Self Assessment is how HMRC collects Income Tax from people who are not fully taxed through PAYE. If you are self-employed, you tell them what came in and what you spent. They work out the bill. You pay it.
That is the whole loop. The rest is dates, a form, and a few traps that catch people the first year.
Who has to file
You send a return if HMRC has asked you to. You also send one if you were self-employed as a sole trader and earned more than £1,000 before expenses in the tax year. Partnerships, some landlords, and a handful of other cases trigger a return too.
If you are not even sure you should be in the system, start with do I need to register as self-employed. Registering and filing are different jobs. You cannot finish a return until you have a UTR.
Once HMRC has you in Self Assessment, they expect a return every year until you tell them you have stopped. A quiet year still needs a return.
The tax year and the return are not the same thing
A tax year runs from 6 April to 5 April. You cannot file for that year until 6 April, because the year is not over yet.
The 2025/26 tax year (6 April 2025 to 5 April 2026) is the one you file during 2026/27. Online deadline: 11:59pm on 31 January 2027. Payment for that year is due at the same moment.
Paper has to arrive by 31 October 2026. File online unless you have a reason not to.
The dates that actually matter
For the 2025/26 return:
- 6 April 2026. You can start the return.
- 5 October 2026. Last day to tell HMRC you need a return if you are new, or if you were registered before but did not need one for 2024/25.
- 31 October 2026. Paper deadline.
- 30 December 2026. File online by this date if you want a bill under £3,000 collected through a PAYE tax code.
- 31 January 2027. Online return and the tax itself.
- 31 July 2027. Second payment on account, if you make them.
The deadline calendar keeps the full list, including penalties.
If you register after 5 October, HMRC will give you a later filing deadline, usually three months from the date on their letter. The payment date does not move. You still have to pay by 31 January.
What you put on the return
You are reporting figures, not writing an essay.
For self-employment that means turnover (everything that came in), allowable expenses, and the profit that is left. Online, those sit on the self-employment pages. On paper they are the SA103 pages attached to the SA100.
If you also have a job, you enter the employment income and the tax already taken from your payslip. HMRC adds the two together and credits the PAYE already paid. Registering when you already have a job covers how that interaction works.
National Insurance is confirmed on the same return. Class 4 is a percentage of profit and is collected with the bill. For 2026/27, Class 2 is treated as paid if your profit is £7,105 or more; below that you can pay it voluntarily to protect your state pension record. It is not a separate invoice during the year.
You do not attach receipts. You do need to produce records if HMRC asks.
How you file
File online on GOV.UK. You need a Government Gateway user ID and your UTR. If this is your first return, register first and wait for the UTR to arrive in the post. Registering in mid-January and hoping to file by the 31st is how people miss the deadline waiting for a letter.
You do not have to finish it in one sitting. Save and come back.
The first time you sign in, HMRC may ask you to prove your identity, usually with a passport or driving licence.
If Making Tax Digital for Income Tax applies to you, the quarterly updates during the year do not replace this. You still make a final declaration by 31 January. Check whether MTD applies.
Then you pay
The online service calculates the bill when you submit. Pay by 11:59pm on 31 January. Faster Payments from your bank is the usual route. You can also pay in the HMRC app.
If you have a PAYE job, you owe less than £3,000, and you filed online by 30 December, HMRC will usually collect it through your tax code over the next year unless you opt out. You cannot part-pay the bill down to £2,999 to sneak under that line.
If the bill is more than £1,000, January also includes the first payment on account towards next year. That is the thing that makes a first January feel like one and a half bills. How much tax to save is the habit that pays for it.
You can estimate the bill before you file.
Records, and how long to keep them
A list of what came in and what went out is enough for most sole traders, backed by bank statements, invoices and receipts. From 2024/25, cash basis is the default: you record money when it actually arrives or leaves, not when you raised the invoice.
Keep the records for five years after the 31 January deadline for that tax year. For the 2025/26 return that is until 31 January 2032.
What you can claim is in what expenses can I claim.
If you are already late
File today. Pay what you can. The penalty ladder starts the day after the deadline, and waiting adds cost. If the problem is the money rather than the form, read what to do if you cannot afford the bill.