How to Categorise Expenses for HMRC: Stop Guessing

Miscategorise a single expense category and HMRC can reject your deduction entirely, even if the cost was 100% legitimate. Here is exactly how to categorise expenses for HMRC so your money stays where it belongs: in your pocket.

The Category System HMRC Actually Uses

When you file your Self Assessment as a sole trader, you use form SA103 (the short version if your turnover is below £85,000; the full SA103F above that). That form has a fixed list of expense boxes. HMRC does not care how your bookkeeping software labels things internally. What matters is which box the total lands in when you submit.

The main categories on SA103F are:

• Cost of goods bought for resale or goods used
• Construction industry: payments to subcontractors
• Wages, salaries, and other staff costs
• Car, van, and travel expenses
• Rent, rates, power, and insurance costs
• Repairs and renewals of property and equipment
• Phone, fax, stationery, and other office costs
• Advertising and business entertainment costs
• Interest on bank and other loans
• Bank, credit card, and other financial charges
• Irrecoverable debts written off
• Accountancy, legal, and other professional fees
• Depreciation and loss or profit on sales of assets
• Other business expenses

Each of these is a distinct bucket. Pouring the wrong costs into the wrong bucket is the root cause of most expense disputes between sole traders and HMRC.

The Three Mistakes That Cost Sole Traders the Most

Mistake 1: Lumping Capital Purchases Into Running Costs

This is the big one. If you are a plumber who bought a £1,200 pipe threading machine, that is not an office cost or a repair. It is a capital asset, and it belongs in the capital allowances section of your return, specifically under the Annual Investment Allowance (AIA), which currently allows 100% first-year relief up to £1 million.

Put it in the wrong box and one of two bad things happens: either HMRC's systems flag the mismatch and you face a compliance check, or you accidentally claim less relief than you are entitled to because capital allowances have different (often more generous) rules than simple expense deductions.

For a sole trader earning £65,000, the difference between correctly claiming AIA on £3,000 of tools versus miscategorising them as repairs could be worth roughly £600 in tax at the basic rate. It is not theoretical. Read more about this in Capital Allowances for Sole Traders: Stop Leaving Money Behind.

Mistake 2: Treating Dual-Purpose Costs as 100% Business

The "wholly and exclusively" test is unforgiving. Your mobile phone bill is a classic example. If you use one phone for both personal and business calls, HMRC expects you to apportion the cost. Claiming 100% of a £600 annual phone contract when 40% of calls are personal is not a grey area; it is an inaccuracy that HMRC can challenge.

The practical fix is either to have a dedicated business phone (then 100% is fine) or to calculate a reasonable business-use percentage and document it. The same logic applies to your home broadband, a car used for mixed purposes, and any subscription that serves both work and personal life.

If you use your home as a base, the Simplified Expenses: The Flat Rate Trick HMRC Allows post covers how HMRC's flat-rate system can sidestep the apportionment headache entirely for home working costs.

Mistake 3: Hiding Everything in "Other Business Expenses"

The "other business expenses" box is not a catch-all for anything you cannot be bothered to classify. HMRC's own guidance notes make clear it is for genuinely miscellaneous costs that do not fit elsewhere. Sole traders who route significant spending through this box attract attention precisely because it is the category with the least inherent logic attached to it.

If your "other" box consistently runs at 30% of your total expenses, expect questions.

Category-by-Category Guidance for Tradespeople

Cost of Goods / Materials

For builders, electricians, plumbers, and painters, this is often the largest category. It covers raw materials, components, and consumables used directly in the job: copper pipe, electrical cable, paint, fixings, timber. It does not include tools that last more than one job (those are capital items) or subcontractor labour (that has its own CIS box if you operate under the Construction Industry Scheme).

Keep delivery receipts. HMRC will want to see that materials tied to specific invoiced jobs are correctly matched, particularly if you are ever subject to a check.

Car, Van, and Travel Expenses

You have two routes here and you must pick one consistently within a tax year.

Actual costs method: Claim the business-use proportion of all vehicle running costs (fuel, insurance, servicing, road tax). You need to log your mileage to establish the business percentage. For a van used 80% for work on a £4,000 annual running cost, you claim £3,200.

HMRC mileage rates (simplified expenses): Claim 45p per mile for the first 10,000 business miles, then 25p per mile thereafter. No receipts for fuel needed; just a mileage log. For a sole trader doing 12,000 business miles a year, that is £4,750 (10,000 × £0.45 + 2,000 × £0.25).

For most sole traders with a single vehicle doing moderate mileage, the mileage rate method is simpler and often delivers a similar or better result. It also keeps HMRC happy because the rate is theirs.

Phone, Stationery, and Office Costs

This category covers business phones, broadband (apportioned), printer ink, paper, postage, and software subscriptions used for the business. Your MTD-compliant record-keeping app, for example, belongs here. So does your accountancy software, your QuickBooks or Xero subscription, or your TapTax subscription.

Note that an iPad or laptop is a capital item if it costs over the capitalisation threshold you use. Many sole traders under-claim here by putting low-cost tech in this box when it would get full relief as a capital allowance anyway.

Repairs and Renewals

This category trips people up because the word "renewals" sounds like it could cover buying new equipment. It does not. Repairs and renewals covers maintaining existing assets: fixing a broken van window, repainting your site office, replacing a worn belt on machinery. It does not cover upgrading or replacing an asset with a better version (that is capital expenditure).

A classic HMRC dispute: a self-employed carpenter replaces an old, broken table saw with a new model and records it as a repair. The new saw is an improvement on what was there before. HMRC will reclassify it as capital expenditure. The tax outcome might be identical if you claim AIA, but the paperwork dispute and potential penalties are not worth it.

Advertising and Business Entertainment

Advertising is fully allowable: your Google Ads spend, your leaflets, your website hosting, your van livery. Business entertainment is almost entirely disallowable under HMRC rules. Taking a client for dinner to win a contract sounds business-like but HMRC does not allow the deduction. The only exception is staff entertaining up to £150 per head per year, and as a sole trader you are not your own employee.

This distinction matters because many bookkeeping tools lump advertising and entertainment together, following the SA103 box name. Do not let software complacency lead you to claim disallowable entertainment costs.

How MTD Changes the Categorisation Game

Under Making Tax Digital for Income Tax, which becomes mandatory for sole traders with income over £50,000 from April 2026 (and over £30,000 from April 2027), you will submit quarterly updates to HMRC rather than a single annual return. Those updates require you to report income and expenses in the same HMRC categories described above, every three months.

This matters for two reasons. First, if your categories are wrong quarterly, HMRC has four opportunities per year to spot the problem rather than one. Second, your MTD software needs to map its internal categories to HMRC's boxes correctly. Not all software does this well. See MTD API Compatible Software: What the Label Actually Guarantees for a frank assessment of what approved software actually does under the bonnet.

The upside: MTD forces good habits. Sole traders who have been vague about categorisation for years will need to sharpen up, and the process of doing so almost always reveals unclaimed deductions.

A Real-World Example: Dave the Electrician

Dave is a self-employed electrician turning over £62,000 a year. His actual business costs look like this:

• £14,000 on cable, fittings, and consumables (materials)
• £3,200 on van running costs, claimed at actual costs with 85% business use (£2,720 claimable)
• £1,800 on a new test meter and kit bag (capital items, claimed via AIA)
• £900 on phone, broadband (70% business), and stationery
• £600 on public liability insurance
• £400 on a client dinner and football hospitality (disallowable entertainment)
• £350 on accountancy fees

Dave's mistake last year was recording the £1,800 equipment purchase under "repairs and renewals" and claiming the £400 entertainment as advertising. The first error had no material tax impact because AIA and expense deductions both gave full relief in year one, but it created a discrepancy HMRC could have queried. The second error was a straightforward overclaim of £400 that Dave was unaware of.

Corrected categorisation on his SA103F delivers exactly the same tax bill for the equipment but removes the overclaim risk entirely. His effective tax liability on £62,000 turnover, after correctly categorised expenses of approximately £18,970, leaves taxable profit of roughly £43,030 and a tax and NI bill of around £10,500. Every pound of miscategorised expense is not just a paperwork issue; it is potential interest, penalties, and an HMRC investigation Dave really does not want.

The Practical System That Actually Works

The most effective approach for a time-poor sole trader is a weekly 15-minute categorisation habit rather than a quarterly panic. Here is what that looks like in practice:

1. Photograph receipts immediately using your MTD app or accounting software. The receipt fades; the photo does not.
2. Assign the HMRC category at point of entry, not at year end. Most MTD apps let you set default categories per supplier, so your materials merchant is always "cost of goods" without you having to think.
3. Flag anything that mixes personal and business use with a note on the percentage split. Do this at the time; you will not remember in January.
4. Keep a separate note for capital purchases over, say, £200. These need to go through capital allowances, and flagging them in-year means your accountant or your software can handle them correctly at year end.
5. Review the "other expenses" box quarterly. If anything is sitting there because you were unsure, sort it before submission. Quarterly updates under MTD will make this a discipline, not a choice.

If you want to understand how the right software can automate most of this, How to Track Expenses as a Sole Trader Without the Shoebox covers the practicalities in depth.

One Question Worth Asking Before You File

Before you submit your Self Assessment (or your next quarterly MTD update), ask yourself one question about each expense line: if HMRC rang tomorrow and asked me to justify this category choice, could I do it in two sentences using the words "wholly and exclusively for my trade"?

If the answer is yes, you are in good shape. If you hesitate, revisit it now. HMRC's compliance checks have become more data-driven since the MTD rollout began, and the days of vague categorisation going unnoticed are running out. Getting your expense categories right is not about being a model taxpayer; it is about not paying more tax than you owe, and not spending a Thursday afternoon justifying a £400 dinner you should have known was disallowable.