Filing your Self Assessment tax return

Each year you must report your income to HMRC by completing a Self Assessment tax return. You can file online using HMRC's service or commercial software. Getting it right avoids penalties and ensures you claim everything you're entitled to.

Who must file a Self Assessment return

You must send a tax return if any of the following apply:

• Self-employed sole trader with gross income over £1,000 from self-employment
• Partner in a business partnership, regardless of income level
• Company director with untaxed income (dividends, benefits in kind, or income not collected through PAYE)
• High earner with total income over £125,140 (even if all taxed through PAYE)
• Capital gains from selling or disposing of assets above the annual exempt amount (£3,000 for 2024/25 onwards)
• Rental income from UK or overseas property (above the £1,000 property allowance)
• Foreign income received from abroad, including overseas pensions
• Untaxed income such as tips, commission, or income from the sharing economy above £1,000
• High Income Child Benefit Charge where you or your partner earn over £60,000 and claim Child Benefit
• Savings income over your Personal Savings Allowance (£1,000 basic rate, £500 higher rate)

If you are unsure, HMRC provides an online tool to check whether Self Assessment applies to you.

Registration and your UTR

If you need to file for the first time, you must register with HMRC by 5 October following the end of the tax year in which you started receiving untaxed income.

Example: If you started freelancing in September 2026 (tax year 2026/27), register by 5 October 2027.

After registering, HMRC sends two items by post:

• Unique Taxpayer Reference (UTR): A 10-digit number, typically within 10 working days (21 days if abroad)
• Activation code: A separate letter to activate your online account, usually within 7 working days of requesting access

Keep your UTR safe. You need it for all Self Assessment dealings, mortgage applications, and some business contracts.

Key deadlines

The tax year runs from 6 April to 5 April. For example, tax year 2026/27 runs from 6 April 2026 to 5 April 2027.

• 5 October: Deadline to register for Self Assessment (if filing for the first time)
• 31 October: Paper return deadline (if you still use paper)
• 31 January: Online filing deadline AND first payment deadline
• 31 July: Second payment on account deadline

Example timeline for 2026/27:

• 6 April 2026: Tax year starts
• 5 April 2027: Tax year ends
• 5 October 2027: Registration deadline for new filers
• 31 October 2027: Paper return deadline
• 31 January 2028: Online filing + payment deadline
• 31 July 2028: Second payment on account

The SA100 form: what you need to complete

The main Self Assessment form (SA100) has several sections. You only complete what applies to you:

Main SA100 sections

• Personal details: Name, address, UTR, National Insurance number
• Employment income: Salary and benefits from P60/P11D (if employed as well as self-employed)
• Self-employment income: Business profits (via supplementary pages SA103S or SA103F)
• Partnership income: Share of partnership profits
• UK property income: Rental income from UK properties
• Foreign income: Income from abroad
• Dividends: Dividend income from shares
• Interest: Bank and building society interest
• Pensions: State and private pension income
• Capital gains: Profits from selling assets
• Other income: Anything not covered elsewhere
• Tax reliefs: Pension contributions, charitable donations, marriage allowance
• Student loan: Repayments due (check you select the correct plan type: Plan 1, Plan 2, Plan 4 Scotland, or Plan 5 postgraduate)

Supplementary pages for self-employed

• SA103S (Short): Turnover under £85,000 and no losses brought forward
• SA103F (Full): Turnover £85,000+ or complex accounts

These pages capture your business income, expenses, capital allowances, and calculate your taxable profit.

Allowable expenses for sole traders

You can deduct expenses that are incurred wholly and exclusively for business purposes. Mixed-use items (such as a phone used for work and personal calls) must be apportioned.

Main expense categories

• Office costs: Stationery, phone bills (business portion), postage, printing, software subscriptions
• Premises costs: Rent, business rates, utilities, property insurance, security, repairs
• Travel: Fuel for business journeys, parking, train and bus fares, hotel accommodation on business trips. Not commuting from home to a regular place of work
• Staff costs: Salaries, employer pension contributions, subcontractor payments, agency fees, employer National Insurance
• Stock and materials: Goods for resale, raw materials
• Financial costs: Bank charges, interest on business loans, hire purchase interest, credit card charges for business transactions
• Professional services: Accountancy fees, legal fees, professional indemnity insurance, trade body subscriptions
• Marketing: Advertising, website costs, business cards, trade journal subscriptions
• Clothing: Protective clothing and uniforms only. Everyday clothing is not allowable, even if you only wear it for work

Simplified expenses

If you are a sole trader or partnership (not a limited company), you can use flat-rate simplified expenses instead of working out actual costs for vehicles, working from home, and living at business premises.

Payments on account

If your tax bill is over £1,000 and less than 80% was collected through PAYE, HMRC requires advance payments towards next year's bill:

• First payment on account: 31 January (due alongside your tax return) - 50% of previous year's bill
• Second payment on account: 31 July - another 50% of previous year's bill
• Balancing payment: 31 January following year - any remaining amount due once actual liability is known

Example: If your 2024/25 tax bill was £4,000:

• 31 January 2027: Pay £4,000 (2025/26 bill) PLUS £2,000 (first payment on account for 2026/27)
• 31 July 2027: Pay £2,000 (second payment on account)
• 31 January 2028: Pay any balance for 2026/27 after deducting the £4,000 already paid on account

Reducing payments on account: If you expect lower income next year, you can apply to reduce through your HMRC online account or form SA303. But if you reduce too much, HMRC charges interest on the shortfall from the original due date.

First year of Self Assessment: No payments on account are due in your first year because there is no previous year's bill to base them on. However, the following January you will owe both your first year's bill and your first payments on account, so budget accordingly.

Record keeping requirements

You must keep records that support every figure in your tax return. HMRC can check your records during an enquiry, and inadequate records can lead to penalties or estimated assessments.

What records to keep

• All sales and income (invoices, till rolls, bank deposits)
• All business expenses with receipts or proof of payment
• Bank statements and chequebook stubs
• P60 and P11D forms (if also employed)
• Dividend vouchers and interest certificates
• Records of assets bought or sold (for capital gains)
• Mileage logs if claiming vehicle expenses

How long to keep records

Keep your records for at least 5 years after the 31 January submission deadline for the relevant tax year.

Example: Your 2026/27 return is due by 31 January 2028. Keep records until at least 31 January 2033.

If you file late, keep records for 15 months after you actually submit the return.

Digital records for Making Tax Digital

From April 2026, sole traders and landlords with qualifying income over £50,000 must keep digital records using MTD-compatible software and submit quarterly updates. Paper records alone will no longer be sufficient if MTD applies to you.

Common Self Assessment mistakes

These errors cause delays, trigger HMRC enquiries, or result in penalties. Avoid them by checking your return carefully before submitting.

• Wrong UTR or National Insurance number: Transposing digits delays processing. Double-check both numbers against your HMRC correspondence
• Claiming non-allowable expenses: Client entertaining, everyday clothing, commuting costs, and fines cannot be deducted. HMRC routinely queries disproportionate expense claims
• Missing trading income: All income from self-employment must be declared, including cash payments, bartering, and income from online platforms. HMRC receives data from payment processors and can cross-reference
• Forgetting bank interest and dividends: Even if tax has been deducted at source or the amount is within your allowance, you must still report it on your return
• Incorrect student loan plan type: Plan 1 (started before September 2012), Plan 2 (started September 2012 onwards), Plan 4 (Scotland), and Plan 5 (postgraduate) have different repayment thresholds. Selecting the wrong one means you pay too much or too little
• Class 2 National Insurance errors: Self-employed people with profits above the Small Profits Threshold (£6,845 for 2026/27) are treated as having paid Class 2 NICs. This is calculated automatically on your return, but check the figure is included
• Mixing personal and business expenses: Only the business portion of mixed-use items is allowable. Keep a clear log for items like phone bills and vehicle use
• Not declaring cryptocurrency and digital asset disposals: Profits from selling, swapping, or gifting crypto assets are subject to Capital Gains Tax and must be reported

Penalties for late filing and payment

Miss the 31 January deadline and penalties escalate quickly.

Late filing penalties

• 1 day late: £100 automatic penalty (applies even if you owe no tax)
• 3 months late: £10 per day for up to 90 days (maximum £900 additional penalty)
• 6 months late: 5% of tax due or £300, whichever is greater
• 12 months late: Further 5% of tax due or £300, whichever is greater. In serious cases (deliberate withholding), up to 100% of tax due

Late payment penalties

• 30 days late: 5% of tax unpaid
• 6 months late: Additional 5% of tax still unpaid
• 12 months late: Further 5% of tax still unpaid

Total exposure: Filing 12+ months late with unpaid tax could cost £1,600+ in fixed penalties, plus up to 15% of the tax owed in payment surcharges, plus interest.

Interest: HMRC charges interest on late payments from the due date. The rate is set at the Bank of England base rate plus 4% and changes when the base rate moves.

Penalties for errors

If HMRC discovers errors in your return, penalties depend on behaviour:

• Careless errors: Up to 30% of the additional tax due
• Deliberate errors: 20-70% of the additional tax due
• Deliberate and concealed: 30-100% of the additional tax due

Voluntarily disclosing and correcting an error reduces the penalty. HMRC treats prompted disclosures (after they have opened an enquiry) more severely than unprompted ones.

Time to pay

If you cannot pay your full bill, contact HMRC before the deadline to arrange a Time to Pay plan. HMRC is more willing to agree a plan if you communicate early. Interest still accrues, but late payment penalties may be deferred.

Amendments and corrections

If you spot an error after filing, you have a 12-month window from the 31 January filing deadline to amend your return online.

Example: For your 2026/27 return (deadline 31 January 2028), you can amend online until 31 January 2029.

After the amendment window closes, you must write to HMRC to claim overpayment relief. There is no online route.

HMRC discovery assessments

Even after your amendment window closes, HMRC can open a discovery assessment if they believe insufficient tax was paid:

• Standard window: 4 years after the end of the tax year
• Careless errors: 6 years
• Deliberate errors: 20 years

Correcting genuine errors voluntarily is not penalised. However, if HMRC discovers the error first, penalties of up to 100% of the additional tax may apply.

Making Tax Digital for Income Tax

MTD for Income Tax Self Assessment is being introduced in phases, replacing the traditional annual return with quarterly digital reporting for affected taxpayers:

• April 2026: Mandatory for sole traders and landlords with qualifying income over £50,000
• April 2027: Extended to qualifying income over £30,000
• April 2028: Extended to qualifying income over £20,000

Qualifying income means combined gross income from self-employment and property before expenses. It does not include employment income, pensions, or savings.

If MTD applies to you, you must use MTD-compatible software to keep digital records and submit quarterly updates within one month of each quarter end. A final declaration by 31 January replaces the traditional tax return.

If your income is below the threshold, you continue using standard Self Assessment until you exceed it.