Self Assessment explained
Who needs to file a tax return, the deadlines you must not miss, penalties for getting it wrong, and how to reduce your bill — in plain English.
Who needs to file a Self Assessment return?
HMRC requires you to file a Self Assessment tax return if any of the following applied in the tax year:
- You were self-employed as a sole trader and earned more than £1,000 (before expenses)
- You were a partner in a business partnership
- You were a company director (unless your only income was PAYE and you have no other tax to pay)
- You had rental income from property
- You had untaxed income over £2,500 — for example from savings interest, investments, or dividends above the allowance
- Your income was over £100,000
- You received Child Benefit and either you or your partner earned over £60,000 (High Income Child Benefit Charge)
- You had capital gains above the annual exempt amount
- You want to claim certain tax reliefs, such as pension contributions or Gift Aid
If you are unsure whether you need to file, it is always better to check — HMRC can charge penalties even if you owe no tax.
Key Self Assessment deadlines
Missing a deadline triggers automatic penalties, so these dates matter:
Register for Self Assessment
Deadline to tell HMRC you need to file a return for the previous tax year. Missing this can result in a penalty.
Paper return deadline
If you file a paper return, it must reach HMRC by 31 October following the end of the tax year.
Online return and tax payment
Deadline to file your online return and pay any tax owed for the previous tax year. This is also the deadline for your first payment on account if applicable.
Second payment on account
If HMRC requires payments on account, the second instalment is due by 31 July. This is an advance payment towards your next tax bill.
Penalties for late filing and late payment
HMRC applies automatic penalties the moment you miss a deadline — even if you owe no tax at all.
Late filing penalties
| How late | Penalty |
|---|---|
| 1 day late | £100 fixed penalty |
| 3 months late | £10 per day (up to 90 days, maximum £900) |
| 6 months late | 5% of tax owed or £300 — whichever is greater |
| 12 months late | A further 5% of tax owed or £300 — whichever is greater |
Late payment penalties
| How late | Penalty |
|---|---|
| 30 days late | 5% of unpaid tax |
| 6 months late | A further 5% of unpaid tax |
| 12 months late | A further 5% of unpaid tax |
HMRC also charges interest on late payments from the day after the deadline. Interest rates change periodically — check HMRC's website for the current rate.
You can appeal a penalty if you have a reasonable excuse — for example a serious illness or a bereavement. An accountant can help you make that case.
Worked examples
Here is what Self Assessment typically looks like for clients we work with in North Devon:
Sole trader — plumber
£38,000 turnover, £9,000 allowable expenses
Taxable profit is £29,000. After the £12,570 Personal Allowance, £16,430 is taxable at 20% basic rate. Class 4 National Insurance also applies on profits above £12,570. Total tax and NI bill: approximately £5,300.
Limited company director
£12,570 salary + £40,000 dividends
The salary uses the Personal Allowance. Dividends above the £500 allowance are taxed: £27,430 at 8.75% (basic rate) = £2,400. The remaining £12,070 falls into the higher-rate band at 33.75% = £4,074. Total dividend tax: approximately £6,474.
Landlord — one rental property
£9,600 rental income, £3,200 allowable expenses
Net rental profit is £6,400. Combined with a £28,000 PAYE salary, total income is £34,400. The rental profit is taxed at 20% basic rate = £1,280. Mortgage interest relief is restricted to the basic rate tax credit.
Side income — freelance designer
£22,000 employed income + £6,500 freelance income
Freelance income above the £1,000 trading allowance is £5,500. After the remaining Personal Allowance (already partly used by employment), approximately £5,500 is taxable at 20% = £1,100. Class 4 NI may also apply depending on total profits.
How to reduce your Self Assessment bill
There are legitimate ways to reduce the amount of tax you owe — but you need to claim them:
Claim all allowable business expenses
Sole traders and partnerships can deduct costs wholly and exclusively for business — tools, mileage, home office, professional subscriptions, and more.
Make pension contributions
Contributions to a personal pension reduce your taxable income. Higher-rate taxpayers can claim additional relief through Self Assessment.
Use your ISA allowance
Income and gains inside a Stocks and Shares ISA are tax-free and do not appear on your return at all.
Claim Gift Aid donations
If you donate to charity under Gift Aid, you can claim back the higher-rate tax relief on those donations through your return.
Check your tax code
An incorrect PAYE tax code can mean you overpay tax through employment. Your Self Assessment return is the place to correct this.
Let us handle your Self Assessment
We prepare and file Self Assessment returns for sole traders, directors, landlords, and anyone with untaxed income across North Devon. Get in touch for a free, no-obligation quote.
Get your free quoteThis guide is general information about how the rules work, not personal tax advice. Tax depends on your individual circumstances and the rules can change. Speak to a qualified adviser before making decisions.