Wholesale receipts, van costs, callout mileage in one app

HMRC lets you deduct any cost that is wholly and exclusively for your plumbing business. The table below maps a typical sole-trader plumber's spend — boilers, bathrooms, gas safety checks, drainage callouts — to the boxes on SA103F. Materials are usually the single biggest line for a plumber: boilers and copper alone can run to tens of thousands of pounds across a year.

Box numbers from HMRC SA103F Notes 2026. SA103S (turnover under £90,000) maps to the same expense categories on boxes 11–19, with Box 23 for AIA and Box 38 for CIS deductions.

• Treating a new van or major pressing tool as "repairs"A new Transit Connect, Vivaro or a £1,400 Rems / Milwaukee pressing kit is a capital purchase, not a repair. Under cash basis (default since 6 April 2024) include the cost with other expenses; under traditional accounting claim it through Annual Investment Allowance — fully deductible in year one.
• Mixing simplified mileage and actual van costs on the same vanPick one method per vehicle and stick with it for that vehicle's life with the business. Claiming 55p/25p AND fuel receipts is double-counting and HMRC will reverse it.
• Forgetting tool insurance is a separate allowable expensePlumbers' vans are a known overnight theft target — copper, brass and pressing tools are easy to move. Tool-specific insurance (covering kit left in the van) is a legitimate business expense in Box 21. Standard van insurance often excludes tools — read your policy.
• Confusion over the VAT Domestic Reverse Charge on commercial workIf you do new-build or commercial CIS work AND you're VAT-registered AND your customer is VAT-registered, the reverse charge applies — you don't charge VAT, the customer accounts for it. For direct domestic customers and non-VAT-registered customers, the reverse charge does not apply and you charge VAT as normal.
• Plain clothes claimed as "work clothing"HMRC's rule is explicit: everyday clothing is never allowable, even if you only wear it for work. Branded uniform, hi-vis, knee pads, safety boots — yes. Plain trousers and t-shirts — no.
• Treating new-trade retraining as CPDGas safety refresher, G3 unvented hot water, Legionella awareness, Part L energy efficiency, ACS reassessment — all allowable because they update your existing plumbing skills. A course in electrics or tiling from scratch is not allowable as it's training in a new trade.
• Daily lunch and coffee claimed as subsistenceSubsistence is only allowable on overnight trips away from your normal working area. A meal deal between callouts is not a business expense.
• Forgetting the materials carve-out on CIS-deducted invoicesIf you work as a CIS subcontractor on new-build sites and you invoice materials separately, split labour and materials on the invoice. CIS comes off labour only — mix them and the contractor will deduct 20% on the materials too.
• Missing card-terminal feesSumUp, Zettle and Square typically deduct 1.5–1.7% before money reaches your bank. Across £18,000 of card takings that's £270–£306 a year of pure expense relief, easy to miss because you never see the fee leave your account separately.
• Skipping Gas Safe registration costThe annual Gas Safe registration fee is a mandatory regulatory cost for gas work — fully allowable in Box 30. Plumbers often pay it from a personal account and forget to claim it because it's a regulator, not a supplier.
• Missing the 5 October registration deadlineIf you went self-employed in the 2025–26 tax year you must register for Self Assessment by 5 October 2026. HMRC can charge a "failure to notify" penalty calculated as a percentage of the tax due.