UK Accountant's 2026 Deadline Calendar

Every key 2026 deadline a UK practice needs to track, in one place. The trick is to diary the period ends, not the deadlines themselves. Every date below is derived from the period it relates to.

Monthly recurring deadlines

These dates don't move. They run on a rhythm regardless of any individual client's year-end, and when a practice misses them it's almost always because nobody owns the recurring slot in the diary.

One nuance the recurring grid hides. PAYE late-payment penalties don't bite on the first slip; they bite on the second. SI 2009/56 catches anyone who's late 'more than once in a tax year', which is how a one-off £400 quarter quietly becomes a pattern that costs four figures.

Corporation Tax key dates

Take a worked example: a private limited company with an accounting period ending 31 March 2026. The CT clock runs from that date, not from when the accounts are signed.

You pay before you file. A quirk that catches new client managers every January. Large companies (taxable profits above £1.5m, divided by group size) pay in quarterly instalments under SI 1998/3175 reg.5, so the calendar starts mid-period rather than after it.

Self-Assessment milestones

The 2024/25 SA cycle finishes in 2026. Paper returns were due 31 October 2025. Online returns and the balancing payment fall on 31 January 2026, with the second payment on account on 31 July 2026. Clients who only became self-employed in 2024/25 will hit their first POA cliff this January, so flag them now if you haven't already.

P11D, P60 and payroll year-end

For 2025/26: P60s to employees by 31 May 2026; P11D and P11D(b) by 6 July 2026; Class 1A NIC payment by 22 July (electronic) or 19 July (post). For the full mechanics see our guide to the 6 and 22 July P11D and Class 1A NIC deadlines. Mandatory payrolling of benefits was deferred to 6 April 2027, so 2026 is still a 'choose your route' year.

Companies House: accounts and confirmation statements

Accounts run on the 9-month clock (21 months for first accounts). The confirmation statement runs on its own incorporation cycle with a 14-day filing window, completely independent of the year-end. We've covered this elsewhere in detail: see what a confirmation statement is and when it is due if a client's review date confuses anyone in the team.

One-off 2026 dates to flag

ATED return and payment fall on 30 April 2026 for any residential property held in a corporate envelope above £500,000. The 2026 SORP applies to charity accounting periods beginning on or after 1 January 2026. From 1 October 2026 the charity audit threshold rises to £1.5m. And MTD ITSA's first wave (income above £50,000) lands in April 2026 — quarterly updates due 7 August, 7 November, 7 February and 7 May each year. See the MTD ITSA complete guide for thresholds, deadlines, and the penalty framework.

How to turn this into a practice-wide workflow

A calendar like this is a reference, not a system. The system is whatever you use to track each client's individual period end and let the deadlines fall out automatically. A shared spreadsheet works up to roughly 40 clients. After that, someone always forgets to update it after an AP shortening, and the calendar above stops matching reality for at least one client at any given time.