If you're self-employed, a landlord, or earn over £100,000, Self Assessment is how HMRC collects the tax you owe each year.
Miss the 31 January filing deadline by a single day and HMRC issues an automatic £100 penalty, regardless of whether you actually owe any tax. That is not a warning letter or a nudge; it is an immediate fine, and it is just the starting point if the return stays unfiled.
HMRC will not automatically tell you that you need to file. The obligation is yours to spot. The clearest trigger is self-employment with trading income above £1,000 in a tax year, but the list is longer than most people realise.
You are required to register and file if, in the 2024/25 tax year, any of the following apply:
If none of those apply and all your income is taxed through PAYE, you almost certainly do not need to file. But if you are unsure, HMRC's online checker or a quick read of recent posts on the TapTax blog can point you in the right direction.
The UK tax year runs from 6 April to 5 April. For the 2024/25 tax year (ending 5 April 2025), the key dates are:
| Deadline | Date | What it covers |
|---|---|---|
| Register for Self Assessment | 5 October 2025 | New filers only |
| Paper return | 31 October 2025 | Filed by post |
| Online return | 31 January 2026 | Filed via HMRC online or approved software |
| Tax payment due | 31 January 2026 | Balancing payment + first payment on account |
| Second payment on account | 31 July 2026 | 50% of prior year's liability |
The paper deadline is easy to miss if you assume online rules apply to everyone. If you are filing on paper, you have three fewer months.
The penalty structure is deliberately escalating, and it moves faster than most people expect.
If you are worried you are already past a deadline, use the late-filing penalty calculator to see exactly what you are facing before you contact HMRC.
HMRC does accept appeals against penalties where you have a reasonable excuse (serious illness, a bereavement, or a system failure on HMRC's own side), but "I didn't know I had to file" rarely qualifies once you have been self-employed for more than a year.
Self Assessment is not just a form; it is a calculation. HMRC uses your return to work out your total taxable income, apply the relevant allowances and reliefs, and arrive at the tax and National Insurance you owe for the year.
Take a freelance graphic designer with £38,000 of invoiced income and £6,500 of allowable business expenses in 2025/26.
| Step | Amount |
|---|---|
| Gross trading income | £38,000 |
| Less allowable expenses | £6,500 |
| Net profit (taxable) | £31,500 |
| Less Personal Allowance (2025/26) | £12,570 |
| Taxable income | £18,930 |
| Income Tax at 20% (basic rate) | £3,786 |
| Class 4 NI at 6% (profit over £12,570) | £1,136 |
| Class 2 NI (if applicable, via return) | £0 (abolished April 2024) |
| Total liability | £4,922 |
This designer would also owe a first payment on account of £2,461 (50% of the liability) on 31 January 2027, alongside the balancing payment for 2025/26. Use the sole trader tax calculator to run these figures for your own income and expense profile.
From April 2026, most sole traders and landlords with income above £50,000 will move to Making Tax Digital for Income Tax (MTD for IT). Instead of one annual return, you will submit quarterly updates of income and expenses through MTD-compatible software, followed by a final declaration.
The annual Self Assessment return as it currently exists will not disappear overnight; those below the £50,000 threshold join later (£30,000 from April 2027, with further expansion planned). But for higher-earning sole traders, the shift is real and soon.
The core principle remains the same: declare your income accurately, claim the expenses you are entitled to, and pay what you owe on time. MTD changes the rhythm and the software, not the underlying obligation.
Self Assessment is simply the mechanism HMRC uses to collect tax that cannot be taken at source; get the deadlines right and the paperwork becomes manageable.
TapTax connects to your bank, categorises expenses automatically, and submits quarterly updates to HMRC. Free plan, no card required.
