How to reduce your tax bill legally: a UK guide

Photo by Sarah Agnew on Unsplash

Nobody enjoys watching a chunk of their salary disappear before it even hits their bank account. But here’s the thing most people don’t realise: the UK tax system is full of legal, government-sanctioned ways to keep more of your own money. These aren’t loopholes or grey areas. They’re allowances and reliefs that HM Revenue & Customs (HMRC) built into the system on purpose.

The problem is that almost none of this is explained to you automatically. Your payslip doesn’t flag that you’re missing out on free pension tax relief. Your bank doesn’t remind you that your ISA allowance resets every April and you haven’t used any of it. You’re expected to just know — and most people don’t.

This guide walks through the legitimate ways to reduce how much tax you pay, from the obvious (pensions, ISAs) to the overlooked (marriage allowance, trading allowance). None of this requires an accountant, an offshore account, or anything remotely dodgy. It’s just using the rules as they’re written.

Tax avoidance isn’t the same as tax evasion — and that matters

This is the misconception that stops people from even looking into this stuff. “Reducing your tax bill” sounds like something only dodgy accountants and offshore companies do. It isn’t.

Tax evasion is illegal — hiding income, lying on a tax return, not declaring earnings. Tax avoidance, in the everyday sense this article means it, is using the allowances and reliefs Parliament deliberately created to encourage certain behaviour: saving into a pension, investing through an ISA, giving to charity, supporting a lower-earning spouse.

HMRC’s own guidance distinguishes between legitimate reliefs and “tax avoidance schemes” (the aggressive, contrived arrangements they actively pursue). Putting money into your workplace pension is not in the same universe as the latter. Claiming the allowances below is exactly what the government intends people to do.

Make sure your tax code is actually correct

Before anything else, check your tax code. It’s the string of numbers and letters (like 1257L) your employer uses to work out how much tax to deduct from your pay. An incorrect tax code is one of the most common reasons people overpay tax without ever noticing.

This happens more often than you’d think — after changing jobs, having more than one income source, or going through emergency tax when starting a new role. You can check your tax code on your payslip or via your personal tax account on gov.uk. If it’s wrong, HMRC can correct it and refund any overpaid tax, sometimes going back up to four years.

This isn’t really “reducing” your tax bill — it’s making sure you’re not paying more than you legally owe in the first place. Worth five minutes of checking.

Use your ISA allowance before you use anything else

An ISA (Individual Savings Account) is a wrapper that shields your savings or investments from tax — no tax on interest, dividends, or capital gains earned inside it. Everyone gets an ISA allowance of £20,000 per tax year (6 April to 5 April), and it resets every year whether you use it or not. Unused allowance doesn’t roll over.

This matters more than people assume. Outside an ISA, interest on savings is taxed once you go over your Personal Savings Allowance (£1,000 a year for basic-rate taxpayers, £500 for higher-rate, £0 for additional-rate). With savings rates higher than they’ve been in years, it’s now genuinely easy for ordinary savers to breach that allowance without realising — and start paying tax on interest they didn’t think of as “income.”

Put money that would otherwise sit in a regular savings account into a cash ISA instead, and that interest is tax-free, full stop.

Pension contributions: the most underused tax relief in the UK

This is the big one, and most people are leaving money on the table without knowing it.

When you pay into a pension, the government tops it up with tax relief at your highest rate of income tax. If you’re a basic-rate (20%) taxpayer, for every £80 you put in, the government adds £20 — making it £100 in your pension. If you’re a higher-rate (40%) taxpayer, you can claim back even more through your tax return (the extra 20% isn’t added automatically — you have to claim it).

Here’s what that looks like in practice:

*Higher and additional-rate relief above the basic 20% usually needs to be claimed through Self Assessment — it isn’t applied automatically by most workplace schemes.

For most employees, the simplest way to do this is through your workplace pension via salary sacrifice, if your employer offers it — this also reduces your National Insurance contributions, not just income tax. Worth asking your HR or payroll team whether this is available.

The annual limit on how much you can pay in while still getting tax relief (the annual allowance) is £60,000 for most people, so this isn’t something most earners need to worry about hitting.

Marriage allowance: the £1,260 most couples never claim

If you’re married or in a civil partnership and one of you earns under the personal allowance threshold (£12,570) while the other is a basic-rate taxpayer, you may be eligible for marriage allowance. It lets the lower earner transfer £1,260 of their unused personal allowance to their partner, reducing the higher earner’s tax bill by up to £252 a year.

It sounds small, but it’s free money sitting unclaimed for an enormous number of eligible couples — largely because nobody tells you it exists. You can backdate a claim up to four tax years, which can mean a lump sum of over £1,000 if you’ve been eligible and haven’t claimed. Apply directly through gov.uk’s marriage allowance page — it takes a few minutes and there’s no cost to apply.

If you’re self-employed or have side income

Self-employed people and anyone with extra income (freelancing, selling online, renting a room) have more levers to pull — and more responsibility to get it right.

• Trading allowance: If your total income from self-employment or casual work is under £1,000 a year, you don’t need to declare it or pay tax on it at all.
• Allowable expenses: Costs genuinely incurred for your business — equipment, travel, a proportion of home costs if you work from home, professional subscriptions — reduce your taxable profit. Keep receipts and records; this is where people either overpay by not claiming enough, or get into trouble by claiming things that aren’t genuinely business costs.
• Rent a Room scheme: If you rent out a furnished room in your own home, the first £7,500 a year is tax-free, with no need to declare it if you earn under that.
• Register and file on time: Self Assessment penalties for late filing start at £100 and increase the longer you leave it, even if you owe nothing. This isn’t a tax-reduction tip exactly, but avoiding unnecessary penalties is the easiest “saving” there is.

Capital gains and dividends: smaller allowances than they used to be, but still worth using

If you sell investments, a second property, or other assets at a profit, you may owe Capital Gains Tax (CGT) above an annual tax-free allowance — currently £3,000. This allowance has shrunk significantly over the last few years, which makes using it deliberately more important than it used to be: if you’re planning to sell investments, spreading the sale across two tax years rather than one can sometimes keep more of the gain under the threshold.

Dividend income (from shares held outside an ISA) also has its own tax-free allowance, currently £500 a year, before dividend tax rates apply. Again — holding investments inside a Stocks and Shares ISA avoids this issue completely, since gains and dividends inside the wrapper aren’t taxed at all.

A quick reality check on what’s realistic for you

Not every option here applies to everyone. A sensible starting order for most people looks like this:

The bottom line

• Check your tax code this week — it takes five minutes via your gov.uk personal tax account and could reveal you’ve been overpaying.
• Find out if your employer offers salary sacrifice for pension contributions — it’s one of the few genuinely “free” ways to reduce your tax bill, and most people never ask.
• Check marriage allowance eligibility today if you’re married or in a civil partnership and one of you earns under £12,570 — and claim backdated years if you qualify.
• Move idle savings into an ISA before your Personal Savings Allowance gets eaten up by rising

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