Self Assessment: The Complete UK Guide 2025/26

Who Needs to File Self Assessment?

You must register for Self Assessment if you are self-employed with income over £1,000, a partner in a business partnership, a company director (unless your only income is PAYE salary with no other complications), or if you have untaxed income such as rental income, significant savings interest above your Personal Savings Allowance, dividend income above £500, or capital gains above £3,000. You also need to file if your income exceeds £100,000 (to reclaim the tapered Personal Allowance via pension contributions), if you received income from abroad, or if HMRC sends you a notice to file. If in doubt, register — the penalty for failing to register when you should have is up to 100% of the tax owed.

"If in doubt, register — the penalty for failing to register when you should have is up to 100% of the tax owed."

Registering: How and When

Register at gov.uk/register-for-self-assessment. The deadline is 5 October following the end of the tax year in which you first need to file. So if you started self-employment in June 2025, you must register by 5 October 2025. You will receive a Unique Taxpayer Reference (UTR) number by post within 10 days — this is your permanent tax identifier and you will need it every year. You will also need a Government Gateway login. If you already have one for PAYE, you can add Self Assessment to it. Allow 2–3 weeks for the registration process before attempting to file.

The Key Deadlines

There are four critical dates. 5 October: register if you are newly required to file. 31 October: paper return deadline (almost nobody uses this now). 31 January: online return filing deadline AND payment of any tax owed AND first payment on account. 31 July: second payment on account. Missing the 31 January filing deadline triggers an automatic £100 penalty even if you owe no tax. After 3 months, the penalty increases by £10 per day (up to £900). After 6 months and 12 months, further percentage-based penalties apply. Interest accrues on unpaid tax at 7.75% from 1 February.

What Income to Declare

Self Assessment captures all income that has not already been taxed through PAYE. Employment income is pre-populated from HMRC data but you should verify it. Self-employment income is your gross receipts before expenses (you then deduct allowable costs to calculate profit). Rental income is rent received minus allowable expenses (mortgage interest now only gets 20% basic rate credit, not full deduction). Savings interest above your Personal Savings Allowance (£1,000 basic rate, £500 higher rate). Dividends above £500. Capital gains from selling assets — shares, second properties, crypto. Foreign income. Any other income not taxed at source.

"Self Assessment captures all income that has not already been taxed through PAYE"

Claiming Expenses as a Self-Employed Person

You pay tax on profit, not turnover, so claiming every legitimate expense is crucial. Allowable expenses include: equipment and tools used wholly for business, business mileage at 45p/mile (first 10,000 miles), 25p above that, the business proportion of your phone and broadband, professional subscriptions, accounting fees, marketing and advertising, subcontractor costs, business insurance, training directly related to your current trade, and a proportion of home costs if you work from home (either the flat-rate £6/week or an apportioned calculation). Keep every receipt. The test for any expense is: was it incurred wholly and exclusively for business purposes?

Payments on Account: The First-Year Shock

Self Assessment has a payment system that catches many first-time filers by surprise. When your total tax bill exceeds £1,000, HMRC requires you to make advance payments toward next year's bill. Each payment on account is 50% of your current year's bill, due on 31 January and 31 July. This means your first ever January filing can require paying 150% of your annual tax bill: the full bill for the year just gone, plus 50% advance for the current year. The solution is to save 25–30% of every self-employed payment received throughout the year in a dedicated savings account. If you expect lower income next year, you can apply to reduce payments on account via your HMRC online account.

Common Mistakes and How to Avoid Them

The most common errors in Self Assessment returns: forgetting to declare all income sources (HMRC cross-references data from banks, platforms, and employers — omissions are increasingly detected automatically), not claiming all allowable expenses (underclaiming is just as costly as overclaiming), missing the payment on account system (see above), failing to claim higher rate pension relief via Self Assessment (this 20% relief is not automatic — you must claim it), and not keeping records (HMRC can investigate returns up to 4 years back, or 6 years if they suspect carelessness). Good record-keeping throughout the year makes the return straightforward; leaving it to January makes it stressful and error-prone.

Getting Help Without Paying Too Much

For a straightforward return (one income source, simple expenses), HMRC's free online system is perfectly adequate. Software like GoSimpleTax (from £29) or TaxCalc makes the process significantly easier and catches errors. For self-employed people with multiple income sources, property income, or complex expenses, a qualified accountant typically charges £150–500 for a self-assessment return and pays for themselves many times over through legitimate tax savings and stress reduction. Avoid unregulated "tax return" services advertised on social media — check that any accountant is registered with ICAEW, ACCA, or CIOT. HMRC also offers free help via phone (0300 200 3310) and webchat.