Quick answer
Before you file Self Assessment, gather your UTR, National Insurance number, income and expense records, bank interest and dividend figures, pension and Gift Aid details, and your Government Gateway login. Having them ready makes filing quick and accurate.
Updated October 2026 · Figures based on 2026/27 rates
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Key dates to put in your diary
The tax year runs from 6 April to 5 April. If you need to register for Self Assessment, the deadline is 5 October after the end of the tax year. A paper return is due by 31 October, while the online return and the payment of any tax due are both due by 31 January. If you have to make payments on account, the second one is due on 31 July. Missing these dates is what triggers penalties, so it helps to work backwards: aim to have your records ready by the end of November and file well before Christmas. See GOV.UK’s Self Assessment tax returns guide for the official deadlines, and our explanation of the SA100 form to understand what the return asks for.
Getting access sorted early
Many last-minute problems are not about tax at all. They are about access. Check that you have your Unique Taxpayer Reference, your National Insurance number and a working Government Gateway login. If you are new to Self Assessment, activation codes are sent by post and can take time to arrive. Our guide on setting up a Government Gateway account covers what you need.
Worked example: pulling the figures together
This is an illustrative example for a freelancer in England with no other income apart from a little bank interest. Assume nothing has been collected at source and there are no earlier payments on account.
| Item | Working | Amount |
|---|---|---|
| Turnover | Sales invoiced in the tax year | £46,000 |
| Allowable expenses | Software, travel, insurance and so on | £6,200 |
| Taxable profit | £46,000 – £6,200 | £39,800 |
| Income tax | (£39,800 – £12,570) x 20% | £5,446.00 |
| Class 4 NI | (£39,800 – £12,570) x 6% | £1,633.80 |
| Total bill | £7,079.80 |
The £400 of bank interest is covered by the £1,000 Personal Savings Allowance for a basic-rate taxpayer, so no extra tax arises, but it should still be declared if it is above the amount HMRC already knows about. Because the bill is over £1,000 and nothing was collected at source, payments on account start. Each one is half the bill, which is £3,539.90. On the first 31 January the cash needed is therefore £7,079.80 plus £3,539.90, or £10,619.70, with a further £3,539.90 due on 31 July. Many first-time filers are caught out by this, so plan for it.
Common mistakes
- Leaving bank statements, invoices and receipts until January.
- Forgetting small income sources such as savings interest, dividends or occasional freelance work.
- Mixing personal and business spending, which makes expenses hard to evidence.
- Not budgeting for payments on account.
- Entering figures for the wrong tax year, especially for those with a year end other than 5 April.
- Throwing records away too soon. Our note on how long to keep tax records explains the five-year rule after the 31 January deadline.
A last pre-filing checklist
- Confirm you are registered and can log in.
- Total your income for the year, by source.
- Total your allowable expenses, and note any capital purchases.
- Gather pension, Gift Aid and student loan details where they apply.
- Check any tax already deducted, such as through PAYE or the Construction Industry Scheme.
- Work out how much to set aside for the tax bill.
How SATR can help
If you would rather hand the job over, our qualified UK accountants prepare and file your return online for a fixed fee from £199 + VAT. See our Self Assessment service for sole traders.
This is general information, not personal tax advice.
Organising records as you go
The easiest return is the one where records were kept all year. A simple routine is enough. Keep a separate bank account for business money if you can, save digital copies of every invoice and receipt in a labelled folder, and spend ten minutes each month updating a spreadsheet or bookkeeping app with income and expenses. At the end of the year you then only need to add up the totals. If you are weighing up whether to use software or hire someone, our comparison of accountants and bookkeepers explains who does what.
Look ahead to Making Tax Digital
From 6 April 2026, Making Tax Digital for Income Tax applies to self-employed people and landlords with qualifying income over £50,000, with the threshold falling to £30,000 from April 2027 and £20,000 from April 2028. Good digital records now make that change far easier, so check current HMRC guidance to see whether and when it applies to you.
Frequently asked questions
How long should I keep my records?
Keep records for at least 5 years after the 31 January filing deadline if you are self-employed or have rental income.
What if I am missing a document?
Tell your accountant early. Many figures can be recovered from bank statements or HMRC’s online account.
What if I do not have all my records by January?
File with the best information you have, marking any figures that are provisional, and correct them later. You can amend a return within 12 months of the 31 January deadline. It is usually better than filing late.
Do I need to include small amounts of interest?
Yes, if you are filing a return you should report taxable interest. Whether any tax is due depends on your Personal Savings Allowance and starting rate band, so check current HMRC guidance.
Can I file before 31 January?
Yes. You can file any time after the tax year ends on 5 April. Filing early gives you more time to plan the payment, although the payment is not due until 31 January.
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The Bottom Line
Filing Self Assessment is straightforward when your records are in order: gather your UTR, income and expense figures, and logins early, file well before 31 January, and budget for payments on account so the first bill does not catch you out. The checklist above covers everything HMRC will ask for.
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