If your only income is a salary or a pension, your tax is usually sorted for you. HMRC collects it automatically through PAYE, and you never see a tax return. So when do you actually have to fill one in?
That's the question this guide answers. We'll walk through every common trigger, the figures that matter for the 2025/26 tax year, and a few illustrative examples so you can see where you'd land.
This is for anyone with a side income, a rental property, dividends, a higher salary, or a nagging feeling they've forgotten something. By the end you'll know whether you need to register, and what the deadlines are if you do.
What is Self Assessment, in plain English?
Self Assessment is the system HMRC uses to collect Income Tax from people whose tax isn't fully handled at source.
If you're an employee, your employer deducts Income Tax and National Insurance from your wages before you're paid. That's PAYE (Pay As You Earn). Pensioners are usually taxed the same way. For most people, that's the end of it.
But some income arrives with no tax taken off. Profit from self-employment, rent from a property, dividends from shares you own. When that happens, HMRC needs you to declare it yourself and pay the tax due. You do that by completing a tax return, either online or on paper.
The phrase "Self Assessment" just means you (not your employer) are responsible for telling HMRC what you earned and working out the tax.
Who must send a Self Assessment tax return?
For the 2025/26 tax year (6 April 2025 to 5 April 2026), you must send a return if any of the following applied:
- You were self-employed as a sole trader and your gross income was more than £1,000 (before deducting expenses).
- You were a partner in a business partnership.
- You had to pay the High Income Child Benefit Charge (more on the £60,000 threshold below) and aren't paying it through your tax code.
- You had to pay Capital Gains Tax, for example on selling a second property or shares.
- You were an off-payroll worker repaying a student or postgraduate loan under the off-payroll rules.
You might also need to file if you had untaxed income that HMRC can't collect another way, such as:
- Rental income from property.
- Tips, commission or other untaxed earnings.
- Savings, investment or dividend income above your allowances.
- Foreign income that's taxable in the UK.
And remember the catch-all: if HMRC sends you a notice to file, you must complete a return, even if you think you owe nothing. You can ask them to withdraw it if it genuinely doesn't apply, but you can't simply ignore it.
Do I need to file if I'm employed under PAYE?
Often, no. If a salary is your only income, PAYE usually settles your tax in full and you have nothing to file.
But being employed doesn't make you exempt. PAYE can't see income your employer doesn't pay you. So you can be a full-time employee and still need a return because of something on the side.
Common reasons an employed person ends up in Self Assessment:
- A freelance or side-hustle income over the £1,000 trading allowance.
- Rental income from a let property.
- Dividends above the £500 dividend allowance for 2025/26.
- The High Income Child Benefit Charge.
The point is simple. PAYE handles your job. Anything outside your job may still need declaring.
The complete Self Assessment checklist
Work through these questions for the 2025/26 tax year. A "yes" to any one of them means you almost certainly need to file.
| Did this apply in 2025/26? | Need to file? |
|---|---|
| Self-employed with gross trading income over £1,000 | Yes |
| Partner in a business partnership | Yes |
| Rental income that isn't covered by an allowance | Usually yes |
| Untaxed savings, dividends or investment income above your allowances | Usually yes |
| Capital Gains Tax to pay (e.g. sold a second property or shares) | Yes |
| You or your partner earn over £60,000 and claim Child Benefit (the HICBC) | Yes, unless paid via PAYE |
| Foreign income taxable in the UK | Usually yes |
| HMRC sent you a notice to file | Yes |
A few of these have figures worth pinning down for 2025/26:
- Trading allowance: the first £1,000 of gross trading or miscellaneous income is tax-free. Below that, you generally don't need to tell HMRC or file.
- Property allowance: there's a separate £1,000 tax-free allowance for property income, working the same way.
- Dividend allowance: £500 for 2025/26. Dividends above this are taxable and may push you into Self Assessment.
- High Income Child Benefit Charge: for 2024/25 onwards, this kicks in once the higher earner in the household has adjusted net income over £60,000, and claws back all the Child Benefit at £80,000.
How does the £1,000 trading allowance work?
The trading allowance is the line that decides whether a casual side income even reaches HMRC's radar.
If your gross self-employed or casual income for the year is £1,000 or less, you usually don't have to register for Self Assessment or report it at all. That covers a lot of small side hustles, the odd bit of freelance work, selling handmade goods, a small content income.
Cross £1,000 of gross income and the picture changes. You need to register for Self Assessment and declare it. At that point you choose, each year, between two options:
- Claim the £1,000 trading allowance and pay tax on the rest, but you can't also deduct your actual expenses.
- Ignore the allowance and deduct your real business costs instead.
You pick whichever leaves you better off. If your expenses are under £1,000, the allowance usually wins. If they're higher, deducting actual costs usually does. Our self-employed tax calculator gives you a quick estimate of the tax due once you're over the threshold.
One trap to watch: the £1,000 test is on gross income, not profit. Earn £1,500 and spend £900 on materials, and you're still over the threshold even though your profit is only £600. You still need to file.
If you're building something more serious, our accounting for sole traders page explains how we handle the bookkeeping and the return for you.
Worked examples: do these people need to file?
Illustrative example 1: Aisha, employed with a small side hustle
Aisha earns £38,000 in a salaried job, all taxed under PAYE. On weekends she sells illustrations online and made £850 gross from it in 2025/26.
Her side income is under the £1,000 trading allowance, so she doesn't need to register or report it. Her PAYE settles the rest. No return needed.
Illustrative example 2: Tom, content creator over the allowance
Tom has a part-time job (PAYE) and also earns money from brand deals and platform payouts. His gross creator income for 2025/26 was £6,200.
That's well over £1,000, so Tom must register for Self Assessment and declare it. He'll compare the £1,000 trading allowance against his real costs (editing software, kit, a share of his phone bill) and use whichever is higher. Our guide for TikTok and content creators covers exactly this.
Illustrative example 3: Priya, an accidental landlord
Priya kept her old flat and now rents it out, receiving £11,400 in rent during 2025/26. Her only other income is her salary.
Rental income above the £1,000 property allowance is taxable, so Priya needs to file and report the rent (less allowable costs like letting fees and certain finance costs). Our accounting for landlords page sets out how property income is taxed. Return needed.
Illustrative example 4: Daniel, higher earner claiming Child Benefit
Daniel earns £67,000 under PAYE and his family claims Child Benefit. Because his adjusted net income is over £60,000 for 2025/26, the High Income Child Benefit Charge applies.
Unless he arranges to pay it through his tax code, Daniel needs to register for Self Assessment to report and pay the charge. Return needed.
What are the Self Assessment deadlines?
If you do need to file, the dates below apply to the 2025/26 tax year. Miss them and HMRC charges automatic penalties and interest, so they're worth diarising.
| Deadline | Date |
|---|---|
| Register for Self Assessment (if you're new to it) | 5 October following the end of the tax year |
| Paper tax return | Midnight 31 October following the tax year |
| Online tax return | Midnight 31 January following the tax year |
| Pay the tax you owe (balancing payment) | Midnight 31 January following the tax year |
| Payments on account (if you make them) | 31 January and 31 July |
So for the 2025/26 tax year, you'd register by 5 October 2026, file online by 31 January 2027, and pay any tax due by the same January date.
Want this handled properly and on time? Zmartly's Self Assessment service registers you, prepares the return, and files it, so you don't have to wrestle with the HMRC portal. Book a free call and we'll tell you in minutes whether you need to file at all.
Frequently asked questions
Do I need to do a Self Assessment if I only have a salary?
Usually no. If a salary is your only income, PAYE collects your tax automatically and you don't need to file. You'd only need a return if you also have untaxed income, a reason like the High Income Child Benefit Charge, or HMRC sends you a notice to file.
How much can I earn from a side hustle before I have to file?
For 2025/26, you can earn up to £1,000 of gross trading income tax-free under the trading allowance, with no need to register or report it. Once your gross income from self-employment or casual work goes over £1,000, you must register for Self Assessment and declare it.
Do I have to file just because I earn over £100,000?
Not automatically. HMRC removed the rule that you must file simply because your income passes a set high-income figure. Whether you need a return depends on the actual triggers, such as untaxed income, the loss of your personal allowance creating an underpayment, or HMRC issuing a notice. If you're unsure at this income level, it's worth checking with an accountant.
What happens if I don't file when I should have?
HMRC charges an automatic penalty for filing a return late, even if you owe no tax, plus further penalties and interest the longer it goes unpaid. If you've missed a return or aren't sure whether you needed one, the sooner you sort it the smaller the bill.
Can I file a Self Assessment voluntarily?
Yes. Some people file even when they don't strictly have to, for example to claim certain tax reliefs, to pay voluntary National Insurance, or to evidence self-employed income for benefits like Maternity Allowance or Tax-Free Childcare.
