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 and what most plumbers get wrong.

When is it due?

For the 2025/26 tax year (April 2025 to April 2026), the deadline for online self assessment is 31 January 2027. The paper deadline is earlier — 31 October 2026. File online. It’s easier, you get longer, and you get an instant calculation of what you owe.

Don’t leave it until January. The earlier you file, the earlier you know what you owe, and the longer you have to budget for it. You don’t have to pay early — payment isn’t due until 31 January regardless of when you file.

What you need before you start

• Your UTR number (on any HMRC letter)
• Your National Insurance number
• Your total plumbing income for the year
• Your total business expenses broken down by category
• Your mileage log if claiming simplified mileage

If you’ve been tracking your jobs and costs in an app like PlumberLog throughout the year, you can generate a tax report that gives you most of these figures in one tap. If you’ve been using a shoebox of receipts, set aside a few hours to add everything up first.

The self employment pages — where everything goes

Once you’re logged into HMRC’s self assessment online, you’ll fill in the self employment section (SA103 or SA103F). Here’s where your plumbing-specific figures go:

Box 15 — Turnover (your total income)

The total amount you invoiced or were paid for plumbing work during the tax year. Include everything — cash jobs, bank transfers, cheque payments.

Box 17 — Cost of goods

Parts and materials you bought specifically for jobs. Copper pipe, fittings, solder, boiler parts, radiator valves, sealant, tape. This is not your tools — it’s the consumable materials that go into the customer’s property and stay there.

Box 20 — Wages and staff costs

If you hired a labourer or subcontractor during the year, their cost goes here. Most sole trader plumbers leave this blank.

Box 24 — Travel costs

Your mileage claim goes here if you’re using simplified expenses (55p per mile for the first 10,000 miles, 25p after that). Also include parking charges, congestion charges, and any public transport for business journeys.

Box 25 — Phone, fax, stationery, office costs

The business proportion of your phone bill (most plumbers claim 50–75%). Also accounting software subscriptions and any other admin costs.

Box 27 — Insurance

Your public liability insurance premium, professional indemnity if you have it, and tool insurance if it’s a separate policy.

Box 29 — Other allowable expenses

This is the catch-all and it’s where most plumber-specific costs go:

• Gas Safe registration fees
• CIPHE membership
• Training courses and CPD
• Workwear and PPE (steel toe boots, hi-vis, knee pads)
• Tool replacement and repair
• Waste disposal charges and skip hire

Common mistakes plumbers make

Forgetting to claim mileage

This is the single biggest missed deduction. A plumber doing 8,000 business miles a year at 55p/mile is losing £4,400 from their expenses — meaning they’re paying roughly £880 more tax than they need to.

Claiming mileage AND van running costs

You can’t do both. It’s one or the other for each vehicle. If you’re using the 55p rate, you cannot also claim fuel receipts, insurance, or servicing for the same van.

Not including Gas Safe fees

Your annual Gas Safe registration is a fully allowable business expense. So is any renewal of your qualifications or any training courses to maintain your competence. Many plumbers forget these because they feel “personal” — they’re not, they’re 100% business.

Mixing personal and business expenses

If you’re claiming for your phone, only claim the business proportion. If you’re claiming for your van, only claim business mileage. HMRC will not accept 100% claims on items that clearly have personal use.