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Self Assessment FAQ

How to fill in a Self Assessment tax return

Log in with your Government Gateway details, start the SA100, add the sections that apply to you, declare your income, claim what you are entitled to, then check and submit before 31 January.

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Filling in your return is straightforward once your records are ready, HMRC’s online system walks you through the rest. Here is the process in four steps.

  1. 1

    Gather your records

    Your UTR and Government Gateway login, plus income and expense records: self-employment figures, P60/P45s, rental income, dividends or interest.

  2. 2

    Log in and start your return

    Sign in to your HMRC online account and start the SA100, plus any supplementary pages that apply, self-employment, property, dividends and so on.

  3. 3

    Enter income and claim expenses

    Declare each income source and claim every allowable expense and relief you are entitled to, this is where an accurate return, not just a completed one, saves you money.

  4. 4

    Check the calculation and submit

    HMRC calculates your tax automatically as you go. Review the figure, submit before 31 January, and pay what is due by the same date.

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Questions

Filling in your return, answered

How do I fill in a Self Assessment tax return?
Log in to your HMRC online account with your Government Gateway details, start the SA100 form, and add any supplementary pages that apply to you, self-employment, property, dividends or foreign income. Enter your income for each source, claim the expenses and reliefs you are entitled to, then review HMRC’s automatic tax calculation before submitting. The whole process is done online and free, though many people use an accountant to make sure nothing is missed.
What information do I need before I start?
Your Unique Taxpayer Reference (UTR) and Government Gateway login, plus records of everything you need to declare: self-employment income and expenses, P60 or P45 details, rental income, dividend vouchers, bank interest, and details of any Capital Gains disposals. Having these ready before you log in makes the return much quicker to complete.
Can I complete a Self Assessment return on paper?
Yes, but the deadline is earlier, 31 October rather than 31 January for online returns, and HMRC now strongly pushes people toward filing digitally. Most people file online because it is faster, gives you longer, and calculates your tax for you as you go.
What if I make a mistake on my return?
You can amend an online return yourself within 12 months of the original filing deadline, simply log back in and correct the figures. If HMRC has already opened an enquiry, or the deadline for amending has passed, you will usually need to write to them or get help correcting it properly.
Do I need to fill in supplementary pages?
Only the ones relevant to you. If you are self-employed you add the self-employment pages; if you have rental income, the property pages; and so on for dividends, foreign income or Capital Gains. HMRC’s online system only shows you the sections that match the answers you give at the start.
Is it worth getting help completing my return?
For a simple return with one income source, most people manage fine alone. Once you add self-employment, property, dividends or Capital Gains, an accountant typically finds reliefs worth more than their fee and removes the risk of an error that triggers an HMRC enquiry.

Related: who needs to file a return · what happens if you miss the deadline

File online: File your Self Assessment tax return, GOV.UK.

Rather not fill it in yourself?

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