Do I Need to Do a Self Assessment? HMRC's Triggers Explained

A lot of people only find out they needed a tax return when a penalty letter arrives. The rules aren't hard, but they're spread across several GOV.UK pages, so I've pulled the triggers together in one place for anyone with a side hustle, a bit of rental income or a one-off sale.

The main reasons you must send a return

In the last tax year (6 April to 5 April), you...	Self Assessment needed?
were self-employed as a sole trader and earned more than 1,000 pounds before relief	Yes
had to pay Capital Gains Tax on something that increased in value	Yes
had to pay the High Income Child Benefit Charge and don't pay it through PAYE	Yes
are an off-payroll worker repaying a student or postgraduate loan	Yes
had other untaxed income, such as rent or some savings income	Maybe; use the checker
only earned through PAYE wages with no other income	Usually no

The 1,000 pound side hustle line

For self-employment the key number is 1,000 pounds, and it's measured before you take off anything you can claim relief on. That links to the trading allowance: if your gross trading income is 1,000 pounds or less, you may not need to tell HMRC at all. But there are cases where you must register even below it: if you made a loss and want relief for it, if you want to pay voluntary Class 2 National Insurance, or if you want to claim Tax-Free Childcare or Maternity Allowance based on your self-employment.

Reasons to register even if you don't have to

• Proving you're self-employed, for example for Tax-Free Childcare or Maternity Allowance
• Paying voluntary National Insurance contributions
• Claiming Income Tax relief on maintenance payments

These come up more than you'd think. A new parent who freelances part time may want Maternity Allowance based on self-employment, and a quiet first year in business can leave a gap in your National Insurance record that voluntary contributions fill. Both need you inside the Self Assessment system, so registering early can be worth it even when you owe nothing.

The deadline that catches new people

This is the catch. If you've never sent a return before, or you registered once but didn't need to file for the last one, you must tell HMRC by 5 October after the end of the tax year. Miss that and you could get a penalty, even before the filing deadline comes round. If you registered years ago and stopped filing, you may need to reactivate your account instead of starting again.

Self Assessment deadlines for 2025 to 2026

What	Deadline
Tell HMRC you need to file (new or returning)	5 October
Paper tax return	11:59pm on 31 October
Online return, if you want tax collected through your tax code	11:59pm on 30 December
Online tax return	11:59pm on 31 January
Pay the tax you owe	11:59pm on 31 January
Second payment on account, if you make them	31 July

You can send the return any time from 6 April, as soon as the tax year has ended. If HMRC writes asking you for a return, you must send it by the deadline in that letter, even if you think you don't owe anything.

How to check in five minutes

1. List every source of money you had between 6 April and 5 April: wages, side hustles, rent, sales, savings.
2. Add up any self-employed income before costs, across every hustle.
3. Open the GOV.UK tool 'Check if you need to send a Self Assessment tax return' and answer the questions. GOV.UK says it doesn't send your details to HMRC.
4. If it says yes and you're new, register by 5 October.
5. If it says no but HMRC already sends you returns, contact HMRC to tell them, or you may still be expected to file.

Common situations, quick answers

Situation	Likely answer
Payslip job plus 600 pounds from weekend dog walking	Covered by the trading allowance; usually no return
Payslip job plus 3,000 pounds from freelance design	Yes, you're self-employed over 1,000 pounds
Sold shares at a gain and owe Capital Gains Tax	Yes
Only a salary, all taxed through PAYE	Usually no

These are my illustrations of the GOV.UK rules, not HMRC rulings. If your case is borderline, the checker is quicker than guessing.

Renting out a room is different

If you rent a furnished room in your own home, the Rent a Room Scheme lets you earn up to 7,500 pounds a year tax-free, or 3,750 pounds if you share the income with someone else. Under that, the exemption is automatic and there's nothing to file. Over it, you must complete a tax return.

My verdict

If you earn anything outside your payslip, assume you might need a return and check it carefully once a year, in April, not in January. The 5 October registration date is the one to put in your diary. For more on the side hustle rules, see my freelancer tips, and if you sell online, selling on Vinted and what eBay takes are good next reads.

Frequently asked questions

Do I need to do a Self Assessment if I earn under 1,000 pounds self-employed?

Usually not, thanks to the trading allowance. You must register anyway if you want loss relief, voluntary Class 2, or to claim Tax-Free Childcare or Maternity Allowance based on self-employment.

When do I need to register for Self Assessment?

By 5 October after the end of the tax year you need to file for, if you're new or didn't file for the last one. Late registration could mean a penalty.

What is the Self Assessment deadline?

For the 2025 to 2026 tax year, online returns and payment are due by 11:59pm on 31 January, and paper returns by 11:59pm on 31 October.

Do I need a tax return for renting a room?

Not if you earn less than 7,500 pounds a year under the Rent a Room Scheme (3,750 pounds if shared). Above that, you must complete a return.

What if HMRC sends me a return but I don't need one?

GOV.UK says you must contact HMRC to tell them. If HMRC asked you for a return, you must send it by the deadline on the letter.