How do I register for CIS as a plumber?

If a contractor pays you to install heating, water or drainage systems, register online as a CIS subcontractor with your Unique Taxpayer Reference (UTR). Contractors then deduct 20%, not 30%, and not from materials you paid for (GOV.UK, October 2026).

The Construction Industry Scheme (CIS) is the system where a contractor takes tax off your pay and passes it to HMRC. Plenty of plumbing work never touches it, because most boiler and repair jobs are paid by homeowners. But installation work for builders, developers and main contractors usually does. This page follows GOV.UK, HMRC's CIS 340 guide and HMRC's CIS manual as checked on 17 October 2026.

Which plumbing and heating work is CIS

GOV.UK says CIS covers installing systems for heating, power and water. HMRC's manual adds that it applies only to installing those systems, not to their repair or maintenance. That exclusion only works for a job contracted on its own: CIS 340 says if a contract also includes CIS work, every payment under it is CIS.

The test is who pays you as much as what you do. A full heating install paid by a homeowner sits outside CIS. The same install paid by a builder sits inside it.

Steps to register as a subcontractor

1. Check you are self-employed and that at least some of your work is for contractors, not homeowners.
2. If you have no UTR, register as a new business for Self Assessment and choose "working as a subcontractor". GOV.UK says this registers you for Self Assessment and CIS at the same time.
3. If you have a UTR, use HMRC's online CIS registration service. You need your legal business name (and trading name if different), your UTR, your VAT number if you have one, the date you started trading and, depending on your circumstances, your National Insurance number.
4. Give each contractor the exact name and UTR you registered with, so HMRC can verify you at 20%.
5. Keep receipts for every material you buy for contractor jobs. They are what keeps materials out of the deduction.

If you cannot register online, GOV.UK links to a postal form for sole traders. Our subcontractor registration guide covers this side in more detail.

What this means for you

Once you are registered and verified, a contractor deducts 20% from each payment, less materials. If you are not registered, or they cannot verify you, the deduction is 30%. GOV.UK says contractors do not take deductions from VAT, plant hired for the job, consumables, or materials you paid for directly. Each month they must give you a statement of what they paid and deducted, within 14 days of the end of the tax month.

Plumbers buy a lot of materials, so this split matters more for you than for most trades. HMRC's manual says the deduction applies to the part of the payment that is not the direct cost of materials. In our reading, any mark-up you add on materials counts as part of the payment and is deducted from, because only what you actually paid comes off. If you have no receipts, the contractor estimates the cost.

If you pay a self-employed mate or another plumber to work on contractor jobs, you are their contractor too. Register as a contractor by following HMRC's new-employer process, before the first payment and no more than 2 months before you start paying people. Verify them with HMRC before you pay, then pay deductions to HMRC by the 22nd of the month (the 19th by post). Our guide to registering as a CIS contractor explains the monthly routine.

A worked example

Dan is a self-employed heating engineer who is not VAT registered. A builder pays him to install a full central heating system in an extension. His invoice shows £1,800 of labour and £1,200 of boiler, radiators and pipe, which is what he paid his merchant, including VAT.

Registering saves Dan £180 of cash flow on this one job. CIS 340's own example follows the same pattern, taking materials off first, including VAT, and deducting 20% from what is left. GOV.UK says the £360 counts as an advance payment towards Dan's tax and National Insurance bill. The same week, a homeowner pays him for a boiler swap with no deduction at all.

Gross payment status

Plumbers with steady contractor work can apply to be paid with no deduction. As a sole trader you need at least £30,000 of turnover in the last 12 months, ignoring VAT and the cost of materials, tax and National Insurance paid on time, construction work in the UK, and a business run through a bank account. In our reading, a plumber who invoices £50,000 before VAT, with £25,000 of materials has £25,000 for this test. GOV.UK says applying for gross payment status also registers you for CIS if you have not registered yet.

Making Tax Digital and CIS

If your self-employment and property income before costs on your 2025-26 tax return was over £30,000, Making Tax Digital for Income Tax (MTD) starts for you on 6 April 2027, unless it already started in April 2026. See our guide to signing up for MTD as a plumber, use the free MTD check to see where you stand, and visit the trades section for more on CIS.