HMRC DEADLINES · CORPORATION TAX
Corporation Tax payment dates, worked through.
Nine months and one day after year-end is the rule everyone learns. The detail. Instalments for big companies, payment on account for previously profitable companies, what changes when your year-end shifts. Is where bills get missed.
The basic rule
Nine months and one day after the accounting period ends.
For companies with profits below £1.5m, Corporation Tax falls due nine months and one day after the end of the accounting period. A 31 December 2025 year-end means 1 October 2026 is the payment deadline. A 31 March 2026 year-end means 1 January 2027.
The CT600 return is due twelve months after period end. Three months after the payment date. But most companies file early because the figures must be calculated to know what to pay anyway.
HMRC pays interest on early payments at base rate minus 1 (currently 3.25%), and charges interest on late payments at base rate plus 4 (currently 8.25%). The asymmetry is real, and the late interest is not deductible for tax.
The big-company rule
Quarterly instalments for profits above £1.5m.
A "large" company (taxable profits over £1.5m, divided by the number of associated companies) pays CT in four quarterly instalments. The first instalment is due 6 months and 13 days after the start of the accounting period. Successive instalments fall at three-month intervals.
For a 31 December 2025 year-end (i.e. period starting 1 January 2025): instalments due 14 July 2025, 14 October 2025, 14 January 2026, 14 April 2026. The third and fourth instalments fall after year-end.
"Very large" companies (profits over £20m) pay even earlier. First instalment four months into the period, then quarterly. This catches growing scale-ups; if you crossed the threshold this year and weren't large in the prior year, you fall under the threshold rule and pay on the standard 9m+1d basis. The next year you'll be on instalments.
The associated-companies trap
The £1.5m threshold divides by the number of associated companies.
A standalone company crosses the threshold at £1.5m of taxable profits. A group with three associated companies crosses at £500k each. "Associated" is broad. Common control, common ownership, and includes companies that pause trading.
The same divisor applies to the 19% small profits rate (£50k divided by associated companies) and the marginal relief upper limit (£250k divided). A small group of four associated companies sees the small profits rate apply only up to £12,500 of profit per company.
If you've recently restructured, sold off a subsidiary, or incorporated a holding company, the associated count may have changed mid-period. Check this before the year-end accounts go in. It changes both your effective rate and your payment cadence.
Year-end changes
Changing your year-end creates a one-off transition period.
If you shorten your accounting period (e.g. from 12 to 9 months to align with a parent), you file a return for the shortened period and CT becomes due 9 months and 1 day after that shorter period ends. Useful if you want to bring forward losses or accelerate relief.
If you extend the period (only allowed once every 5 years and only up to 18 months), CT for the period over 12 months is split into two returns. One for the first 12 months, one for the remainder , each with their own payment date.
Common questions
- What if I haven't filed the return by the payment date?
- You still have to pay an estimate. HMRC accepts that final figures may not be available. Pay your best estimate by the due date, then top up (or claim a refund) when the return is filed. Late payment interest runs from day 1 on any shortfall.
- Can I get a Time to Pay arrangement on Corporation Tax?
- Yes, but HMRC's online TTP self-serve doesn't currently cover CT. CT arrangements have to be agreed directly with HMRC's Business Payment Support Service. We can talk you through the options first. Read about Time to Pay or get a quote. Interest still accrues on the unpaid balance.
- When is CT actually a problem worth funding?
- Most often when a profitable trading year coincides with a capex year. The profit is real, the tax is real, but the cash is sitting in equipment or stock or working capital. Lenders respond well to this story when it's framed honestly.
- Does HMRC apply penalties on late CT?
- Late payment of CT carries interest only. No fixed penalty. Late filing of the CT600 carries fixed penalties (£100, then £200, then 10% of the tax due if more than 18 months late). These are separate from the payment regime.
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