Your Money
Most people only think about tax when they have done something wrong. Here is what you need to know before that.
Figures reflect UK data as of 2026. Always verify specifics before making financial decisions.
The UK tax system is designed so that most employed people never have to think about it — their employer handles everything. But the moment you have any income outside a standard job, the rules change. Understanding when you need to act, and what happens if you do not, is the difference between a minor admin task and an unexpected fine.
If you are employed, your employer deducts income tax and National Insurance from your pay before it reaches your bank account. This is PAYE (Pay As You Earn). Your employer pays these amounts directly to HMRC on your behalf. Most employees never need to file a tax return, check their own tax calculation, or interact with HMRC at all.
If you only have one PAYE job and no other income, you do not need to do anything. HMRC gets its money automatically.
You are required to register for self-assessment and file a tax return if:
The deadline for online self-assessment returns is 31 January for the previous tax year ending 5 April. Miss it and the penalties start immediately.
The first £1,000 of income from self-employment or casual work is entirely tax-free under the trading allowance. You do not need to register with HMRC or declare it.
If your side income exceeds £1,000 in a tax year, you must register as self-employed with HMRC — it is a simple online process — and complete a self-assessment tax return for that year. You pay tax on your profits, not your total income. Revenue minus allowable expenses equals your taxable profit.
As a self-employed person, you pay income tax on your profits — not your total revenue. Allowable expenses that reduce your taxable profit include:
In addition to income tax, self-employed people pay Class 4 National Insurance — 6% on profits between £12,570 and £50,270, and 2% above that. Keep receipts for everything. They are your evidence if HMRC ever queries your expenses.
If you are employed, National Insurance is deducted through PAYE automatically. In 2025/26, the rates are:
If your annual business turnover exceeds £90,000, you must register for VAT and charge 20% on most goods and services you sell. This threshold is unlikely to apply at 18, but it is worth knowing it exists — missing the registration deadline carries penalties of its own.
£100 automatic fine for missing the 31 January deadline — regardless of whether you owe any tax.
An additional £10 per day, up to a maximum of £900 on top of the initial £100 fine.
Interest accrues on any unpaid tax from the 31 January deadline. The longer you leave it, the more you owe.
If you are married or in a civil partnership, and one partner earns below the personal allowance (£12,570) while the other pays basic-rate tax, the lower earner can transfer £1,260 of their personal allowance to their partner. This saves the higher earner up to £252 per year. It is not relevant at 18 but worth knowing for the future.
Tax is not complicated if you understand the structure. The three most common mistakes young people make are: not registering with HMRC when they start earning side income, not keeping receipts for allowable expenses, and missing the 31 January deadline. All three are completely avoidable. If in doubt, register early — HMRC does not penalise people for registering when they did not need to.
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