What can a self-employed plumber claim?
The boilers, cylinders and radiators you supply, the pipe and fittings on the van, Gas Safe registration and the five-year reassessment, the analyser and press tool, and the van itself all come off your profit before tax. Here is how supply-and-fit work changes the sums, why the VAT threshold arrives sooner than it does for other trades, and a worked year for a heating engineer.
HMRC-recognised
CIS deductions handled
No monthly subscription
What a self-employed plumber can claim
Boilers, cylinders and radiators you supply
A combi you fit on Tuesday, the unvented cylinder for Thursday, towel rails and radiators for a full system: each one is bought by you and sold on to the customer as part of the job. The purchase goes under Goods for resale (HMRC's goods used box), and the full price on your invoice, mark-up included, is turnover. Registering the warranty with the manufacturer is part of finishing the job and adds nothing to the tax picture. If the customer buys the boiler themselves and you only fit it, nothing goes in this box and your invoice is labour.
Pipe, fittings and what sits on the van
Copper and plastic pipe, press and compression fittings, valves, inhibitor, a magnetic filter, flux and solder are Goods for resale too. Stock you buy ahead on the merchant account is deducted when you pay for it under the cash basis, so a van restocked in March is a cost of that tax year even if half of it is still in the racking on 5 April. PTFE, jointing paste, abrasive cloth, blades and gas for the torch are used up rather than sold on, and belong in Other expenses.
Gas Safe and the five-year reassessment
You cannot touch a gas appliance for payment without being on the Gas Safe Register, so the yearly registration is a cost of being in business, recorded as Other expenses, and so is the fee for notifying each installation to building control through the register. The ACS reassessment every five years keeps a qualification you already use current, which is allowable training, and a course on heat pumps or unvented systems when you already fit heating counts the same way. Learning a different trade from scratch does not.
Press tool, analyser and test kit
A press-fit tool and its jaws, a pipe bender, a flue gas analyser, pressure testing kit, a pipe freezer, a drain camera and a core drill are equipment. In FileThat they go under Repairs and maintenance, the box HS222 gives tools under the cash basis, and each is deducted in the tax year you pay for it however much it cost. The analyser's yearly calibration and a new sensor go in the same box, because keeping kit working is maintenance.
Old boilers, scrap and the tip
Carting the old boiler and cylinder away is part of most heating jobs. What the transfer station charges you to take them, and registering as a waste carrier so you can carry it in your own van, are costs of the work: both in Other expenses. What the scrap yard pays you for copper cylinders and cast iron is the other way round. It is income of the business, even in cash, so record it as a sale rather than letting it disappear into your pocket.
Insurance, software and the office
Public liability, cover for the tools in the van overnight and employer's liability if you take on a mate are Insurance. A job management app for quotes and service reminders, the work share of your mobile, the accountant and bookkeeping software all come off the profit too. Van livery, a website and the listing that brings in boiler enquiries are Advertising costs.
The rules behind every claim: allowable expenses for sole traders.
The van, the merchant and the day's jobs
Pick one of two methods for the van and do not switch while you own it: HMRC's flat mileage rates, 55p a mile for the first 10,000 business miles in the tax year and 25p after that, or the business share of what it really costs to run, from diesel and insurance to tyres and the service. A van bought outright is deducted under the cash basis in the year you pay, less any share for private use, and from then on only that van's real running costs can be claimed, never the flat mileage rate; a car is never deducted that way.
A heating engineer's week moves house to house: a boiler swap takes a day or two, a service an hour, a leak whatever it takes. Working from home, each drive to a customer and each detour to the merchant for a cylinder is business travel. Where it changes is the long contract. First-fixing pipework on plot after plot of one development for most of a year can be treated as where the business is carried on, and the drive there becomes ordinary commuting.
A mileage log with the date, the address and the miles is enough. In FileThat each mileage entry works out the deduction at the flat rates, and the fuel and running costs for a van on actual costs take a business-use percentage so only the business share is filed.
Both methods, and which drives count: can I claim mileage.
CIS and VAT
Installing a heating system is CIS
Finance Act 2004 names the installation in any building of systems of heating, water supply, drainage and sanitation, so when a builder or developer pays you to pipe and fit a full central heating system, a bathroom or the plumbing on a new build, they take 20% off your labour, or 30% if you are not registered, and never off materials. Turnover is the gross amount before the deduction. Enter the figures from each monthly statement the contractor gives you in FileThat so the tax they have paid for you counts against your bill.
A boiler swap on its own is not
HMRC's CIS manual says replacing a central heating boiler, even though it is a significant part of the system, is not the installation of a system of heating, so on its own it is outside the scheme, and so are adding a radiator, fixing a leak and servicing. The deduction comes back in when the boiler or repair is part of a mixed contract of works that includes making good to the building. A homeowner is never a CIS contractor whatever the job, and a letting agent or other business outside construction only becomes one after spending £3 million on construction in any 12 months.
Half the invoice can be materials
On supply-and-fit work the boiler, cylinder and pipe you bought can be half of what you bill, and all of it comes off before the contractor works out the deduction, VAT included if you are not VAT registered. Put it on its own line with the cost. If you do not, or cannot show what you paid, the contractor is allowed to put its own figure on them and take 20% from the rest, and if it values them low, more of the 20% lands on what was really a £3,000 boiler, money you do not get back until the return is filed.
VAT arrives sooner for supply-and-fit
You must register for VAT once taxable turnover in any rolling 12 months passes £90,000, or as soon as you expect to pass it in the next 30 days. Because every boiler and cylinder you charge on counts in turnover, a heating engineer on £45,000 of labour who also supplies £45,000 of kit is at the threshold, where a labour-only trade on the same labour is half way there. Register within 30 days of the end of the month you went over. Once registered, work for a VAT-registered contractor under CIS usually falls under the domestic reverse charge, recorded in FileThat under Construction industry - reverse charge sales.
See what you could get back: the CIS refund calculator.
A supply-and-fit heating engineer's year
Take a heating engineer in England who, through 2026-27, supplies and fits boilers, cylinders and radiators for homeowners and invoices them £58,000, roughly half of it the kit itself. They also install the heating systems in a builder's extensions for £22,000, showing £12,000 of boilers and materials on those invoices, so the builder deducts 20% of the £10,000 labour, £2,000, and pays over £20,000. Scrap from old cylinders and boilers brings in £600. Turnover is £80,600.
Costs for the year: £39,000 of boilers, cylinders, radiators, pipe and fittings (the £12,000 charged on to the builder among them), £1,800 of kit (a press tool, an analyser and pressure testing gear), £120 to calibrate the analyser, £400 for Gas Safe registration and notifications, 9,000 miles between jobs (£4,950 at 55p), £650 of insurance, £300 for the work share of the phone, £350 of tip charges and the waste carrier registration, £240 of job software and £300 for the accountant. Together that is £48,110, so the profit is £32,490.
The Personal Allowance takes the first £12,570, leaving £19,920. Income Tax at 20% on it is £3,984, and Class 4 National Insurance at 6% adds £1,195.20, a bill of £5,179.20. HMRC holds the £2,000 the builder deducted, so £3,179.20 is left to pay by 31 January 2028. Because less than 80% of the year's tax was taken at source and the balance is over £1,000, two payments on account of £1,589.60 each follow for the next year, the first on the same day.
Two things the materials do to this year. Turnover of £80,600 sits £9,400 short of the £90,000 VAT threshold, and the test is any rolling 12 months, so a cold autumn with a few more boilers would tip them over. And Making Tax Digital looks at turnover before expenses on an earlier year's return, so once this year's return is filed a profit of £32,490 is beside the point: £80,600 is over every threshold. Scottish rates and bands are different, so a Scottish taxpayer's bill comes out a little differently.
What you cannot claim
Trousers that are not protective
Knee pads, safety boots, gloves and a polo shirt printed with your business name are allowable. The work trousers, hoodie and trainers you could wear to the pub are ordinary clothes, whatever a loft or a drain does to them.
A meal deal between jobs
Lunch from the garage between a service and a boiler swap is food you would have bought anyway, so it is private. A meal is a business cost only when the work takes you away overnight or well outside your usual pattern.
Your own boiler or bathroom
A combi for your own house, or a bathroom for your parents at cost, is private even when the kit comes through the trade account at trade price. Record it and mark it disallowable so it stays out of the profit.
Fines, interest and drawings
The parking ticket outside a customer's flat, interest on tax paid late, the money you take out for yourself, and the tax charged on what is left at the end of the year never reduce it.
The detail behind each: boots, knee pads and branded tops, eating on the job and miles, parking and tickets.
Straight answers
Is Gas Safe registration tax deductible?
Yes. Gas work for payment is only legal for someone on the Gas Safe Register, so the annual registration fee is a cost of carrying on your trade, recorded as Other expenses, and the per-job fee for notifying an installation through the register goes with it. The ACS reassessment every five years is allowable as well: it keeps current a qualification you already use rather than giving you a new one, which is the line HMRC draws for training. Keep the invoices with the records.
Do I pay CIS on a boiler swap?
Not when the swap is the whole job. HMRC's manual says replacing a boiler is not the installation of a system of heating, so a like-for-like swap, an extra radiator or a repair is outside CIS even when a business pays for it, and a homeowner is never a contractor at all. The deduction applies when a contractor pays you to install a full heating system or the plumbing in a new build or extension, or when the boiler forms part of a wider contract that also makes good the building.
When do I have to register for VAT as a heating engineer?
When your taxable turnover for the last 12 months, counted at the end of any month rather than at 5 April, passes £90,000, or when you expect it to pass that in the next 30 days alone. Turnover means everything you invoice, including the boilers and cylinders you charge on and the gross amount before any CIS deduction, not your labour or your profit. You have until 30 days after the end of the month you went over to register, and you are registered from the first day of the second month after it.
Can I claim van stock I have not used yet?
Yes, under the cash basis. An expense counts when you pay for it, not when the fitting goes into a customer's wall, so pipe, fittings and a spare pump bought on the merchant account in March are a cost of the year you paid even if they are still on the van at 5 April. There is no stock adjustment to make. Only if you later use something from the van on your own house does it need recording as disallowable.
Is the money the scrap yard pays me for old boilers taxable?
Yes. Weighing in copper cylinders, brass fittings and old boilers is part of how a heating business earns, so what the yard pays is income of the trade and belongs in your sales, even when it is paid in cash or into a personal account. The flip side is that the tip charges and waste carrier registration you pay to dispose of what cannot be sold are allowable, so record both sides rather than netting them off in your head.
Can I claim a flue gas analyser and its calibration?
Yes. The analyser is equipment, recorded as Repairs and maintenance, and under the cash basis the whole cost is deducted in the year you pay for it, whatever it cost. The yearly calibration and a replacement sensor are maintenance of that equipment and go in the same box. The same treatment covers a press tool, a pipe bender, pressure testing kit and a drain camera, so a year with a lot of kit bought can take a visible chunk off the tax bill.
Do self-employed plumbers have to use Making Tax Digital?
If your qualifying income is over the threshold, yes. It has been £50,000 since 6 April 2026, and falls to £30,000 from April 2027 and £20,000 from April 2028. Qualifying income is turnover before any expenses come off, your self-employment plus any property you let, which for supply-and-fit work means every boiler, cylinder and length of pipe you charged on counts, along with the gross CIS amount before the deduction, so a heating engineer reaches it on far less labour than a labour-only trade does.
Keep every claim, file it with HMRC
The income and expenses ledger is free all year, built for plumbers working from a phone on site. Each new quarterly update costs from £3.99, with 4 free credits to start and no monthly subscription.
Get started free