The short answer
You MUST file Self Assessment if:
- Self-employed income > £1,000 (gross, not profit) in the year
- Rental income > £1,000 (gross)
- Untaxed income > £2,500 (interest, foreign income, dividends outside ISA)
- Total income > £150,000 (additional-rate taxpayer)
- You owe Capital Gains Tax (sales of shares, crypto, second properties)
- You're a partner in a partnership
- You're a company director and HMRC has asked you to file
- You receive Child Benefit and either you or your partner has income > £60,000 (HICBC)
You SHOULD consider filing voluntarily if:
- You're a higher-rate taxpayer making personal pension contributions outside salary sacrifice
- You make Gift Aid donations as a higher- or additional-rate taxpayer
- You've invested in EIS, SEIS or VCT
- You're claiming Marriage Allowance the recipient hasn't yet processed
Self-employment threshold — £1,000
If you have any self-employed income (freelance writing, consulting, eBay reselling, Etsy, OnlyFans, rented parking spaces, paid YouTube revenue), the threshold is £1,000 of gross turnover in the tax year. Below that, the Trading Allowance covers it and you don't need to declare anything. Above that, Self Assessment is mandatory — even if you make a loss.
"Gross" means total receipts. £1,200 of gross income with £1,400 of expenses still triggers a filing obligation, even though you made an £200 loss. The side hustle SA checker walks through the most common borderline cases.
The Child Benefit / HICBC trap
The High Income Child Benefit Charge claws back Child Benefit when one parent's income exceeds £60,000. The charge is 1% of the family's total Child Benefit per £200 of income above £60,000 — fully clawed back at £80,000.
If you've claimed Child Benefit AND either parent earns over £60,000, the higher earner must file Self Assessment to declare and pay back the charge. You don't have to — and arguably shouldn't — stop claiming Child Benefit; even if you'll repay it all via HICBC, the qualifying years for State Pension that come from claiming Child Benefit can be worth thousands later. The HICBC calculator shows the exact charge.
Common error: assuming HICBC is the lower-earning parent's responsibility. It's whichever parent has the higher income, regardless of who claims the Child Benefit.
Voluntary filing — pension relief above 20%
If you contribute to a personal pension (SIPP, group personal pension, or any "relief at source" arrangement) AND you're a higher- or additional-rate taxpayer, the basic 20% relief is added at source — but the additional 20-25% has to be reclaimed via Self Assessment.
The reclaim is straightforward (one form, one number) but you have to file. For a higher-rate taxpayer making £5,000/year in personal pension contributions, the missed reclaim is £1,000/year. Most HMRC reconciliation does NOT pick this up automatically — it's on you to file.
Two ways around: switch to salary sacrifice (the relief happens at source via NI saving), or file SA each year. Either works; salary sacrifice is operationally easier.
Voluntary filing — Gift Aid above basic rate
Gift Aid donations work like pension contributions on the relief side: 20% basic-rate relief goes to the charity automatically, but the additional 20-25% for higher- and additional-rate taxpayers is reclaimed via Self Assessment.
For a higher-rate taxpayer donating £100/month (£1,200/year), the missed Gift Aid reclaim is £300/year. Add a charity bonus of about £80 and you have an actual cost of £820 net for £1,500 of charitable contribution. Without filing SA, you donate £1,200 and the charity receives £1,500 (still good!), but you don't get the £300 back.
Voluntary filing — EIS, SEIS, VCT
EIS (Enterprise Investment Scheme), SEIS (Seed EIS), and VCT (Venture Capital Trust) all offer income tax relief on investments — 30% for EIS and VCT, 50% for SEIS. The relief is claimed via Self Assessment only.
If you've made any of these investments, filing is essentially mandatory in practice — the relief is the entire point. The EIS/SEIS calculator and VCT calculator show the relief stack.
The cost of filing — how hard is it really?
For an employee with one job, one salary, one P60, and one or two reliefs to claim (pension, Gift Aid), Self Assessment takes about 30-60 minutes online. The form is mostly pre-populated from HMRC's record of your PAYE position; you add the additional reliefs and submit.
For self-employed, multiple property landlords, or anyone with foreign income, complexity climbs. The Self Assessment first-timer guide walks through the simple cases. The self-employed toolkit covers business-related complexity.
Filing deadlines:
- Paper return: 31 October following tax year
- Online return: 31 January following tax year
- Paying any tax owed: 31 January (same as online filing)
- Second payment on account (if applicable): 31 July
Late filing penalty: £100 for being one day late, rising fast. The January SA checklist covers timing.
Sources
HMRC: Check if you need to send a Self Assessment tax return · Self Assessment tax returns.