Before you start: what to gather
Filling in the return is far quicker when everything is in front of you first. Spend ten minutes collecting these before you sign in:
- Your Unique Taxpayer Reference, the 10 digit UTR from HMRC, and your Government Gateway user ID and password.
- Your P60, P45 and any P11D if you were also employed during the year.
- A total of your self employed income, and a list of your allowable business costs by category.
- Bank interest, dividend vouchers, and any rental income and costs if you let out property.
- Records of pension contributions, Gift Aid donations and student loan deductions.
- Details of anything that reduces the bill, such as the £1,000 trading allowance or capital allowances on equipment.
How to fill in your tax return, step by step
- Sign in and open the right year. Go to your HMRC online account, choose Self Assessment, then select the tax year you are reporting. The tax year runs from 6 April to 5 April, so the 2025/26 return covers 6 April 2025 to 5 April 2026.
- Tell HMRC which sections apply. The first screens ask what kind of income you had. Tick employment, self employment, property, and so on. Only the sections you tick will appear, so this keeps the form as short as it can be. Take care here, because missing a source is the most common reason a return is wrong.
- Enter your employment income. If you had a job, copy the pay and tax figures straight from your P60 or P45. Add taxable benefits from your P11D, such as a company car or private medical cover.
- Complete the self employment pages. Enter your total turnover, then your allowable costs. If your turnover is below £85,000 you can enter costs as a single total; above that you list them by category. Claim the £1,000 trading allowance instead of costs if your expenses were tiny, whichever leaves you better off.
- Add property income. Enter rent received and allowable costs such as letting agent fees, repairs, insurance and the 20% finance cost credit on mortgage interest. The £1,000 property allowance works the same way as the trading allowance.
- Add everything else. Put in bank interest, dividends, pension income and any other taxable income. Small amounts still count, and HMRC often already holds some of this data, so leaving it out will flag a mismatch.
- Claim your reliefs. Enter personal pension contributions, Gift Aid, and any other reliefs you are entitled to. These can pull higher rate taxpayers back below a threshold and save real money, so do not skip them.
- Check the calculation. HMRC now shows your tax and National Insurance for the year. Compare it against your own rough figure. Your first £12,570 of income is usually tax free through the personal allowance, then 20% applies up to £50,270. If the number looks wildly off, a box is almost certainly in the wrong place.
- Submit, then save the receipt. Once you are happy, submit the return. Save or print the submission receipt and the tax calculation. That receipt is your proof of filing.
- Pay what you owe. Pay your bill by 31 January. If your bill is over £1,000 you will usually also make payments on account, which are advance instalments toward next year, due 31 January and 31 July.
A worked example
Sana is a self employed graphic designer. In 2025/26 she invoiced £38,000 and had £6,000 of allowable costs, so her profit is £32,000. Her first £12,570 is covered by the personal allowance. The remaining £19,430 is taxed at 20%, giving £3,886 of income tax, plus Class 4 National Insurance on her profit above the threshold. Because her bill is over £1,000, she also makes a first payment on account toward 2026/27. Knowing this in advance, she puts money aside each month rather than facing a surprise in January.
Common mistakes that cost people money
- Forgetting a source of income. A second freelance client, a bit of interest, a few dividends. HMRC cross checks against banks and employers, so gaps get noticed.
- Guessing expenses instead of recording them. Round numbers and missing receipts are exactly what triggers questions. Claim what you can prove.
- Missing reliefs. Higher rate taxpayers who pay into a pension often overpay tax simply because they never claim the extra relief on the return.
- Leaving it to late January. The online deadline is 31 January and the penalty is £100 the moment you miss it, even if no tax is due.
Not sure a box is in the right place?
If your return has more than one income source, property, dividends or a first year of self employment, one wrong entry can cost you far more than a fixed fee. TaxTune prepares, checks and files the whole thing for you, and tells you exactly what to pay and when.
Frequently asked questions
What is the deadline to fill in a tax return?
For an online return the deadline is 31 January following the end of the tax year. A paper return is due earlier, by 31 October. You must also pay any tax you owe by 31 January. Missing the online deadline brings an automatic £100 penalty even when no tax is due.
Do I need an accountant to fill in my tax return?
You can file it yourself, and for a simple single income return that is perfectly reasonable. Once you have self employment, property, dividends or higher rate pension relief in the mix, an accountant usually saves more than the fee by claiming everything you are entitled to and avoiding penalties.
How do I fill in the self employment section?
Enter your total turnover for the year, then your allowable business costs. Below £85,000 turnover you can enter one combined costs figure; above that you list them by category. HMRC then works out the tax and Class 4 National Insurance on your profit.
What if I make a mistake after submitting?
You can amend an online return up to 12 months after the filing deadline by signing back in and correcting the figures. If it is later than that, you write to HMRC. Correcting an error early is always better than waiting for HMRC to find it.
What are payments on account?
If your tax bill is over £1,000, HMRC asks you to pay half of next year's expected bill in advance, in two instalments due 31 January and 31 July. It feels like a double bill the first year, then evens out. We explain this clearly in our payments on account guide.
How much does it cost to have TaxTune file my return?
We work on a fixed fee agreed up front, so there are no surprises. The price depends on how many income sources you have. You can see indicative prices on our Self Assessment page or ask for a quote in a two minute message.
Let us take the tax return off your plate
If reading the steps above made you think "I would rather someone just did this properly", that is exactly what we do. Send us your figures, we prepare and check the return, show you the bill before anything is filed, and submit it to HMRC on a fixed fee agreed in advance. No jargon, no January panic.