.
- Late CT600 filing starts at £100 for being one day late, rising to £200 at three months, and then 10% of any unpaid tax at both six and twelve months. HMRC can also issue its own estimate of what you owe — usually not in your favour — if the return still hasn’t landed after six months.
- Late Companies House accounts carry an automatic, escalating fine that increases the longer the delay runs, and the penalty doubles if you’re late two years running.
Notably, missing the CT600 filing deadline doesn’t push back your payment deadline — you’re still expected to pay what you owe by the 9-months-and-1-day mark, even if the return itself isn’t ready yet.
Large companies: a different set of dates
If your taxable profits exceed £1.5 million (this threshold is divided between any associated companies you control), the standard 9-month-and-1-day rule doesn’t apply. Instead, you pay in quarterly instalments across the accounting period itself — starting in month 7 for most large companies, or month 3 for very large companies with profits over £20 million. If you’re approaching that threshold, it’s worth reviewing your instalment obligations well before you cross it, since the payment pattern changes significantly.
Dormant companies aren’t exempt
If your company isn’t trading, you might assume none of this applies — but Companies House and HMRC obligations continue regardless. We’ve covered what a dormant company still needs to file to avoid being struck off in a separate guide, since the requirements are lighter but not zero.
Staying ahead of your deadlines
The easiest way to avoid a scramble is to work backwards from your year-end the moment your accounting period closes, rather than the month before a deadline hits. If you’ve already registered and want the mechanics of the return itself — what goes on it, what reliefs you can claim — href = “https://smartfiling.co.uk/ct600-corporation-tax-explained/”> our guide to the CT600 covers that in detail.
If you’d rather not track three separate dates across two different government departments, our Corporation Tax filing service handles the calculation, the CT600, and the payment timeline for you, so nothing slips through because a deadline landed in a busy month.
