Self Assessment for Plumbers: Expenses You Can Claim and Records to Keep

Most plumbers don't mind the tax bill. What they hate is the week in January spent digging through a van glovebox for a fuel receipt from last May. Your bill comes down to two things: how much you earned, and how much of what you spent you can prove.

This guide covers the basics for a sole trader plumber in the UK: registering, the dates that matter, what counts as an expense, and a routine that keeps the paperwork under control. It isn't tax advice for your exact situation, so where the detail gets fiddly we link to GOV.UK rather than guess.

Registering With HMRC as a Sole Trader

If you work for yourself, you report your profit to HMRC through Self Assessment. You must tell HMRC by 5 October after the end of the tax year in which you started trading. The tax year runs from 6 April to 5 April, so someone who started in June 2026 has until 5 October 2027. Don't leave it that late, because registration takes time to come through.

Start with GOV.UK's guide to setting up as a sole trader, and use check if you need a tax return if you're unsure. The wider picture is in how to set up a plumbing business.

The Dates That Matter

For the 2025 to 2026 tax year (6 April 2025 to 5 April 2026), GOV.UK currently gives these deadlines:

• 31 October 2026: paper return must reach HMRC.
• 30 December 2026: online return deadline if you want a small bill collected through your tax code.
• 31 January 2027: online return deadline, and the date the tax you owe is due.
• 31 July: the second payment on account, if you make them (see below).

Late filing and late payment bring penalties and interest. Bookmark GOV.UK's deadlines and penalties pages, because dates roll forward every year. You can file any time after 5 April, so filing early means knowing your bill months ahead.

What You Can Claim as an Expense

An allowable expense is a cost of running the business that you deduct from income before tax. GOV.UK's example: £40,000 turnover with £10,000 of allowable expenses means Income Tax on £30,000. The full list is on its allowable expenses page. For a plumber, the usual suspects are:

• Materials and stock for jobs: pipe, fittings, fixings, boilers, consumables.
• Tools and equipment. How you claim them depends on your accounting method (see below).
• Van and travel costs: fuel, parking, servicing, insurance and repairs, or the flat mileage rate instead.
• Insurance: public liability, employer's liability if you employ people, tools cover. Our guide to public liability insurance for plumbers explains each.
• Training courses related to your business, such as refreshers.
• Phone and internet: only the business share of the bill.
• Software and subscriptions: accounting apps, trade journals, quoting tools.
• Advertising: website, van signwriting, listings.
• Bank charges, and staff or subcontractor costs.
• Protective clothing: safety boots, overalls, gloves. GOV.UK is clear that everyday clothing such as ordinary shoes or jeans doesn't count, even if you only wear it to work.

Two rules catch people out. If something is used for both business and personal reasons, you claim only the business part. And money you take out of the business for yourself isn't an expense.

Simplified Expenses vs Actual Costs

Instead of adding up every van running cost, you can claim a flat rate per business mile, which covers fuel, insurance, repairs and servicing. GOV.UK's rates for cars and vans were 45p a mile before 6 April 2026. For the 2026 to 2027 tax year it's 55p for the first 10,000 business miles, then 25p after that.

Made-up example: a Leeds plumber drives 8,000 business miles in 2025 to 2026. At 45p that's £3,600. If their real fuel, servicing, insurance and repair bill for the van was higher, they could claim actual costs instead. HMRC has a simplified expenses checker to compare the two. Once you use the flat rate for a vehicle, you stick with it for as long as it's in the business, and you can't claim both for the same vehicle.

There's also a flat monthly rate for working from home on admin if you do 25 hours or more a month there. The tables are on GOV.UK's simplified expenses pages.

Cash Basis or Traditional Accounting

From the 2024 to 2025 tax year, cash basis is the default. You record income when the money arrives and expenses when you pay, so you don't pay tax on invoices that haven't been settled. You can opt out and use traditional accounting, where income and costs go in by invoice date.

The method affects how bigger purchases like a van are treated. Rather than guess, read GOV.UK's capital allowances page or ask an accountant before a big purchase.

The £1,000 Trading Allowance

If your gross trading income for the year is £1,000 or less, you may not need to tell HMRC. If it's more, you can deduct a flat £1,000 instead of actual expenses, but not both. A working plumber's real expenses almost always beat £1,000, so it mostly matters for the first weeks of trading or occasional weekend jobs. The rules, including when you can't use it, are on GOV.UK's tax-free allowances page.

If You Work as a Subcontractor (CIS)

If you plumb for a contractor, the Construction Industry Scheme probably applies. The contractor deducts money from your payments and passes it to HMRC. GOV.UK says the deduction is 20% if you're registered and 30% if you're not. These count as advance payments toward your tax and National Insurance, so report them on your return and they reduce what you owe.

Keep every payment and deduction statement. GOV.UK's guide for CIS subcontractors covers registering and gross payment status. It also matters when you're paid by the day, which we touch on in how to price plumbing jobs.

Payments on Account

The first year can sting. HMRC usually wants the tax for the year just ended and a first payment towards the next year, both on 31 January. A second payment on account follows on 31 July.

Each payment is half of last year's tax bill. Earn more than expected and you pay the difference as a "balancing payment" the next January. Earn less and you can ask HMRC to reduce them. You don't have to make them if last year's tax was under £1,000, or if you paid more than 80% outside Self Assessment. GOV.UK's payments on account page has worked examples.

Move a share of every payment you receive into a separate savings account. Our guide to getting paid on time helps with the other end of that problem.

Records You Need to Keep

GOV.UK says sole traders must keep records of business income and expenses, for at least five years after the 31 January submission deadline of the relevant tax year. Keep:

• invoices you issued, and bank statements showing the money in
• receipts for every expense, paper or photographed
• mileage notes (date, from, to, miles, job) if you use the flat rate
• CIS deduction statements, if you subcontract
• evidence of job dates and customers, such as your diary or booking confirmations

That last one is where call records help. If you use something like Slotby to answer and book jobs, the call summaries and confirmation texts give you a dated trail of who rang and what was booked. Handy when you can't remember if a job was in March or April. It's a side benefit, and the pricing page explains how it works.

A Simple Monthly Routine

Thirty minutes at the end of each month beats a weekend in January:

1. Photograph receipts the day you get them.
2. Log mileage after each job.
3. Update your app or spreadsheet with money in and money out.
4. Match invoices to payments and chase anything overdue.
5. Move tax money into a separate account.
6. Back up your records somewhere other than your phone.

If your income is climbing, check Making Tax Digital for Income Tax. We cover the thresholds in our VAT guide, and GOV.UK's eligibility page has the current dates and a checking tool.

Common Mistakes

• Registering late, or assuming HMRC will find you.
• Mixing personal and business spending in one account.
• Claiming everyday clothing or personal phone use.
• Claiming mileage and fuel for the same vehicle.
• Spending the tax money.
• No receipts. If you can't show it, don't claim it.
• Losing CIS statements, so deducted tax isn't credited.

Quick Checklist

• Registered with HMRC, UTR stored safely.
• 31 January in your calendar.
• Business bank account separate from personal.
• Receipts photographed weekly, mileage logged per job.
• Accounting method chosen (cash basis is the default).
• Tax money saved from every payment.
• Records kept for five years after the 31 January deadline.

Next, see when to register in our VAT guide, or what changes with your first employee in should a plumber hire staff.