CIS deductions, van expenses, Gas Safe registration costs and Making Tax Digital explained plainly for UK plumbers in 2025/26.
If you sub-contract on building sites, HMRC is probably already collecting tax from you before you see a penny: a CIS-registered plumber paid GBP 30,000 in labour by a contractor has GBP 6,000 deducted at source, sitting with HMRC until you claim it back through Self Assessment. Fail to claim it and you effectively pay tax twice on the same money. That single mistake costs more plumbers more cash than any other error in the trade.
As a sole-trader plumber you pay Income Tax on your profit (turnover minus allowable expenses), plus Class 4 National Insurance Contributions. For 2025/26 the first GBP 12,570 of profit is covered by the personal allowance and taxed at zero. From GBP 12,571 to GBP 50,270 the Income Tax rate is 20%; above GBP 50,270 it rises to 40%. Class 4 NIC sits on top at 6% between GBP 12,570 and GBP 50,270, then drops to 2% above that.
Your working life as a plumber usually splits three ways: domestic call-outs where the householder pays you direct; larger project work for builders and developers; and sub-contracting on construction sites where CIS kicks in. Each income stream hits the same Self Assessment return, but the CIS stream brings a complication that the others do not.
When a contractor pays you for site work, they deduct CIS only from the labour element of your invoice. Materials you supply are paid gross. This matters because it determines how much has been withheld and therefore how much you can reclaim.
Use the CIS tax calculator to work out exactly what has been deducted across the year and what you should get back. HMRC matches the figures against the contractor's monthly CIS returns, so your Self Assessment submission and the contractor's records must align. If a contractor has deducted the wrong amount, chase the correction before you file.
If you are not yet registered with HMRC as a CIS sub-contractor, register immediately. The difference between a 20% and a 30% deduction rate on, say, GBP 40,000 of annual labour is GBP 4,000 of additional cash tied up with HMRC for up to 22 months. That is a significant working-capital hit for a one-person operation.
This is where your taxable profit shrinks to something fair. Every item below is genuinely specific to your trade, not a generic list lifted from an accountancy website.
| Expense | What you can claim |
|---|---|
| Tools and equipment | Pipe cutters, blow torches, pressure test equipment, hand tools, power tools, multimeters for electrical testing |
| Materials and consumables | Copper and plastic pipe, fittings, solder, PTFE tape, flux, valves, radiators bought for resale to the customer |
| Van running costs or mileage | Fuel, insurance, servicing, road tax, MOT, finance interest on a van loan (actual-cost method), or 45p/mile for first 10,000 miles then 25p/mile (simplified method) |
| Public liability and tools insurance | Essential cover; the full premium is allowable |
| Gas Safe registration | Annual registration fee, reassessment costs, and any revalidation training required to maintain your licence to practise |
| Trade body and professional fees | WaterSafe, CIPHE membership fees, SNIPEF subscriptions, and similar |
| Work clothing, boots and PPE | Boots, overalls, gloves, safety glasses. General clothing that could be worn off-site does not qualify |
| Merchant accounts and card fees | Trade-account admin charges, credit-card processing fees, card reader subscription costs |
| Phone and broadband | The business-use proportion of your mobile and home broadband |
| Advertising and quoting costs | Website hosting, job-management software (Commusoft, Jobber, etc.), postage |
For materials that you buy and sell on, remember only the cost of goods that you have actually supplied forms part of your allowable expenses; you are taxed on the margin, not the full sale price, because the sale price is in your turnover too.
Your van is almost certainly your single largest expense after materials. HMRC gives you two methods and you must pick one per vehicle per tax year and stick to it.
The simplified mileage method pays 45p for each of the first 10,000 business miles and 25p per mile thereafter. It is clean, needs only a mileage log, and suits plumbers with lower-mileage patterns or a high-value van with large depreciation that the flat rate does not reflect.
The actual-cost method captures fuel, insurance, servicing, road tax, MOT, breakdown cover and finance interest. If you bought the van outright, you claim a capital allowance (the Annual Investment Allowance) on the purchase price instead of depreciation. You must also keep all receipts and apply a private-use reduction if the van is used for any personal journeys.
The mileage calculator lets you compare both methods against your own figures before you commit. Once you have used actual costs for a van, you cannot switch to mileage rate for that vehicle.
VAT registration is compulsory once your taxable turnover in any rolling 12-month period exceeds GBP 90,000. That is not a tax-year test; it is a rolling window. A plumber who invoices GBP 8,000 a month including materials can cross the threshold mid-year without realising it.
Materials are the danger here. A bathroom refurb with GBP 4,000 of sanitaryware and fittings looks like a big materials invoice but all of it counts towards your rolling total just the same as labour. If you are approaching GBP 90,000, monitor your last 12 months of sales every month, not once a year at filing time. Register with HMRC within 30 days of the date you exceeded the threshold; late registration brings a penalty and a backdated VAT debt.
Once registered, domestic customers cannot reclaim VAT so your quotes become more expensive to them in practice. Many plumbers in this position consider the VAT Flat Rate Scheme, though the trade-specific percentage means it is worth modelling carefully before committing.
Samir is a CIS-registered sole-trader plumber. In 2025/26 his invoices total GBP 55,000. Of that, GBP 18,000 relates to materials he has supplied; the remaining GBP 37,000 is his labour.
His contractor deducts 20% CIS from the labour element only:
Samir's allowable expenses for the year:
Taxable profit: GBP 55,000 minus GBP 24,750 = GBP 30,250
Income Tax on GBP 30,250 (after GBP 12,570 personal allowance):
Class 4 NIC on GBP 30,250 (above GBP 12,570):
Total tax and NIC due: GBP 3,536 + GBP 1,061 = GBP 4,597
But Samir already has GBP 7,400 sitting with HMRC as CIS deductions. His Self Assessment return produces a refund of GBP 2,803 rather than a bill. That refund disappears entirely if he forgets to enter his CIS deductions on the return.
Use the sole trader tax calculator to model your own figures with your actual expenses and CIS deductions before filing.
Making Tax Digital for Income Tax (MTD for ITSA) replaces the annual Self Assessment return with quarterly digital submissions for most self-employed people. From April 2026, plumbers with self-employment or property income over GBP 50,000 must file four quarterly updates per year through MTD-compatible software, plus a final end-of-period statement. From April 2027 the threshold drops to GBP 30,000.
For a plumber this means keeping digital records of every invoice and expense as you go, not reconstructing the year from bank statements and receipts in January. TapTax connects to HMRC's MTD API and lets you log income and expenses from your phone between jobs. The full MTD for sole traders guide covers the transition timeline, the software requirements and what happens if you miss a quarterly deadline.
CIS complicates MTD slightly because the deductions your contractor has made must still be reported accurately at year-end. The quarterly updates capture income and expenses; the CIS credit is applied in the final declaration. Make sure whatever software you use handles CIS sub-contractor reporting, not just CIS contractor reporting.
Forgetting to claim CIS deductions. This is the biggest one. The money has already left your contractor and gone to HMRC. If you do not enter the CIS deducted figure on your Self Assessment return, HMRC keeps it and you have paid tax twice on the same labour. Always collect your monthly CIS statements from every contractor you have worked for.
Mixing mileage methods. Switching from mileage rate to actual costs, or vice versa, mid-year on the same van is not allowed. If you realise actual costs would give a larger deduction, you must wait until the next tax year to switch.
Counting materials as pure profit. Some plumbers report the full invoice value as profit without deducting the cost of materials. The materials are an expense; only the margin is taxable income.
Ignoring the rolling VAT window. A strong quarter of commercial work can push the rolling 12-month total past GBP 90,000 before the tax year is out. Check your running total monthly if you are anywhere near the threshold.
Claiming non-specialist clothing. A branded polo shirt that could realistically be worn off-site does not qualify. Boots, hard hats, hi-vis jackets and proper PPE do.
A CIS-registered plumber with GBP 7,400 withheld by contractors can receive the whole amount back as a Self Assessment refund. But only if they claim it.
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