Filing First Self Assessment SA100

How to file your first UK Self Assessment tax return

Filing your first Self Assessment tax return (officially known as Form SA100) is a mandatory step if you earn untaxed income in the UK. Whether you have started freelancing, launched a sole trader business, let out a rental property, or earned dividend income, HMRC requires you to report your earnings for the tax year (6 April to 5 April).

The process requires registering with HMRC to get a 10-digit Unique Taxpayer Reference (UTR), gathering your financial records, selecting the right supplementary schedules, and submitting online by 31 January.

Step 1: Register with HMRC and get your UTR number

Before you can file Form SA100, you must register for Self Assessment with HMRC. The statutory deadline to register is 5 October following the end of the tax year in which you earned untaxed income.

• Sole traders: Register for Self Assessment and Class 4 National Insurance online via your GOV.UK account.
• Landlords and others who are not self-employed: Register using Form SA1 if you receive untaxed income, such as rent.
• Company directors: Register using Form SA1 if you receive income that is not taxed through PAYE, such as dividends above the £500 dividend allowance.

Once registered, HMRC posts your 10-digit Unique Taxpayer Reference (UTR). Keep this number secure as it is required on every tax return and HMRC payment.

Step 2: Understand Form SA100 and supplementary pages

Your Self Assessment return consists of the core SA100 main form plus specific supplementary pages depending on your income sources:

Step 3: Gather your records before starting

Do not start filling in your return without your complete paper or digital evidence pack. You will need:

• P60 certificate: From your employer showing total pay and tax deducted for the tax year.
• P45 document: If you left a job during the tax year.
• P11D form: Showing taxable company benefits (e.g. medical insurance or company cars).
• Sales and expense records: Invoices, bank statements, and expense receipts if you are self-employed.
• Rental income records: Letting agent statements, mortgage interest certificates, and maintenance receipts for landlords.
• Dividend vouchers: Showing net dividend payments and company details.

Step 4: Complete and submit online by 31 January

Online filing gives you an instant tax calculation. You can file from 6 April after the tax year ends. The online filing deadline and payment deadline are 31 January following the end of the tax year. Paper returns must reach HMRC by 31 October.

GOV.UK lists these late filing penalties:

• £100 immediately, even if no tax is due.
• After 3 months: £10 a day, up to £900.
• After 6 months: a further 5% of the tax due or £300, whichever is greater.
• After 12 months: another 5% of the tax due or £300, whichever is greater.

How to do a Self Assessment tax return online

1. Sign in to HMRC online services with your UTR and sign-in details. Use GOV.UK: sign in to file your Self Assessment.
2. You may need to prove your identity with photo ID, such as a passport or driving licence.
3. If you filed before but stopped, reactivate your account first.
4. Choose the tax year you want to file for.
5. Answer the questions about your income. These add the right supplementary pages to your return.
6. Enter your income, expenses and other figures.
7. You do not have to finish in one go. You can save your return and come back to it.
8. Check the tax calculation.
9. Submit your tax return and keep the submission reference.

HMRC's free online service cannot be used by partnerships, trusts, estates or non-residents. They need HMRC-recognised commercial software or paper forms.

Filing a tax return when you are employed and self-employed

One tax return covers both your employment and self-employment. Put your PAYE pay and tax from your P60 (or P45) on the employment page SA102. Put your self-employed profit on SA103.

HMRC adds the income together, applies one personal allowance and deducts tax already paid through PAYE. Class 4 National Insurance applies only to your self-employed profit. Use our employed and self-employed tax calculator to estimate your tax.

HMRC Self Assessment postal address

For paper returns and letters, use:

Self Assessment
HM Revenue and Customs
BX9 1AS
United Kingdom

Check GOV.UK for courier addresses. Most people file online and do not need to post anything.

Making Tax Digital for Income Tax

From 6 April 2026, sole traders and landlords with qualifying income over £50,000 must keep digital records and send quarterly updates through compatible software. They still make a final declaration after the tax year. See who MTD for Income Tax applies to.

What happens after you submit Form SA100?

1. Tax Calculation: HMRC calculates your total Income Tax and Class 4 National Insurance, deducting any tax already paid under PAYE.
2. Payment on Account Check: If your tax bill exceeds £1,000 and less than 80% was collected at source, HMRC will add a 50% advance payment on account towards the next tax year.
3. Payment Deadline: Pay your balancing tax bill and first payment on account by 31 January.

Read our guide on Self Assessment deadlines and penalties or see how our Self Assessment tax return service prepares and files your return accurately.