HMRC deadlines 2026: what your firm needs to know

For accountancy firms managing multiple clients, staying on top of HMRC deadlines is one of the most critical — and most stressful — aspects of the job. A missed deadline does not just mean a penalty for the client. It means a difficult conversation, potential reputational damage, and the kind of regulatory attention that no firm wants. This guide covers the key dates for 2026 and offers practical advice on staying ahead of them.

Self Assessment

The Self Assessment cycle is the rhythm that defines the year for many accountancy firms. The key dates remain consistent from year to year, but the consequences of missing them have become increasingly severe.

The deadline for filing paper Self Assessment returns for the 2024/25 tax year was 31 October 2025. Online returns must be filed by 31 January 2026, which is also the deadline for paying any tax owed for that year. The second payment on account for 2024/25 is due on 31 July 2026, and the first payment on account for 2025/26 is due on 31 January 2027.

For firms with a large Self Assessment client base, the January deadline creates an intense period of activity. The key to managing it successfully is starting early. Firms that begin chasing client information in September and October find January far less stressful than those that leave it until December.

Corporation Tax

Corporation Tax returns must be filed within 12 months of the end of the accounting period they cover. The tax itself is due nine months and one day after the end of the accounting period. For companies with a standard 31 March year-end, this means the tax payment is due by 1 January and the return must be filed by 31 March of the following year.

Larger companies — those with taxable profits above a certain threshold — must pay Corporation Tax in quarterly instalments. The dates for these payments depend on the company's accounting period and can be complex to track, particularly for firms managing clients with non-standard year-ends.

VAT

VAT returns and payments are due one calendar month and seven days after the end of each VAT period. For businesses on standard quarterly VAT periods, this means four filing deadlines per year. The exact dates depend on the business's VAT stagger group, which determines whether their quarters end in March/June/September/December, April/July/October/January, or May/August/November/February.

Making Tax Digital for VAT is now fully in effect, which means all VAT-registered businesses must maintain digital records and submit returns using compatible software. Firms need to ensure that their clients are compliant with MTD requirements, which adds an ongoing obligation beyond the filing deadlines themselves.

PAYE and payroll

Employers must submit Full Payment Submissions to HMRC on or before each payday. The final submission for the tax year — the final FPS or an Employer Payment Summary — must indicate that it is the final submission and be filed by 19 April. P60s must be provided to employees by 31 May.

Monthly PAYE payments to HMRC are due by the 22nd of each month for electronic payments, or the 19th for postal payments. Small employers who pay quarterly have different deadlines. Benefits in kind must be reported on P11D forms by 6 July, and Class 1A National Insurance contributions on those benefits are due by 22 July.

Making Tax Digital for Income Tax

MTD for Income Tax Self Assessment is being phased in from April 2026 for self-employed individuals and landlords with qualifying income above certain thresholds. This represents a significant change in reporting obligations, requiring quarterly updates to HMRC through compatible software rather than a single annual return.

Firms with self-employed clients and landlords need to be prepared for this transition. The quarterly reporting deadlines will add to the compliance calendar, and both firms and their clients will need to adapt their record-keeping practices to meet the new requirements.

Staying on top of deadlines

The sheer number of deadlines facing a multi-client firm makes manual tracking impractical. A firm with 200 clients might be managing over a thousand individual deadlines across Self Assessment, Corporation Tax, VAT, PAYE, and other obligations. Each client may have different year-ends, different VAT periods, and different reporting requirements.

Effective deadline management requires a systematic approach. The most reliable firms use automated compliance calendars that track every deadline for every client, send alerts well in advance, and escalate when deadlines are approaching without the required work being completed. This kind of proactive monitoring is difficult to achieve manually but straightforward to automate.

The cost of getting it wrong is significant. Late filing penalties, late payment interest, and the administrative burden of dealing with HMRC enquiries all eat into a firm's time and reputation. Investing in robust deadline tracking is not a luxury — it is a fundamental part of running a compliant, professional practice.