Updated 2026-05-21
Self Assessment is the HMRC annual tax return for self-employed, side income, CIS, and rental income. Deadlines: register by 5 October, file online by 31 January, file paper by 31 October. £100 late penalty. The return covers the tax year (6 April–5 April) and is filed after 6 April of the following year.
Self Assessment — the tax return for self-employed, CIS subcontractors and anyone with income outside PAYE. You file it online through gov.uk: deadline 31 January, late filing means an automatic £100 penalty (even if you are due a refund). The process has 7 steps: UTR → Government Gateway → SA100 + the supplementary pages you need → check → submit. An accountant charges £150–400 for the same thing.
Self Assessment yourself — 7 steps, one evening.
An accountant charges £150–£400 for the same thing. If you have one job, clear expenses and at least B1 English — you can do it. Deadline: 31 January 2027 for 2025/26.
The whole process — top to bottom.
Click on a step below to read the details. Or scroll — each step is explained in its own section.
Соберите документы
P60, P45, CIS statements, банковские проценты, дивиденды, доход от аренды.
Получите UTR
Если первая декларация — регистрация на gov.uk. ~15 рабочих дней по почте.
Government Gateway
Заведите личный кабинет HMRC. Сохраните пароль офлайн — восстановление долгое.
Заполните SA100
Основная форма. Доходы, расходы, allowances. Online — проще, чем бумага.
Дополнительные секции
SA103 для self-employed, SA105 для аренды, SA106 для иностранного дохода.
Проверьте расчёт
HMRC покажет итог: должны вы или вам. Сверьте с собственной таблицей.
Подайте и оплатите
Дедлайн online: 31 января. Бумажный: 31 октября. Штраф £100 за просрочку.
Are you required to file — or not?
HMRC does not notify you automatically. The responsibility is yours. If you must file and you do not — penalties start.
Any self-employed person: CIS construction workers, Uber/Bolt, Deliveroo, cleaners, tutors, Vinted/eBay sellers.
Construction workers whose contractor withholds CIS deductions — SA is mandatory to reclaim any overpayment.
A share in any business partnership requires SA.
Rental income, dividends, foreign income, capital gains (CGT).
Whatever the source — PAYE alone is no longer enough.
Any income from abroad, including pension, rent, dividends.
One employer, correct tax code, no side income.
Covered by the trading allowance — SA not needed.
If covered by the Personal Savings Allowance (£1,000 basic rate) — SA not needed.
If you are filing for 2025/26 for the first time — register by 5 October 2026. This is the registration deadline, not the filing deadline.
Get your UTR — the only thing that comes by post.
UTR (Unique Taxpayer Reference) — a 10‑digit number. Without it you cannot log into Self Assessment. It arrives by letter to your registered address.
Self-employed / sole trader
gov.uk/register-for-self-employment → “Register as self‑employed”. You will receive your UTR AND register for Class 2/4 NI at the same time.
Form CWF1 →Not self-employed but still need to file
For example, rental income or foreign income. Use form SA1 instead of CWF1.
Form SA1 →Many sources say “5–10 days” — that is wrong. HMRC sends the UTR by standard post. The real time is about 15 working days. If you have lived at your address for less than a year — the letter may get lost. Register early.
Government Gateway → GOV.UK One Login.
From 9 February 2026 HMRC is moving to GOV.UK One Login. New users create an account through One Login. Existing users — still through Gateway for now, the transition will be gradual.
Go to gov.uk/log-in-register-hmrc-online-services
If you already have a Government Gateway account — log in as usual. If not — create one through GOV.UK One Login.
What you need for verification
Your NI number is mandatory. Plus one of: UK passport, BRP, or payslips from the last 3 months. Sometimes HMRC asks for extra confirmation by post — keep that in mind for your timing.
Add Self Assessment to your account
After logging in: Services → “Self Assessment” → “Activate”. You will need your UTR. Activation can take up to 24 hours.
SA100 + supplementary pages.
SA100 is the main form. You add the supplementary pages you need. The choice depends on your sources of income.
Main form
Personal details, PAYE employment income (from your P60), savings interest, dividends, charitable giving. Mandatory for everyone.
Self-employment (short)
For most sole traders with turnover < £85,000. CIS workers — also use this one.
Self-employment (full)
Turnover > £85,000 or a complex expense structure.
Property income
Rental of UK property, including renting out a room.
Foreign income
Income from other countries: rent, dividends, salary, pension from abroad.
Residence & remittance
If you are non-domiciled or used the remittance basis. You should get advice from an accountant.
Income, expenses, CIS deductions.
The main rule for expenses: “wholly and exclusively” — only for business, nothing personal. HMRC checks.
What you are allowed to include as expenses
CIS deductions in SA103S
In form SA103S there is a special field “CIS deductions suffered”. Enter the EXACT amount from all your CIS statements for the year — not an estimate.
HMRC matches your figure against the data from contractors. A mismatch → delay or refusal.
The contractor must give you a statement within 14 days after the end of the tax month. If they do not — that is a breach. Send a written request, and if necessary complain to HMRC.
Check and submit — not at the last minute.
After you submit, the SA302 appears in your account within 72 hours — not immediately. If you need the SA302 for a mortgage, do not leave the submission until the last day.
After you submit your return, the SA302 (tax calculation) appears in your HMRC online account only after 72 hours. If you are getting a mortgage and need the SA302 — file your SA well in advance, not on the day of your mortgage adviser meeting.
Check all amounts
Gross income = total of all P60s, P45s, CIS statements. Expenses = only real ones, with documents. NI number accurate.
Check the tax calculation
HMRC shows the final amount before you submit. If the figure is unexpectedly large — recalculate your expenses. Do not submit without checking.
Enter your bank details
For a refund — sort code + account number of a UK account. BACS transfer is faster than a cheque.
Submit and save the confirmation
After you submit, HMRC gives you a reference number. Save a screenshot or PDF. This is your proof of filing.
Penalties — automatic, without warning.
HMRC does not remind you. A £100 penalty arrives on the day you are late — even if you are due a refund. Penalties for late filing and late payment are separate.
These are two independent penalties. You can file on time but pay late — then you will only get the late payment penalty (5% of the tax at 30 days, 6 months and 12 months overdue). You cannot “save” on filing if you do not have the money to pay.
If you are self-employed or a landlord with income > £50,000 — from 6 April 2026 you must keep digital records and submit quarterly updates through MTD-compatible software. This changes the rules completely: one annual SA return is no longer enough. The threshold > £30,000 will apply from April 2027.
Compatible software on gov.uk →Collect everything before you open the form.
Filling in SA without documents is a risk of mistakes. Collect everything in advance, take photos and keep them for at least 5 years (HMRC requirement).
What next.
Where the numbers come from.
- gov.uk/self-assessment-tax-returns — SA overview
- gov.uk/register-for-self-assessment — registration
- gov.uk/self-assessment-forms-and-helpsheets — all SA forms
- gov.uk/self-assessment-tax-returns/deadlines — deadlines
- gov.uk/self-assessment-penalties — penalties
- gov.uk — Making Tax Digital for Income Tax
- gov.uk — SA100 form and notes
- gov.uk — CIS overview (SA103S CIS field)