How to File a UK Self Assessment Tax Return (First Time)

To file your first UK Self Assessment: register with HMRC by 5 October following the tax year you need to file for; receive your UTR (Unique Taxpayer Reference) by post within 10 working days; set up your online tax account at gov.uk; gather your income, expenses and tax-related documents for the tax year; complete the online return by 31 January; and pay any tax owed by the same deadline. Late filing triggers a £100 penalty automatically. The process takes 1–4 hours for a typical first-time filer once everything’s gathered.

This is the step-by-step process for 2026/27.

Step 1: Confirm you need to file

You need to file Self Assessment if any of the following apply for the 2026/27 tax year:

• Self-employed / side hustle income above £1,000 gross.
• Property letting income above £1,000 gross.
• Untaxed income (e.g. dividends above £500 outside an ISA) above £2,500.
• Capital gains above £3,000, OR total disposals over £50,000.
• High Income Child Benefit Charge applies (income above £60,000 with child benefit claimed).
• You earn over £150,000 PAYE.
• HMRC has sent you a notice to file.

If none apply, you don’t need to file. Check HMRC’s “do I need to file” tool.

Step 2: Register for Self Assessment

If this is your first time, you need to register with HMRC.

Deadline: 5 October following the end of the tax year. For 2026/27, the registration deadline is 5 October 2027.

The process:

1. Go to gov.uk: register for Self Assessment.
2. Choose your registration type (self-employed, not self-employed but needing to file, partnership, etc.).
3. Provide National Insurance number, date of birth, address, contact details.
4. For self-employed: provide business start date and brief business description.

HMRC processes your registration and posts a Unique Taxpayer Reference (UTR) to you — typically within 10 working days.

You’ll also receive a letter explaining how to file online.

Step 3: Set up your online tax account

Once you have the UTR:

1. Go to gov.uk: file your Self Assessment.
2. Sign in with your Government Gateway ID (create one if needed).
3. Use the UTR + activation code (from HMRC’s letter) to activate online filing.
4. Complete identity verification (passport or driving licence).

You now have access to your Self Assessment online account.

Step 4: Gather your documents

Before starting the return, collect:

Employment income (if applicable)

• P60 from your employer (issued after tax year end, ~April/May).
• P11D if you had benefits-in-kind.
• P45 if you left a job during the year.

Self-employed income

• Sales records for the tax year (invoices, online platform statements).
• Expense records (receipts, bank statements showing business expenses).
• Mileage logs if claiming vehicle expenses.

Property income

• Rental income statements from letting agents or your own records.
• Mortgage interest statements (for the 20% credit).
• Expense receipts (repairs, agent fees, insurance, etc.).

Investment income

• Dividend vouchers or platform tax statements.
• Interest statements from banks (if above PSA).
• Capital gains records for any disposals.

Other income

• State Pension statements.
• Private pension withdrawals.
• Foreign income statements.

Tax-deductible items

• Pension contributions (for higher-rate relief claim).
• Gift Aid donation receipts.
• Marriage Allowance transfer details.

Step 5: Complete the online return

The Self Assessment online form has multiple sections, you only complete the relevant ones:

Main return (SA100)

The core form, with general questions about your circumstances. Always complete.

Supplementary pages

You add these as needed:

• SA102: Employment income.
• SA103: Self-employment income (full or short version depending on turnover).
• SA104: Partnership income.
• SA105: Property income.
• SA106: Foreign income.
• SA108: Capital gains.
• Other specialised pages for specific income types.

The online system asks you a series of questions and adds the relevant supplementary pages automatically.

For a first-time filer with PAYE + side hustle:

• Main return + SA102 (employment) + SA103 (self-employment).

Step 6: Review and submit

After completing the form:

• Review the summary calculation HMRC generates.
• Confirm your tax bill (the amount you owe or are owed).
• Submit the return.

You’ll receive an on-screen confirmation and a confirmation email. Keep this for your records.

Step 7: Pay any tax owed

The payment deadline is 31 January following the tax year end.

Payment options:

• Online banking (Faster Payments, BACS, CHAPS).
• Direct Debit via your HMRC online account.
• Debit card (small surcharge).
• Bank transfer using HMRC’s payment details (see your online account for the reference number).

You must use your UTR followed by “K” as the payment reference (e.g. 1234567890K). Without this, HMRC may not allocate the payment correctly.

Deadlines and penalties

For 2026/27 tax year:

• Register: by 5 October 2027.
• File paper return: by 31 October 2026 (rarely used — most file online).
• File online return: by 31 January 2029.
• Pay tax: by 31 January 2029.

Penalties for missing deadlines:

Delay	Penalty
1 day late	£100 automatic penalty
3 months late	Additional £10/day for up to 90 days (£900 max)
6 months late	Additional 5% of tax owed (minimum £300)
12 months late	Another 5% of tax owed

Late payment also triggers interest (currently around 7%+) and possible surcharges.

The penalties stack up quickly. Filing on time even with an unpaid balance is much cheaper than filing late.

Payments on Account

If your Self Assessment tax bill exceeds £1,000 (and certain other conditions), you owe Payments on Account:

• First payment on account: 50% of last year’s tax bill, due 31 January alongside the balancing payment.
• Second payment on account: 50% of last year’s tax bill, due 31 July.
• Balancing payment: any difference between estimated and actual tax due, with the next return.

A worked example for first-time filer with £2,000 of self-employed tax:

• 31 January 2029: pay £2,000 (the 2026/27 balance) + £1,000 (first POA for 2026/27) = £3,000 total.
• 31 July 2028: pay £1,000 (second POA for 2026/27).
• 31 January 2029: balance for 2026/27 + first POA for 2027/28.

This cash-flow shock catches many first-time filers off-guard. Budget for 1.5× your first year’s tax bill in January.

Common first-time mistakes

A few things first-timers often get wrong:

1. Underestimating preparation time

Gathering documents and figuring out the form takes 1–4 hours typically. Don’t start the day before the deadline.

2. Not claiming all allowable expenses

For self-employed: don’t forget mileage, share of home utilities, professional subscriptions, training costs, equipment.

3. Missing the higher-rate pension relief

If you contributed to a personal pension and you’re a higher-rate taxpayer, claim the additional relief through Self Assessment.

4. Forgetting interest and dividend income

Even small bank interest above your PSA is taxable. Include it.

5. Mis-categorising income

Side hustle vs trading vs investment — categorise correctly. HMRC treats each differently.

Getting help

Several resources for first-time filers:

• HMRC online help and chat: through the personal tax account.
• HMRC helpline: 0300 200 3310 (Self Assessment).
• Citizens Advice: free general help on tax.
• Low Incomes Tax Reform Group: free help for lower-income filers.
• Specialist accountants: for complex cases (typically £200–£500/year for sole trader returns).

For simple side hustles, DIY is usually fine. For property letting, self-employment with multiple income sources, or any complex situation, an accountant pays for itself.

After submission

After you submit:

• HMRC processes the return (usually instantly online).
• They confirm the tax due / refund owed.
• For overpayment: you get a refund (typically 2–4 weeks).
• For underpayment: pay by 31 January.

You’ll receive a tax calculation (SA302) confirming the return — useful for future reference (and required by some mortgage lenders).

Internal links

• Do I need to register as self employed for a side hustle?
• What is the trading allowance and how does it work?
• What expenses can I claim as self employed UK?

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This guide is information, not regulated financial advice. UK Self Assessment rules can change — confirm on gov.uk before relying on specific guidance.