HMRC Self Assessment explained for 2026: who has to file, the deadlines that matter, how your bill is worked out, and how to register.
Self Assessment has a reputation it mostly deserves. The language is dense, the deadlines are scattered across the calendar, and the bill often lands larger than people expect because of a rule nobody explains until it bites.
None of it is complicated once you see the shape of the thing. HMRC Self Assessment is a reporting system, not a separate tax. Here is how it works, what you will owe, and the dates that actually matter this year.
It is the system HMRC uses to collect income tax from money that was not already taxed before it reached you.
If you are employed, your employer takes tax off through PAYE before you are paid, so HMRC already has its money and you never think about it. If you are self-employed, or you have rental income, or dividends, or a side business, nobody has taken anything off. Self Assessment is how you report that income and settle up once a year.
That is the whole concept. You are not being taxed twice, and you are not being singled out. You are just doing the job your employer would otherwise do for you.
You must send a return if, in the last tax year, any of these applied:
You may also need to file if you had untaxed income from renting out property, tips and commission, savings interest, dividends, or foreign income.
That £1,000 figure is the one people get wrong most often. It is turnover, not profit. If you invoiced £1,400 doing weekend photography and spent £900 on kit, you are over the threshold even though you only cleared £500. It is also a per-person figure across all your self-employed work, not per client and not per side project.
If you genuinely cannot tell, HMRC has a checker tool that walks through it in about two minutes.
Two separate charges come out of the same profit figure, which is why the total tends to surprise people who only budgeted for income tax.
First you work out profit: income minus allowable expenses. Then income tax applies to profit above your personal allowance, and Class 4 National Insurance applies on top.
For the 2026/27 tax year the numbers are:
| Charge | Threshold | Rate |
|---|---|---|
| Income tax | £12,571 to £50,270 | 20% |
| Income tax | £50,271 to £125,140 | 40% |
| Class 4 NI | £12,570 to £50,270 | 6% |
| Class 4 NI | Above £50,270 | 2% |
Worked example. You are a freelance designer. You invoice £38,000 across the year and have £6,000 of allowable expenses, so your profit is £32,000.
That is an effective rate of about 15.8% on the full £32,000, which is lower than most people fear. The sting is not the rate. It is the timing.
If your bill is more than £1,000 and less than 80% of your tax was collected at source, HMRC asks you to pay towards next year’s bill in advance, in two instalments.
So our designer does not pay £5,051.80 on 31 January. They pay the £5,051.80 plus half of it again as an advance payment, which is £7,577.70 in one go. Then another £2,525.90 lands on 31 July.
This is the single most common reason a first Self Assessment bill feels like an ambush. It is not a penalty and it is not an error. You are paying one and a half years of tax in your first January because you are moving from paying in arrears to paying in advance. It evens out afterwards, but only if you knew it was coming.
Here is what I would do the moment you start earning untaxed income: move a flat 25% of every payment into a separate savings account and do not treat it as yours. For most people at this profit level that covers the tax, the National Insurance and the first payment on account without any drama.
The return you are filing now covers the 2025/26 tax year, which ran from 6 April 2025 to 5 April 2026.
Miss the filing deadline and you get an automatic £100 penalty, even if your bill is zero and even if you are owed a refund. It escalates from there.
The 5 October date is the quiet one. It is not the filing deadline, so people ignore it, but it is the deadline to register, and registering is what generates the UTR number you cannot file without.
Registering is how you tell HMRC you exist as a taxpayer outside PAYE. You do it once.
Two paper waits stacked on top of each other is why leaving registration until January goes badly. If you register in early October you have three comfortable months. If you register on 20 January you may physically not have the codes in time, and the penalty still applies.
Keep the UTR somewhere permanent. You will need it every year, and for mortgage applications, and any time you ring HMRC.
The personal allowance is still £12,570, frozen since 2021 and currently scheduled to stay frozen through April 2028. The higher rate threshold is still £50,270. Because both are held flat while earnings rise, more self-employed people cross into higher rate each year without their work changing at all.
Class 4 National Insurance stayed at 6% and 2%. Class 2 remains effectively abolished for anyone with profits above the small profits threshold, which is £7,105 for 2026/27. Below that you can still pay voluntarily at £3.65 a week to protect your State Pension record, and for many people with a low profit year that is a genuinely good deal.
The trading allowance is unchanged at £1,000, so if your side income is under that you do not need to report it at all.
If you want to know what a given month of invoices actually leaves you with after tax and National Insurance, the NetPay app tracks it as you earn rather than making you find out in January. Free to download.
Sources: HMRC Self Assessment tax returns, Self-employed National Insurance rates, Income Tax rates and Personal Allowances
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You must file if you were self-employed as a sole trader and earned more than £1,000 before expenses, if you were a partner in a business partnership, if you owed Capital Gains Tax, or if you had to pay the High Income Child Benefit Charge and do not pay it through PAYE. Untaxed income such as rent, tips, savings interest or dividends can also pull you in. HMRC has a checker tool if you are unsure.
For the 2025/26 tax year, you must register by 5 October 2026 if you have never filed before. Paper returns are due by 31 October 2026. Online returns and the payment itself are both due by 11:59pm on 31 January 2027. Missing the filing deadline triggers an automatic £100 penalty even if you owe nothing.
You pay income tax on profit above your £12,570 personal allowance, at 20% up to £50,270 and 40% above that. On top you pay Class 4 National Insurance at 6% on profit between £12,570 and £50,270, then 2% above. A £32,000 profit in 2026/27 produces roughly £5,052 in total.
A UTR is your ten digit Unique Taxpayer Reference, the number HMRC uses to identify you. You get one automatically when you register for Self Assessment, and it arrives by post within about two weeks. You cannot file a return without it, which is the main reason registering late causes problems.
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.
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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 (19 September 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
