How to legally reduce your Self Assessment tax bill
There's a difference between avoiding tax you genuinely don't owe and dodging tax you do. Everything here is the first kind: ordinary, legitimate ways the tax system is designed to let you lower your bill. None of it is clever or aggressive. It's mostly about claiming what you're entitled to and making a few decisions before the tax year ends.
1. Claim every allowable expense
For the self-employed, this is the biggest lever and the most commonly under-used. Your tax is charged on profit, not turnover, so every legitimate business cost you claim reduces the profit that gets taxed, and reduces your Class 4 National Insurance at the same time. Office costs, travel, a proportion of home and phone use, professional fees, tools, software and stock all count. The test is that the cost is "wholly and exclusively" for the business. Our guide to allowable expenses goes through the main categories.
2. Pay into a pension
Pension contributions are the most powerful tax planning tool most people have. You get tax relief at your marginal rate: a higher-rate taxpayer effectively turns £100 of take-home into £166 in the pension once relief is counted. If your income is in the £100k to £125k taper zone, the effective relief can reach 60%. The money is locked away until pension age, so it's not a way to cut a bill you need to pay next month. For long-term tax efficiency nothing else comes close.
3. Use your allowances before they reset
Several allowances work on a use-it-or-lose-it basis each tax year:
- The £1,000 trading allowance: if your self-employment income is under £1,000, you may not need to report it at all.
- The Dividend Allowance and Personal Savings Allowance: structure how you take income to use them.
- Marriage Allowance: if one spouse earns under the Personal Allowance, they can transfer £1,260 of it to a basic-rate partner, worth up to £252 a year.
4. Give through Gift Aid
Charitable donations under Gift Aid extend your basic-rate band and reduce your adjusted net income. For higher-rate taxpayers that means extra relief through your tax return, and like pensions, Gift Aid can help pull you out of the Personal Allowance taper.
5. Time income and costs sensibly
If you have control over when you invoice or when you buy equipment, timing can matter. Bringing a big deductible purchase into the current year, or pushing some income into the next, can keep you under a threshold like £50,270 or £100,000 where the rate jumps. This is about smoothing across the 5 April line, not hiding anything. The income is still declared, just in the year it genuinely falls.
6. Consider your business structure
At higher profit levels, some sole traders look at whether incorporating as a limited company would be more tax-efficient, taking a mix of salary and dividends. It comes with extra admin, filing and costs, and it isn't right for everyone, so it's a conversation to have with an accountant rather than a default move.
What not to do
Steer well clear of anything that sounds too good to be true: contrived "tax avoidance schemes", disguised remuneration, or backdating expenses that never happened. HMRC pursues these hard, and the penalties dwarf any saving. The legitimate routes above are more than enough for almost everyone.
Check the impact before you commit
Before you make a pension contribution or a big deductible purchase, model it. Put your income into the estimator, note the tax, then adjust the figures and compare. Seeing the two numbers side by side turns tax planning from a vague good intention into a concrete decision. For anything significant, confirm it with a qualified accountant who can see your full position.