Self Assessment Checklist 2025/26 — Everything You Need Before January 2027

Key deadlines for 2025/26

• 5 April 2026: End of the 2025/26 tax year
• 5 October 2026: Deadline to notify HMRC if you started self-employment in 2025/26 and are not yet registered for Self Assessment
• 31 January 2027: Online filing deadline and payment deadline for tax owed
• 31 July 2027: Second payment on account (if applicable)
• Late filing: £100 automatic penalty on day one. Additional £10 per day after 3 months.

Income records to gather

Self-employment income: Total income from all business activities. If you use Xero or QuickBooks, this is your Profit and Loss report for the year. If you are on the cash basis, this is money received, not invoiced.

PAYE employment income: If you also worked as an employee, you will need your P60 from each employer. If you left a job during the year, your P45.

Rental income: If you receive rental income from UK property, you will need a summary of rents received and allowable costs (mortgage interest is now restricted to 20% relief for higher-rate taxpayers).

Investment income: Dividends, savings interest above the Personal Savings Allowance, and capital gains from selling assets.

State benefits: Certain state benefits including the State Pension, Jobseeker's Allowance, and Carer's Allowance are taxable.

Expense records to gather (self-employment)

Gather receipts, bank statements, or software exports covering:

• Office costs (stationery, software, printer)
• Travel (mileage log, public transport, parking)
• Professional fees (accountant, bookkeeper, legal costs)
• Marketing and advertising
• Equipment purchased (check Annual Investment Allowance rules)
• Phone and internet (business proportion only)
• Training and professional development
• Working from home (flat rate or actual costs)
• Bank charges on business account

Bank statements: 12 months of business bank statements — the reconciled version from Xero or QuickBooks, or your actual bank export.

Gift Aid and pension contributions

Both Gift Aid donations and pension contributions can reduce your tax bill and should be included on your return. Gift Aid payments made in 2025/26 can also be carried back to 2024/25 if it would reduce your tax bill.

Payments on account

If your tax bill for 2025/26 exceeds £1,000, HMRC will ask you to make payments on account toward 2026/27. The first payment is due 31 January 2027 (alongside your 2025/26 tax). The second is due 31 July 2027. This catches many sole traders off guard — your January payment is effectively one and a half times the bill you expect.

What to check before filing

• Have you included all income sources, including small one-off payments?
• Have you claimed all allowable expenses including mileage and use of home?
• Does your self-employment income match your bookkeeping records?
• Have you checked your payments on account from last year — they reduce what you owe in January?
• Is your HMRC Government Gateway login working? HMRC now requires two-factor authentication.

The risk of getting it wrong

HMRC can open an enquiry into your Self Assessment up to 12 months after the filing deadline for errors — and up to 6 years where the error is careless. Understating income or overclaiming expenses can result in penalties of up to 100% of the tax owed for deliberate errors.

The best protection is clean, reconciled bookkeeping records throughout the year, not a last-minute scramble in January.