Self Assessment Tax Return 2025/26: Key Dates and Deadlines

Self Assessment: An Overview

Self Assessment is the system HMRC uses to collect Income Tax from people whose tax is not fully deducted at source through PAYE. This includes sole traders, landlords, company directors, partners in a business partnership, and anyone with more complex tax affairs. If you need to file a Self Assessment tax return for the 2025/26 tax year (which runs from 6 April 2025 to 5 April 2026), it is essential that you know the key dates to avoid penalties and interest charges.

Registration Deadlines

If you are new to Self Assessment and need to register for the first time, you must do so by 5 October 2026. This applies if you became self-employed, started receiving rental income, or had any other reason to file a return during the 2025/26 tax year. To register, you need to complete form SA1 (or CWF1 if you are self-employed) on GOV.UK. HMRC will then issue you a Unique Taxpayer Reference (UTR), which you will need to file your return. Registration can take several weeks, so do not leave it until the last minute.

Filing Deadlines

The filing deadlines for the 2025/26 tax year are as follows:

• 31 October 2026 — Deadline for paper tax returns. Note that very few people file paper returns these days, and HMRC strongly encourages online filing.
• 31 January 2027 — Deadline for online tax returns. This is the deadline most people need to be aware of. If you file after this date, you will receive an automatic £100 late filing penalty.

If you file more than three months late, daily penalties of £10 per day begin (up to a maximum of £900). After six months, there is a further penalty of 5% of the tax due or £300, whichever is greater. After twelve months, another 5% or £300 penalty applies.

Payment Deadlines

Paying your tax bill on time is just as important as filing on time. The key payment dates are:

• 31 January 2027 — Balancing payment for the 2025/26 tax year, plus the first payment on account for 2026/27. The payment on account is 50% of the previous year's tax bill and is required if your tax liability was £1,000 or more.
• 31 July 2027 — Second payment on account for 2026/27.

If you pay late, HMRC charges interest from the due date (currently at a rate linked to the Bank of England base rate plus 2.5%). There are also late payment penalties: 5% of the unpaid tax at 30 days, a further 5% at 6 months, and another 5% at 12 months.

Payments on Account Explained

Payments on account are advance payments towards next year's tax bill. They are designed to spread the cost of your tax over the year rather than having one large bill. Each payment on account is 50% of your previous year's total tax liability. If your income drops significantly, you can apply to reduce your payments on account through your online Self Assessment account, but be aware that if you reduce them too much, you will face interest on the underpayment.

Tips for Filing Early

While the deadline is 31 January, there are significant advantages to filing your return early — even as early as April or May:

• You know your tax bill sooner — This gives you more time to plan and save for the payment. No nasty surprises in January.
• Faster refunds — If you are owed a refund, filing early means you receive it sooner.
• Reduced stress — HMRC's systems often slow down in January due to heavy traffic. Filing early avoids this entirely.
• Time to correct errors — If you spot a mistake after filing, you have time to amend your return before the deadline.
• Your accountant's availability — January is the busiest month for accountants. Filing early means your return gets more attention.

Filing early does not mean you have to pay early — the payment deadline remains 31 January regardless of when you file.

What You Need to File Your Return

Gather the following information before starting your return: your UTR and National Insurance number, P60 and P11D from your employer (if applicable), bank interest certificates, dividend vouchers, records of all self-employment income and expenses, rental income and expenses, pension contributions, Gift Aid donations, student loan details, and any foreign income. Having everything ready before you start will make the process much quicker and reduce the risk of errors.

How London Accountant Can Help

We prepare hundreds of Self Assessment tax returns every year for individuals, sole traders, landlords, and company directors across London. Our fixed-fee service includes gathering your information, preparing and reviewing your return, filing it with HMRC on your behalf, and advising you on any tax-saving opportunities. We aim to complete returns within five working days of receiving your information, and we handle any follow-up queries from HMRC. Contact us today to get started.