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Self Employed Plumber Tax Return — Step by Step Guide 2026

Updated June 2026 · 10 min read · PlumberLog Team

Filing a self assessment tax return for the first time as a self employed plumber is daunting — not because it’s difficult, but because nobody explains it in plumber-specific terms. This guide walks through the actual process, with real examples of where plumber-specific expenses go.

When is it due?

For the 2025/26 tax year, the online deadline is 31 January 2027 (paper: 31 October 2026). File online — it’s easier and gives an instant calculation.

What you need before you start

• Your UTR number and National Insurance number
• Your total plumbing income for the year
• Your total business expenses broken down by category
• Your mileage log — see New 55p Mileage Rate 2026 for how it’s calculated

The self employment pages — where everything goes

Box 15 — Turnover

Everything you invoiced or were paid — cash, bank transfers, cheques.

Box 17 — Cost of goods

Copper pipe, fittings, solder, boiler parts, radiator valves — not your tools.

Box 20 — Wages and staff costs

Any labourer or subcontractor you hired.

Box 24 — Travel costs

Your mileage claim (55p/mile for the first 10,000, 25p after), plus parking, congestion charges and tolls. Full method comparison in Can I Claim My Van as a Self Employed Plumber.

Box 25 — Phone, stationery, office costs

The business proportion of your phone bill, plus software subscriptions.

Box 27 — Insurance

Public liability, professional indemnity, and tool insurance.

Box 29 — Other allowable expenses

• Gas Safe registration fees
• CIPHE membership
• Training courses and CPD
• Workwear and PPE
• Tool replacement and repair
• Waste disposal charges and skip hire

Common mistakes plumbers make

Forgetting to claim mileage

The single biggest missed deduction — 8,000 miles at 55p/mile is £4,400 unclaimed.

Claiming mileage AND van running costs

It’s one or the other for each vehicle.

Not including Gas Safe fees

Your annual registration and CPD training are 100% business expenses.

Mixing personal and business expenses

Only claim the business proportion of your phone or van.

How to make next year painless

Log every job, every cost, every mile — daily, not retrospectively. If you’re near the £50,000 threshold, this habit also gets you ready for Making Tax Digital, which replaces the annual return with quarterly updates.