When is corporation tax due? For most companies, corporation tax is due nine months and one day after the end of the accounting period. The corporation tax return (CT600) is due 12 months after the period ends. A client with a 31 March 2026 year end must pay by 1 January 2027 and file by 31 March 2027.
The dates themselves are simple. The harder part for practices is delivering them across a full client list when year ends cluster, information arrives late and the same people are also handling MTD, VAT, PAYE and Self Assessment. This guide covers the key dates and the capacity pressures that tend to cause problems.
When Is Corporation Tax Due, and What Is the Corporation Tax Return Deadline?
The statutory dates for a standard 12-month accounting period are:
- Payment: Nine months and one day after the period end.
- Corporation tax return deadline: 12 months after the period end.
- Statutory accounts at Companies House: Nine months after the period end for private companies.
Periods longer than 12 months are split into two for tax purposes, each with its own return and payment date. HMRC’s guidance on corporation tax accounting periods sets this out in full. For rates, thresholds and the full penalty schedule, see our guide to corporate income tax in the UK.
Why Do Corporation Tax Deadlines Strain Practice Capacity?
Three features of the regime create pressure, even for well-run practices.
- The payment date comes first: Tax is due three months before the return, so the computation effectively has to be ready earlier than the filing date suggests. Clients who expect to pay at filing are caught out.
- Accounts and payment collide: Companies House accounts fall due the day before the tax payment date. Finalised accounts, tax computation and a payment figure are therefore needed at the same point.
- Year ends cluster: March, and to a lesser extent December, bring a surge of work that rarely matches staffing levels. Peaks also overlap with other obligations sometimes.
Also read our refresher guide on the UK tax system.
What Are the Most Common Filing Challenges for Practices?
Most delays trace back to a handful of causes:
- Late or incomplete client information: Missing records, unreconciled accounts and unanswered queries hold up the corporation tax return more than technical issues do.
- Large company instalments: Companies above the profit threshold pay quarterly, with the first instalment falling six months and 13 days after the period begins. Associated companies reduce the threshold, so group structures need checking early.
- Computation and iXBRL work: Adjusting profits, claiming capital allowances and tagging accounts is detailed, time-consuming work.
- Review bottlenecks: Senior staff or partners often become the constraint when every return needs sign-off at the same time.
- Competing priorities: MTD preparation, VAT, CIS and payroll deadlines draw on the same team.
How Can Practices Manage Corporation Tax Capacity?
A few measures make a huge difference.
- Build a deadline map: Record each client’s year end, size category and associated companies, and review it at least annually.
- Set internal deadlines ahead of statutory ones: Work back from the payment date, not the filing date.
- Request information early: Send checklists several weeks before year end, not after.
- Standardise preparation: Consistent templates and review checklists reduce rework and speed up sign-off.
- Protect senior review time: Schedule review slots in advance and route routine returns through a lighter process.
- Add flexible support for peaks: Many practices use Befree’s tax services to extend their team during busy periods, with preparation handled in the practice’s own systems and sign-off remaining with the firm.
How to File a Corporation Tax Return Without Last-Minute Pressure
What Happens If a Client Misses a Deadline?
Need Extra Capacity for Corporation Tax Season?
If your practice is stretched at peak times, a dependable support team can protect quality and deadlines. Contact Befree to discuss how we can help.