UK Self Assessment for first-timers

Step 1 — Confirm you actually need to file

About 12 million people file Self Assessment returns each year. The most common triggers for first-time filers in 2026/27 are:

• Self-employment / sole-trader income above £1,000 in the tax year (the trading allowance threshold).
• Rental income above £1,000 (the property allowance threshold).
• Dividends above £10,000 in the year, even if all from UK companies.
• Savings interest above £10,000. Below £10,000, HMRC reconciles via tax code adjustment without a return.
• Total income above £150,000.
• The High Income Child Benefit Charge applies — i.e. your or your partner's adjusted net income is above £60,000 and Child Benefit is being claimed.
• Capital gains above the £3,000 annual exempt amount, OR disposal proceeds over £12,000 (4× AEA), even if the gain is below £3,000.
• Income from abroad — foreign salary, foreign pensions, foreign dividends, etc.
• You're a partner in a partnership.
• You're claiming relief on pension contributions outside PAYE, or claiming higher-rate relief on a SIPP.
• You've received untaxed income over £2,500 from any source not covered by PAYE.

Use the side-hustle checker for the side-income decision and the readiness checklist to walk through every trigger systematically.

Step 2 — Register with HMRC (allow 4-6 weeks)

If you've never filed before, you must register first. Registration is at gov.uk/register-for-self-assessment. There are slightly different forms depending on the trigger — sole trader, partnership, "not self-employed but otherwise required" — but they all eventually produce a Unique Taxpayer Reference (UTR): a 10-digit number that's your tax-system fingerprint forever afterwards.

HMRC posts a separate activation code for the online Government Gateway service after registration. Allow up to 10 working days for the code to arrive. The activation code expires after 28 days of issue, so don't lose it.

Deadline to register for a tax year is 5 October following the end of that tax year. So for a return covering 2026/27 (the year ending 5 April 2027), you must register by 5 October 2027. Most first-timers register much later than this and incur penalties; see the deadlines section below.

Step 3 — Gather your records

Before opening the form, collect every document for the tax year:

• P60 from each employer (one per job, year-end summary)
• P45 from any job you left during the year
• P11D for any taxable benefits in kind (company car, private medical, etc.)
• Self-employment income records — invoices, receipts, mileage log, business bank statements
• Dividend vouchers from any UK shares held outside ISAs
• Bank/building society interest summaries for non-ISA accounts (look for "consolidated tax certificates")
• Rental income records — rent received, allowable expenses, mortgage interest paid
• Pension contribution evidence if claiming higher-rate relief
• Charity donation receipts if claiming Gift Aid relief
• Crypto / share disposal records if you sold any taxable assets
• Foreign income summaries with the GBP equivalent at the date received
• Any notice from HMRC about a previous tax code adjustment

For the self-employed, HMRC requires you to keep records for at least 5 years after the 31 January submission deadline. Bank app statements, photos of receipts, and a simple spreadsheet are usually enough — you don't need fancy software unless your turnover is approaching the VAT threshold.

Step 4 — File online before 31 January

For most first-timers, the online return is the right approach. It's free, walks you through the right supplementary pages based on your answers, and tells you the exact tax bill at the end. Log in at gov.uk/log-in-file-self-assessment-tax-return with your Government Gateway credentials.

The form is divided into sections:

• SA100 — main return (everyone fills this)
• SA102 — employment (one per job)
• SA103S or SA103F — self-employment (Short or Full)
• SA104 — partnerships
• SA105 — UK property
• SA106 — foreign income
• SA108 — capital gains
• SA110 — tax calculation summary

The online system shows you only the supplementary pages you need based on your answers to the upfront "About you" section. Save and resume as often as you want before submission.

Step 5 — Pay by 31 January

The same 31 January deadline applies to:

• The full balance of tax owed for the previous tax year
• The first "payment on account" for the current tax year, if your previous tax bill was £1,000+

Payments on account come as a surprise to most first-timers. If you owe £4,000 in January for 2026/27, HMRC also expects you to pay £2,000 alongside (the first 50% of estimated 2027/28 tax) — making the January bill effectively £6,000. The second £2,000 payment on account is due 31 July. Plan cash flow accordingly.

Payment options: online banking via Faster Payments (instant), Direct Debit (set up several days ahead), card payment via HMRC's portal, or by post for cheques (allow 3 working days). The bank account details are at gov.uk/pay-self-assessment-tax-bill.

Deadlines for the 2026/27 return

Penalties for missing deadlines

• Initial late filing: £100 immediately, even if no tax is due.
• 3 months late: £10/day for up to 90 days (max £900) on top of the £100.
• 6 months late: 5% of tax due or £300 (whichever is higher).
• 12 months late: Another 5% or £300.
• Late payment: 5% of unpaid tax after 30 days, another 5% at 6 months, another 5% at 12 months.
• Interest: currently around 7.75% on late-paid tax (HMRC late payment interest rate).

HMRC will accept "reasonable excuse" appeals (serious illness, bereavement, IT system failure, etc.) but the bar is high. Don't rely on getting penalties cancelled.

Common first-timer mistakes

• Not registering until December. The activation code takes 10 working days — register by mid-December at the latest, not 28 January.
• Forgetting to include savings interest. Banks no longer deduct tax at source; you must declare interest above the Personal Savings Allowance even if you've never thought about it before.
• Not claiming higher-rate pension relief. Higher-rate taxpayers contributing to a SIPP get only basic-rate (20%) relief at source. The remaining 20% must be claimed via Self Assessment — typically £400-£800 of refund per £1,000 contributed.
• Forgetting to declare foreign income. Even small amounts of foreign dividends from US tech stocks count; HMRC and the IRS share data routinely.
• Skipping the payment-on-account. Treating the January bill as just last year's tax leaves you blindsided when HMRC also asks for half of next year's estimate.
• Mismatching crypto disposals. HMRC treats crypto disposals as CGT events — every swap, sale, or "spend" is a disposal. The crypto tax calculator handles share-pool accounting.
• Claiming "tax-free" expenses for self-employment that aren't. Mileage, working-from-home, and clothing all have specific rules; not everything that looks business-related is allowable. Use the self-employed toolkit.

FAQs

Related calculators and tools

Self Assessment readiness checklist — walks through every trigger and document. January 2027 cycle checklist — current-year specific. Side-hustle checker — single-question test for casual income. Tax calculator — model the bill before filing. Sole trader calculator — for the self-employed.