If you need to file a Self Assessment tax return for the 2025/26 tax year (6 April 2025 to 5 April 2026), this guide covers everything you need to know — from key dates and deadlines to the records you'll need and the most common mistakes that trigger HMRC enquiries.

Key Dates for 2025/26 Self Assessment

Mark these dates in your calendar:

  • 6 April 2025 — Start of the 2025/26 tax year
  • 5 April 2026 — End of the 2025/26 tax year
  • 6 October 2026 — Deadline to register for Self Assessment if you haven't filed before
  • 31 October 2026 — Paper tax return deadline
  • 31 January 2027 — Online tax return deadline and payment deadline for balance due
  • 31 July 2027 — Second payment on account deadline

Filing late results in an automatic £100 penalty, even if you don't owe any tax. After 3 months, daily penalties of £10 per day kick in (up to 90 days). After 6 months, there's a further 5% penalty on tax owed, and another 5% after 12 months. Don't miss the deadline.

Who Needs to File a Self Assessment Return?

You need to file a Self Assessment tax return if any of the following apply to you:

  • Self-employed sole traders with income over £1,000 (the trading allowance)
  • Partners in a business partnership
  • Landlords with rental income over £1,000 (the property allowance) or with expenses to claim
  • Directors of limited companies (unless the company is dormant and you have no other income to declare)
  • Higher-rate taxpayers with income over £50,270 who need to declare untaxed income
  • Anyone with capital gains above the annual exempt amount (£3,000 for 2025/26)
  • Recipients of foreign income or income from trusts
  • Anyone HMRC asks to file — if you receive a notice to file, you must submit a return even if you don't think you owe tax

What Records Do You Need?

Gather these before you start your return:

  • P60 from your employer showing salary and tax paid
  • P11D for any benefits in kind (company car, health insurance, etc.)
  • Bank statements for all business and rental accounts
  • Invoices and receipts for business expenses you want to claim
  • Rental income records including tenancy agreements and letting agent statements
  • Dividend vouchers or statements showing dividend income
  • Pension contribution certificates
  • Gift Aid declarations for charitable donations
  • Capital gains records including purchase and sale documents for assets
  • Student loan statements showing your plan type and any direct repayments

HMRC requires you to keep records for at least 5 years after the 31 January deadline. For Self Assessment relating to the 2025/26 tax year, keep records until at least 31 January 2032.

Common Mistakes and How to Avoid Them

1. Forgetting to Include All Income

HMRC receives information from employers, banks, and other sources. If your return doesn't match their records, you'll likely face an enquiry. Make sure to include all employment income (even from short-term jobs), bank interest, dividends, and rental income. HMRC's Connect system cross-references millions of data points — don't assume small amounts will go unnoticed.

2. Claiming Expenses You're Not Entitled To

The rules on allowable expenses vary depending on whether you're self-employed, a landlord, or an employee. Common mistakes include claiming personal expenses as business costs, over-claiming use-of-home expenses, and claiming the full cost of items that are only partly used for business. When in doubt, check HMRC's guidance or use AI-powered software that knows the rules.

3. Getting the Rental Income Figures Wrong

Landlords frequently make errors with mortgage interest (only the basic rate tax reduction applies since April 2020), replacement of domestic items relief (which replaced the old wear-and-tear allowance), and splitting income between joint owners. If you own property jointly with a spouse, the default split is 50/50 unless you've made a Form 17 declaration.

4. Forgetting Payments on Account

If your Self Assessment tax bill is more than £1,000, HMRC will require you to make payments on account — advance payments towards next year's bill. Each payment on account is half of the previous year's tax bill. Many taxpayers are caught off guard by the July payment on account and face cash flow problems.

5. Not Claiming All Available Reliefs

Many taxpayers miss out on legitimate reliefs. Check whether you qualify for: marriage allowance transfer (worth up to £252), pension tax relief on personal contributions, EIS and SEIS income tax relief, trading and property allowances (£1,000 each), and capital gains tax reliefs such as Business Asset Disposal Relief.

How TaxStats AI Automates Self Assessment

TaxStats AI takes the pain out of Self Assessment. Here's how:

  • Upload your documents — the AI reads P60s, P11Ds, bank statements, and receipts, extracting the data automatically
  • Automatic categorisation — income and expenses are categorised by type and SA100 supplementary page
  • Tax calculation — the AI applies the correct rates, allowances, and reliefs for the 2025/26 tax year
  • Error checking — before you file, the AI reviews your return for common mistakes and inconsistencies
  • Plain English guidance — don't understand a question on the return? Ask the AI and get a clear explanation

Whether you're a sole trader, landlord, or have complex affairs, TaxStats AI handles the calculations while you stay in control of the decisions.