HMRC Self-Assessment 2026: Deadlines, Mistakes & Reliefs Most People Miss
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If HMRC sends you a Self-Assessment notice — or you have self-employment, rental income, or untaxed income over £1,000 — getting this right can save you hundreds or even thousands in tax each year. Here’s the 2026 essentials.
Key Deadlines
- 5 October: Register for Self-Assessment if you’re new to it
- 31 October: Paper return deadline
- 31 January: Online return deadline AND balancing payment due
- 31 July: Second payment on account due (if applicable)
Missing the January deadline triggers an automatic £100 penalty even if you owe £0. After 3 months, daily penalties start adding up.
The Three Most Common Mistakes
1. Missing the trading allowance. You can earn up to £1,000 in side income without paying tax or filing. Above that, you have a choice: deduct actual expenses or claim the £1,000 allowance. Whichever is bigger.
2. Forgetting interest income. If you earn over the Personal Savings Allowance (£1,000 basic rate, £500 higher rate, £0 additional rate), the excess is taxable. Most people forget about this until HMRC adjusts later.
3. Not claiming working-from-home expenses. If part of your home is used for self-employed work, you can claim a portion of utilities, broadband and rent. Either use the £6/week flat rate or actual costs.
Reliefs Most People Miss
- Marriage Allowance. A non-working or low-earning spouse can transfer £1,260 of their Personal Allowance to a basic-rate-paying spouse. Worth up to £252 per year. Claim retroactively for up to 4 prior years.
- Pension contributions for higher-rate taxpayers. The extra 20% relief on top of basic-rate isn’t automatic — claim it on your return.
- Gift Aid for charitable donations. As a higher-rate taxpayer, you can claim the difference between basic and higher rates on Gift Aided donations.
- Capital losses. You must report capital losses to use them against future gains. Losses are useless if you don’t declare them.
Side Hustle / Self-Employed Specifics
- Mileage allowance: 45p/mile for the first 10,000 business miles, 25p after. Much simpler than tracking fuel + maintenance + depreciation.
- Home office: Either the flat-rate £6/week or actual costs apportioned to the room/hours used for business.
- Equipment & software: Anything purchased wholly for the business is deductible. Phone bills proportional to business use.
When to Use an Accountant
Their £200–500 fee makes sense if:
- You have multiple income sources (employment + rental + side business)
- Your taxable income is well into the higher-rate band
- You’re new to Self-Assessment and could use the year-one setup
- You sold a property or substantial investment in the year
For straightforward employment + small rental or side income, the HMRC online system is usable.
Things to Know About Payments on Account
If your tax bill exceeds £1,000, HMRC asks for advance payments in January and July equal to half your prior year’s tax bill each. This catches new self-assessors off-guard — you can end up paying 150% of one year’s tax in a single January.
⚠️ Watch Out: Don’t file early but pay late. HMRC charges interest from 1 February regardless of when you filed. File when ready, but plan the cash flow.