How to Reduce Your Self Assessment Tax Legally

Let me be absolutely clear from the start: there is a world of difference between tax avoidance and tax evasion. Tax evasion is illegal — it involves deliberately hiding income or lying on your return. Tax avoidance, on the other hand, means using the reliefs, allowances, and deductions that the law provides for. HMRC expects you to claim what you are entitled to. Not claiming legitimate expenses is not being cautious — it is overpaying your tax bill unnecessarily. I am Penny, your AI bookkeeper at Accounted, and in this guide I am going to show you every legal way to reduce your Self Assessment tax bill.

Claim Every Allowable Business Expense

If you are self-employed, your taxable profit is calculated as your income minus your allowable business expenses. Every legitimate expense you claim reduces your taxable profit, which in turn reduces your Income Tax and National Insurance.

The golden rule is that an expense must be incurred "wholly and exclusively" for business purposes. Here are the categories most sole traders should be claiming:

Office and Working from Home Costs

If you work from home, you can claim a proportion of your household running costs. You have two options:

Simplified expenses (flat rate): HMRC lets you claim a flat rate based on the number of hours you work from home each month:

• 25-50 hours per month: £10 per month
• 51-100 hours per month: £18 per month
• 101+ hours per month: £26 per month

Actual costs: Calculate the proportion of your home used for business and claim that percentage of your rent/mortgage interest, council tax, electricity, gas, water, broadband, and insurance. This requires more record-keeping but can result in a higher claim.

Travel and Vehicle Costs

You can claim the cost of business travel, but not your regular commute. If you use your own car, the simplest approach is the HMRC mileage rate:

• 45p per mile for the first 10,000 miles per year
• 25p per mile after that

Keep a mileage log showing the date, destination, purpose, and distance for every business journey.

Professional Services and Subscriptions

Accountancy fees, bookkeeping costs, legal fees related to your business, professional body subscriptions, trade journal subscriptions — all claimable.

Equipment and Tools

Computers, phones, printers, tools, machinery — anything you purchase for your business. For items used partly for personal purposes (like a laptop), you can claim the business proportion.

Marketing and Advertising

Website costs, business cards, online advertising, signage, networking event fees — all legitimate business expenses.

Insurance

Professional indemnity insurance, public liability insurance, and business contents insurance premiums are all allowable.

Training

Training courses that update or maintain your existing skills are deductible. Courses that teach entirely new skills (such as retraining for a different career) generally are not, though there can be grey areas.

For a comprehensive list of every expense you can claim, read our tax deductions for sole traders guide. You might be surprised how many things you have been missing.

Use Your Personal Allowance Effectively

Every UK taxpayer has a Personal Allowance of £12,570 — the amount you can earn before paying any Income Tax. This seems straightforward, but there are several nuances worth understanding.

Marriage Allowance

If you are married or in a civil partnership and one partner earns less than £12,570 while the other is a basic rate taxpayer, you can transfer up to £1,260 of the lower earner's unused Personal Allowance to the higher earner. This saves the higher earner up to £252 per year in tax.

You can claim Marriage Allowance going back up to four years, so if you have not been claiming it, you could be owed over £1,000.

Apply through the GOV.UK Marriage Allowance page.

The £100,000 Trap

If your income is between £100,000 and £125,140, you are in a particularly punishing tax bracket. Your Personal Allowance reduces by £1 for every £2 earned over £100,000, creating an effective marginal tax rate of 60% in this band.

Strategies to avoid this include:

• Making additional pension contributions to bring your adjusted net income below £100,000
• Timing business expenditure to reduce your profit below the threshold
• Making charitable donations under Gift Aid

Maximise Pension Contributions

Pension contributions are one of the most powerful tax reduction tools available. When you contribute to a personal pension, you receive tax relief at your marginal rate.

How It Works

If you are a basic rate taxpayer and contribute £800 to your pension, the pension provider adds £200 in basic rate tax relief, making a total contribution of £1,000. It costs you £800 but £1,000 goes into your pension.

If you are a higher rate taxpayer, you can claim an additional 20% relief through your Self Assessment return. So that £1,000 pension contribution effectively costs you only £600.

Additional rate taxpayers can claim 25% additional relief, making the net cost just £550 for a £1,000 pension contribution.

Annual Allowance

You can contribute up to £60,000 per year to your pension and receive tax relief (or 100% of your earnings, whichever is lower). If you have unused allowance from the previous three years, you can carry it forward.

Practical Impact

If your taxable profit is £55,000 and you make a £5,000 pension contribution, your adjusted income drops to £50,000 — keeping you entirely within the basic rate band and avoiding 40% tax on that £5,000. That saves you £1,000 in Income Tax compared to not making the contribution, plus you have £5,000 growing in your pension pot.

Use the Trading Allowance

If your self-employment income is relatively small (perhaps from a side hustle alongside employment), you may be able to use the £1,000 trading allowance instead of deducting actual expenses.

If your total self-employment income is under £1,000, you do not need to report it at all. If it is over £1,000, you can choose to deduct the £1,000 allowance instead of your actual expenses. This is only beneficial if your actual expenses are less than £1,000.

Claim Capital Allowances

When you purchase assets for your business (equipment, vehicles, machinery), you can claim capital allowances rather than deducting the cost as a revenue expense.

The Annual Investment Allowance (AIA) lets you deduct the full cost of qualifying assets up to £1,000,000 per year. For most sole traders, this means you can deduct the entire cost of equipment purchases in the year you buy them.

For cars, the rules are different. You can claim:

• 100% first-year allowance for new electric or zero-emission cars
• 18% writing-down allowance for cars with CO2 emissions of 50g/km or less
• 6% writing-down allowance for cars with higher emissions

If you are using the cash basis of accounting, you deduct the cost of most assets when you pay for them, so capital allowances do not apply to most purchases. However, cars are an exception — you still use capital allowances for cars even under the cash basis.

HMRC has full guidance on capital allowances at GOV.UK — Capital allowances.

Utilise Loss Relief

If your business makes a loss in a tax year, you do not just lose that money from a tax perspective. You can use the loss to reduce your tax bill in several ways:

• Carry the loss forward to set against future profits from the same business
• Set it against other income in the same tax year (such as employment income)
• Carry it back to set against income from the previous tax year

Loss relief can be particularly valuable in the early years of a business when start-up costs might exceed income. If you made a loss in your first four years of trading, you can carry it back up to three years.

Make Charitable Donations Through Gift Aid

Charitable donations made under Gift Aid reduce your taxable income for Self Assessment purposes. This is especially valuable for higher rate and additional rate taxpayers.

When you donate £100 under Gift Aid, the charity claims back £25 in basic rate tax. But if you are a higher rate taxpayer, you can claim additional relief of £25 through your Self Assessment return, making the effective cost of the donation just £75.

Time Your Income and Expenses

While you should never manipulate your accounts dishonestly, the timing of legitimate transactions can make a difference to your tax bill.

If you use the cash basis, income is taxed when received and expenses are deducted when paid. This means:

• If you are approaching the end of the tax year and expect to be in a higher tax bracket next year, consider delaying invoices until after 5 April (though be careful not to damage client relationships)
• If you have planned business purchases, making them before 5 April means the expense falls in the current tax year
• If your income fluctuates, consider whether bringing expenses forward or pushing income back could keep you in a lower tax bracket

This is perfectly legitimate tax planning, but it requires careful consideration. If in doubt, get professional advice.

Consider Your Business Structure

If your profits are consistently high, it may be worth considering whether operating as a sole trader is still the most tax-efficient structure. Converting to a limited company can sometimes reduce your overall tax bill through a combination of corporation tax and dividends.

However, this is not a simple calculation. Limited companies come with additional administrative requirements, costs, and responsibilities. The tax savings need to outweigh these additional burdens. As a general rule, it starts to become worth considering when your profits consistently exceed £40,000-£50,000, but every situation is different.

Use Tax-Free Allowances You Might Be Missing

Several tax-free allowances are commonly overlooked:

• Personal Savings Allowance: Basic rate taxpayers can earn £1,000 in savings interest tax-free; higher rate taxpayers get £500
• Dividend Allowance: The first £500 of dividend income is tax-free
• Property Allowance: The first £1,000 of property income is tax-free (similar to the trading allowance)
• Rent a Room Relief: If you let out a furnished room in your home, the first £7,500 of income is tax-free
• ISA allowance: Up to £20,000 per year can be invested in an ISA, with all income and gains tax-free

How Accounted Helps You Pay Less Tax

One of the biggest reasons people overpay their tax is poor record-keeping. If you do not track your expenses properly, you cannot claim them. And if you cannot claim them, you pay more tax than necessary.

With Accounted, I automatically categorise your transactions, flag potential business expenses, and keep a running total of your deductions. I also estimate your tax liability throughout the year so you can see in real time how expenses affect your bill.

Visit our features page to see how it works, or sign up to start reducing your tax bill today. Every pound of legitimate expense you claim is money back in your pocket.

A Word of Caution

While all of the strategies in this guide are perfectly legal, it is important to be honest and accurate in your Self Assessment return. HMRC has sophisticated systems for detecting anomalies, and aggressive or artificial tax avoidance schemes can backfire badly.

Stick to claiming legitimate expenses, using the allowances and reliefs that Parliament has provided, and keeping accurate records. That is all you need to do to ensure you are not paying a penny more than you should. For more on how your tax bill is calculated, see our guide on how to calculate your tax bill as a sole trader.

Tax planning is not about gaming the system. It is about understanding the rules and making sure you claim everything you are entitled to. And that is something every taxpayer should be doing.

Accounted files your Self Assessment directly to HMRC, with your return pre-populated from your records. See Self Assessment filing →