A calm Self Assessment checklist for first-timers: UTR, Government Gateway, income records, expenses, pension, and donations, so your first return is painless.
The first Self Assessment is the one that feels scary. After that it is just admin. Almost all of the stress comes from sitting down to file and realising you are missing a number, a login, or a document, then scrambling for it at the worst possible moment. The fix is boring but effective: gather everything first, then file. Here is the checklist of what you need before you open the return, so your first time through is calm rather than frantic.
A quick note on timing. For the 2025/26 tax year, the online deadline is 31 January 2027 and the paper deadline is 31 October 2026. Most first-timers file online, because it gives you three extra months and works out the tax for you as you go. This checklist assumes you are filing online.
The UTR is your ten-digit Self Assessment reference, and you cannot file without it. If you registered for Self Assessment, HMRC will have sent it to you, and it also appears in your personal tax account and on previous HMRC letters. If you have only just registered, the UTR can take around ten working days to arrive, so check you actually have it well before deadline day. No UTR means no filing, so this is the first thing to confirm.
You file through your HMRC online account, which needs a Government Gateway user ID and password. First-timers often have the UTR but have never set up or activated the online account, so do this early. If you have an account but have not used it in a while, log in now to check the password still works and that you can get through the identity checks, which sometimes ask for details from a passport, payslip, or P60.
You will need your National Insurance number to hand, both for logging in and for the return itself if you are self-employed and paying Class 2 or Class 4 National Insurance. It is on your payslip, your P60, or in the HMRC app.
If you had a normal job alongside your self-employed or other income, HMRC wants to see that employment income too, so your return shows the full picture. Your P60 shows your pay and tax for the year from an ongoing job, and a P45 covers a job you left during the year. Having these means you can enter your employment income and the tax already deducted accurately, so you are not taxed twice on money that PAYE already handled.
Add up everything you earned from the work you are declaring. For self-employment that means your total gross income for the tax year, before expenses. Pull it from your invoices, your bank statements, or the summaries that platforms like delivery apps or marketplaces provide. If you had other income to declare, such as rent, dividends, or untaxed interest, gather those totals too. Accuracy here matters more than anything, because this is the figure the tax is based on.
This is where people either save money or leave it on the table. You can deduct costs that are wholly and exclusively for your work, which lowers the profit you pay tax on. Common ones include tools and materials, business mileage, a reasonable share of home running costs if you work from home, phone and internet use, professional fees and insurance, and equipment. Gather your receipts and totals now. If your expenses are small and simple, the £1,000 trading allowance might be worth more than claiming actual costs, so it is worth comparing the two.
If you pay into a personal pension, you may be able to claim higher-rate relief through your return, so have the totals ready. This does not apply to contributions already taken from a workplace salary before tax, but personal pension payments you made from taxed income can reduce your bill if you are a higher-rate taxpayer. It is a commonly missed box that can be worth real money.
Charity donations made under Gift Aid work the same way. Basic-rate relief is handled at the point of donation, but higher-rate taxpayers can claim the extra relief through Self Assessment. If you gave regularly to charity under Gift Aid, add up the total for the year and have it ready for the relevant box.
The single best habit for first-timers is to keep all eight of these in one place, whether that is a physical folder or a folder on your phone. When everything is together, the actual return usually takes under an hour for simple affairs, because you are copying numbers across rather than hunting for them. It also means that if HMRC ever asks you to back up a figure, you can, which is the whole point of keeping records.
First-timers often freeze because they are terrified of making a mistake. In reality, an honest error is not a crisis. If you realise after filing that you got something wrong, you can amend your return up to twelve months after the deadline. HMRC reserves its real penalties for people who do not file, pay late, or deliberately hide income, not for someone who transposed a figure and fixed it. File to the best of your knowledge, keep your records, and correct anything you spot later.
If your situation is simple, one stream of self-employed income and some straightforward costs, most people can file themselves without paying for help. The online return guides you box by box and calculates the tax for you. An accountant earns their fee once things get more involved, for example if you run a limited company, have large or unusual expenses, deal with capital gains, or simply value the time saved and the reassurance. For a first, simple return, doing it yourself is a perfectly reasonable choice.
Two dates matter for the 2025/26 tax year. The paper deadline is 31 October 2026 and the online deadline is 31 January 2027. The same 31 January date is also when any tax you owe must be paid, so filing early does not bring the payment forward, it simply tells you sooner what the bill will be. That is actually an argument for filing in good time: the earlier you file, the longer you have to set the money aside before you have to hand it over.
First-timers are sometimes caught out by payments on account. If your Self Assessment bill is over £1,000 and most of your tax is not already collected through PAYE, HMRC asks you to pay towards next year’s bill in advance, in two instalments. In practice that can mean your first January bill is larger than you expected, because it includes both the tax for the year just gone and a payment on account towards the next one. It is not a penalty and it is not double taxation, it is prepayment, but it is a genuine cash-flow surprise if nobody warned you. Knowing it is coming is half the battle.
The simplest way to avoid a nasty January is to put money aside as you earn. A rough rule for many basic-rate sole traders is to set aside somewhere between a fifth and a third of profit for tax and National Insurance, in a separate pot you do not touch. When the bill arrives, the money is already there. Estimating your bill as you go, rather than waiting for the return to tell you, turns Self Assessment from a shock into a formality.
If you want a sense of what you will owe before you file, the NetPay app helps you estimate the tax on income your payslip does not cover, so you can set the money aside in good time. Free to download.
See your real take-home pay in seconds
NetPay does this maths for every shift, invoice and payslip, automatically.
Download the app now
You need your UTR, a Government Gateway login, your National Insurance number, records of all your income including any P60 or P45 from employment, records of self-employed income and allowable expenses, and details of pension contributions and charity donations. Gather these before you start and the return itself is quick.
For the 2025/26 tax year, the online filing and payment deadline is 31 January 2027. The paper deadline is earlier, on 31 October 2026. Most first-timers file online because it gives more time and calculates the tax for you.
You can claim costs that are wholly and exclusively for your self-employed work, such as tools, materials, business mileage, a share of home costs if you work from home, professional fees, and equipment. Keep receipts and records to back up every figure you claim.
Not necessarily. If your affairs are simple, one source of self-employed income and some straightforward expenses, most people can file themselves. An accountant is worth considering once things get more complex, such as a limited company, large expenses, or capital gains.
Want to see your actual take-home pay?
NetPay UK works out your real net pay after tax, NI, pension and salary sacrifice, for hourly, shift and variable-income workers. Free to download.
Download the app now
A note on financial advice: NetPay UK calculates take-home pay based on official HMRC tax rules. This article reflects rules in force at the time of publication (6 October 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
