Corporation tax deadlines for small limited companies

If you run a small limited company, Corporation Tax has two clocks that confuse many directors: when you pay, and when you file. Add Companies House accounts into the mix and it is easy to focus on the wrong date.

This article sets out the standard deadlines for private limited companies, walks through a worked example, and flags practical habits that keep HMRC and Companies House happy. Always confirm your own dates in your HMRC business tax account — first periods after incorporation and accounting period changes can differ.

The three deadlines that matter

For most small private limited companies with a 12-month accounting period:

• Companies House annual accounts — due 9 months after the accounting reference date.
• Corporation Tax payment — usually due 9 months and 1 day after the end of the Corporation Tax accounting period.
• Company Tax Return (CT600) — normally due 12 months after the end of the accounting period.

Payment is typically due about three months before the CT600 filing deadline. You often need a reliable estimate of the tax bill before the statutory accounts and return are fully finalised — which is why tidy books and an early year-end pack matter.

Worked example (31 March year end)

Assume your company’s accounting period ends on 31 March 2026:

• Companies House accounts — normally by 31 December 2026
• Corporation Tax payment — normally by 1 January 2027
• CT600 filing with HMRC — normally by 31 March 2027

Shift every date if your year end is different. A 30 September year end, for example, pushes Companies House to 30 June the following year, payment to 1 July, and the CT600 to 30 September. Write the three dates in your board calendar as soon as the year ends — do not rely on memory.

Accounting periods versus “the year end”

Your Companies House accounting reference date and your Corporation Tax accounting period usually align, but Corporation Tax accounting periods cannot exceed 12 months. If you lengthen your first financial year toward the 18-month Companies House maximum, HMRC will normally split Corporation Tax into two periods — with two payment and filing deadlines. If you change your year end, tell HMRC promptly so deadlines stay correct.

New companies should also register for Corporation Tax with HMRC within the required window after starting to trade. Missing registration does not remove the liability; it only removes your early warning system.

How to file and pay

Company Tax Returns must be filed electronically. Most small companies file through an accountant using commercial software, or via approved products themselves. HMRC has been withdrawing older free online filing options for unrepresented companies, so DIY filers should check which channel remains available for their next return well before the deadline.

Corporation Tax must be paid electronically. Keep your UTR and payment references handy, and do not wait for a paper demand if you already know a liability is due. Paying on time with a best estimate, then adjusting after final computations, is usually better than paying nothing while you wait for perfect numbers — but take advice if you are unsure how HMRC will treat an under- or over-payment in your case.

Rates in outline (not a substitute for a computation)

Corporation Tax is charged on taxable profits. In recent years the structure has included a small profits rate and a main rate, with marginal relief between the bands, and thresholds that can be affected by associated companies. Rates and bands can change with Finance Acts — treat any percentage you see online as a prompt to check the rules for your accounting period, not as a final answer. Your CT600 computation is what counts. Confirm current rates with HMRC or your adviser when you are budgeting cash for the payment date.

Penalties and interest — why the calendar matters

• Late CT600 filing attracts flat-rate penalties that escalate the longer the return is outstanding, with further tax-geared penalties in some cases.
• Late payment attracts interest.
• Late Companies House accounts attract separate penalties based on how late you are.

Missing one deadline does not excuse the others. If cash flow is tight, speak to your accountant early about Time to Pay with HMRC — silence until the due date is the expensive option.

A practical timeline for directors

1. At year end — freeze cut-off, gather records (see our year-end checklist).
2. Within 1–2 months — get draft figures so the tax estimate is credible.
3. Well before month 9 — finalise accounts for Companies House and confirm the CT payment amount.
4. By 9 months + 1 day — pay Corporation Tax.
5. By 12 months — file the CT600 and supporting iXBRL accounts/computations with HMRC.

Companies that treat month eight as “when we start thinking about accounts” regularly collide with the payment date. Bring bookkeeping up to date monthly and year end becomes a review, not an excavation.

How Muftals Accountancy helps

We prepare limited company accounts, Corporation Tax computations and CT600 filings for small companies from our Chislehurst base and remotely across the UK. If you are unsure of your next payment or filing date, contact us with your year-end date and we will map the calendar with you. Local directors can also read more on our accountants in Chislehurst page. Confirm critical dates with HMRC or your adviser if anything about your accounting periods is non-standard.