Filing First Self Assessment SA100
Step-by-step guide to filing your first UK Self Assessment return (Form SA100): UTR registration, supplementary pages, records, and 31 January deadline.
In one sentence
Form SA100 is the main UK Self Assessment tax return form used alongside supplementary schedules to report untaxed income by 31 January.
Quick answer
- Register by 5 October to receive your 10-digit UTR number
- Form SA100 core return plus supplementary pages (SA102 employment, SA103 self-employment, SA105 property)
- Online filing and payment deadline is 31 January
- Automatic £100 penalty applies for late filing even if no tax is owed
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 high earners: Register using Form SA1 if you are not self-employed but receive untaxed income.
- Company Directors: Register using Form SA1 if you receive untaxed director dividends or salary above statutory thresholds.
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:
| Form code | Form title | Who needs to complete it? |
|---|---|---|
| SA100 | Main Tax Return | Everyone filing a Self Assessment return (interest, pensions, Gift Aid) |
| SA102 | Employment | If you had a PAYE job or company directorship during the tax year |
| SA103S / SA103F | Self-Employment | Sole traders (Short form for turnover under £90,000; Full form over) |
| SA105 | UK Property | Residential or commercial landlords receiving rental income |
| SA108 | Capital Gains Tax | If you sold property, shares, crypto, or business assets over allowances |
| SA109 | Residence & Remittance | If you are non-UK resident, dual resident, or claim remittance basis |
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 is faster and gives an instant tax calculation. The deadline for online submissions and tax payments is Midnight on 31 January following the end of the tax year.
Paper returns must reach HMRC by 31 October, three months earlier. If you miss the 31 January online deadline, HMRC issues an automatic £100 late filing penalty, even if you owe zero tax.
What happens after you submit Form SA100?
- Tax Calculation: HMRC calculates your total Income Tax and Class 4 National Insurance, deducting any tax already paid under PAYE.
- 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.
- 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.
Filing your first tax return?
Book a 20-minute call with an experienced UK accountant. We will review your income sources, check allowable expenses, and ensure your SA100 return is filed correctly.
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