How to Lower Your Self-Assessment Bill (Legally) With Allowable Expenses

Every pound you spend on legitimate business expenses is a pound that reduces your tax bill. 

Yet thousands of self-employed people overpay tax each year simply because they don’t claim everything they’re entitled to. Some miss entire categories of expenses, while others chronically under-claim.

Of course, this isn’t about aggressive schemes or bending rules. It’s about knowing what HMRC allows and claiming it properly. 

Here’s how to properly reduce your self-assessment tax bill so you only pay what you should pay. 

How Expenses Reduce Your Tax

First off, you don’t pay tax on your total income. You pay tax on your profit after deducting allowable expenses.

Here’s a simple example: you earn £60,000 and have £5,000 in allowable expenses. After your personal allowance of £12,570, your taxable income is £42,430. Without those expenses, your taxable income would be £47,430.

That £5,000 difference saves you £1,000 if you’re a basic rate taxpayer (20%), £2,000 as a higher rate taxpayer (40%), or £2,250 as an additional rate taxpayer (45%). 

The more you earn, the more each legitimate expense saves you. Actual savings may vary once National Insurance and other thresholds are taken into account, but the principle holds.

The Golden Rule: Wholly and Exclusively

HMRC’s test for allowable expenses is straightforward. Costs must be ‘wholly and exclusively’ for business purposes. This doesn’t mean you can’t claim mixed-use assets; however, you can claim only the business portion.

So if your mobile phone bill is £200 annually and you can demonstrate a reasonable business proportion of £70, you claim £70. 

If you use one room in your four-room house as an office and your electricity bill is £400, you can claim £100. The key is being able to justify your calculation with supporting evidence if HMRC ever asks.

Maximising Your Home Working Expenses

Working from home offers substantial tax relief that many people underutilise. You can claim a proportion of household costs, including heating, electricity, water, council tax, mortgage interest (not capital repayments), internet, and phone bills.

There are two methods to calculate this. The simplified expenses route allows you to claim a flat rate if you work at least 25 hours per month from home:

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

These rates apply for 2024/25 and 2025/26, giving you £120 to £312 annually. This method is simple but often results in a smaller claim than calculating actual costs.

The actual costs method requires more work but usually yields better results. Calculate the business proportion of each bill using either the number of rooms (if you use one room out of five, claim 20% of eligible costs) or the time spent working (if you work four days a week, claim 4/7ths of the business proportion).

For example, if your electricity bill is £1,120 annually and you use one of four rooms for business, that’s £280. If you work five days a week, you’d claim £200 (5/7ths of £280). On heating, council tax, internet, and other eligible costs, this quickly adds up to over £1,000 in annual claims.

Important: If you claim a room is used exclusively for business, get advice first – exclusive business use can sometimes affect Capital Gains Tax relief when you sell your home.

Getting Vehicle Expenses Right

Vehicle expenses represent one of the largest potential deductions for many self-employed people, but also one of the most commonly miscalculated. You have two options: simplified mileage rates or actual costs.

The mileage method is straightforward. For 2024/25 and 2025/26, claim 45p per mile for the first 10,000 business miles and 25p thereafter. This covers all running costs, including fuel, insurance, repairs, and depreciation. You just need to keep a log of business mileage.

For someone doing 8,000 business miles annually, that’s £3,600 in tax-deductible expenses. At 10,000 miles, it’s £4,500. These figures can transform your tax position.

If you use mileage rates, you can usually still claim parking, tolls, congestion charges, and similar travel costs separately on top of your mileage allowance.

The actual costs method lets you claim the business proportion of all vehicle expenses: fuel, insurance, road tax, MOT, repairs, and breakdown cover. If you use your vehicle 60% for business, claim 60% of all costs. This method often works better for high-mileage drivers or owners of expensive vehicles, but it requires meticulous record-keeping.

What you cannot claim: Travel that is essentially ordinary commuting is not allowable. Business travel, such as trips to clients, suppliers, and temporary work locations, is generally allowable.

Office Costs and Equipment

Office expenses cover more than you might think. Stationery, postage, printer ink, phone bills, and mobile contracts all qualify. Software subscriptions used for business purposes also count, including accounting packages, design tools, project management software, and cloud storage.

Under the cash basis (which most small businesses use), most equipment you buy for the business is expensed. This includes computer equipment, laptops, tablets, phones, desks, chairs, filing cabinets, and other office furniture. Cars are treated differently – capital allowances rules apply to vehicles.

Don’t forget the small items that add up, such as USB drives, external hard drives, computer mice, keyboards, cables, and chargers. If you’re buying it for business use, track it and claim it.

Professional Fees and Subscriptions Often Missed

Many self-employed people overlook professional subscriptions entirely. 

If you’re a member of a professional body or trade organisation relevant to your business, the membership fee is tax-deductible. This includes unions, industry associations, and professional institutes.

Subscriptions to trade journals, professional publications or industry-specific online resources also qualify. These must relate to your current business, not training for a new venture.

Accountant fees are fully deductible, as are costs for solicitors, surveyors, architects, or any other professional adviser used for business purposes. Bank charges on business accounts, overdraft fees, credit card interest on business cards, and loan interest for business borrowing all count.

Insurance premiums for professional indemnity, public liability, business insurance, and equipment insurance are all allowable. If you’re required to hold specific insurance to operate in your industry, those premiums reduce your tax bill.

Marketing and Advertising Costs

Your marketing expenses deserve careful attention. Website costs, including domain registration, hosting fees, design and development work, and e-commerce platform subscriptions, are all deductible. Email marketing tools, social media management software, and SEO services qualify too.

Traditional advertising still counts: newspaper adverts, magazine features, directory listings and printed materials like business cards, flyers, and brochures. Providing free samples to potential customers is permissible, as are costs for promotional items with your branding.

What you cannot claim: Entertaining clients, event hospitality, or meals with potential customers. HMRC draws a firm line on entertainment expenses.

Training and Development

You can claim for training courses that maintain or improve skills you already use in your business. 

A graphic designer can claim for courses on new design software. A tradesperson can claim for refresher courses on updated regulations. A marketing consultant can claim for workshops on the latest digital marketing techniques.

What you cannot claim: Training that helps you start a new business or branch into entirely new areas. The training must relate to your current business activities.

Staff and Subcontractor Costs

If you employ people, you can claim salaries, bonuses, pensions, benefits, and the employer’s National Insurance contributions. Agency fees, subcontractor payments, and freelancer costs all count.

Staff training costs are allowable, as are the costs of hiring temporary or seasonal workers.

What you cannot claim: Childcare costs or payments for nannies and domestic help, even if they enable you to work.

The Expenses People Forget

Here are the often-overlooked allowable expenses that could be costing you money:

• Bad debts: If you use traditional (accruals) accounting and you’ve already included an invoice as income, you may be able to deduct it once it’s genuinely irrecoverable. If you use the cash basis, unpaid invoices are not yet taxed, so the treatment is different.
• Protective clothing: Uniforms, specialist work clothing, and safety equipment all qualify. Regular business attire doesn’t count, but industry-specific clothing does.
• Legal and regulatory costs: Licences required for your trade, regulatory fees, and professional registrations are all deductible.
• Business use of home phone: If you have a landline and use it for business, claim the business portion.
• Cleaning costs: If you clean business premises or work areas, these costs count.
• Security costs: Burglar alarms, security systems, and monitoring for business premises.
• Waste disposal: Trade waste collection for business premises.
• Repairs and maintenance: Fixing business equipment, maintaining business premises, or repairing business vehicles.

Capital Allowances for Larger Purchases

Some purchases fall under capital allowances rather than allowable expenses. The Annual Investment Allowance gives 100% relief on most qualifying plant and machinery up to £1 million. Some commercial vehicles (like vans) can qualify, but cars are excluded.

This means if you buy a van for £15,000, computer equipment for £3,00,0 and tools for £2,000 in the same tax year, you can deduct the full £20,000 from your profits, potentially saving £4,000 to £9,000 in tax depending on your rate.

What you cannot claim under AIA: Cars are excluded from the Annual Investment Allowance and have different capital allowances treatment.

Record-Keeping: Your Insurance Policy

You must keep records of all expenses you claim for at least five years from the 31 January following the tax year. This doesn’t mean carrying shoeboxes of receipts around, but it does mean having a system.

Digital tools make this straightforward. Photograph receipts with your phone, use accounting software to categorise expenses, and sync your bank accounts to track spending automatically. 

The time invested in proper record-keeping pays back many times over through accurate claims and peace of mind if HMRC ever enquires.

How Double Point Can Help

At Double Point, we don’t just prepare your self-assessment – we actively help you identify every legitimate expense that reduces your tax bill whilst keeping you fully compliant.

We’ll help you claim what you’re entitled to and keep the records HMRC expects.

Whether you’re a sole trader who’s been underestimating home working costs, someone missing out on professional subscription claims, or a high-mileage worker not maximising vehicle expenses, we’ll show you exactly where you can save.

Stop overpaying tax because you don’t know what you can claim. Book a consultation with Double Point today to discover how much you could save through effective expense management.

Disclaimer: This article is provided for general information purposes only and does not constitute professional tax, legal, or financial advice. Tax rules and allowances are subject to change, and individual circumstances vary. Whilst every effort has been made to ensure accuracy at the time of publication, Double Point accepts no liability for any reliance placed on this information. You should always consult with a qualified chartered accountant or tax adviser regarding your specific situation before making any decisions based on this content. HMRC rules can be complex, and what applies to one taxpayer may not apply to another.