Does CIS apply to plumbers and heating engineers?

Yes, for installing a whole heating, water or sanitation system and for fitting kitchens and bathrooms. As of October 2026, HMRC's manual treats a boiler replacement, an extra radiator, repairs and servicing as outside CIS on their own.

The Construction Industry Scheme (CIS) covers installing systems of heating, drainage, sanitation, water supply and power supply in a building, under section 74 of the Finance Act 2004. HMRC's CIS manual and its CIS 340 guide set out where plumbing and heating work falls. This page follows those sources as checked on 17 October 2026.

What HMRC counts as a system

HMRC's manual page on building service systems defines each one:

• Heating includes central heating, hot air convection and storage heating systems.
• Power supply includes gas, such as a mains gas supply or liquefied gas, as well as electrical wiring.
• Drainage is anything that takes waste water away from a building, such as guttering, downpipes, waste pipes, overflows and external drainage, up to the point they join the main sewer.
• Sanitation means toilets, urinals, hospital sluices and their pipework.
• Water supply can include mains pipework to the property, the stopcock, water meters and the hot and cold pipework to taps, baths, showers and tanks, when done as part of installing the water supply system.

Installing water meters, water softeners and other water treatment systems is also within CIS.

Repairs, boilers and extra radiators

Only installing a system is covered. HMRC's manual lists these as plain repairs, outside CIS: soldering a leaking pipe, patching a leaking boiler, replacing a defective tap, replacing a broken wash basin, fixing a leaking radiator, and swapping standard radiator valves for thermostatic ones.

The boiler is the one that surprises people. The manual says that even where the replaced item is a major part of the system, such as a central heating boiler, replacing it is not installing a system of heating, so it falls outside CIS. Replacing the whole system is different: the manual says replacing a system counts as installing one.

Extending a system is outside too. The manual's example is adding an extra radiator to a central heating system. CIS 340 also lists the repair, servicing and maintenance of heating, drainage, sanitation and water supply systems as excluded.

Three things bring this work back into CIS:

• the repair contract includes making good to the building, such as replastering after a leak
• the work sits in a mixed contract with covered work; the manual's example is one contract to repair a broken gutter, normally excluded, and replace broken roof tiles
• the system is extended under the same contract as an extension to the building

Plumbing and heating jobs, in or out

Kitchen and bathroom fitting appears in HMRC's list of finishing operations, and CIS 340 lists installing and repairing kitchens and bathrooms as included. The manual's example of a contract relating to construction is one to supply and install a fitted kitchen, which is caught because of the installation.

Gas work

Because power supply systems include gas, installing a gas supply in a building is covered, and CIS 340 includes installing gas mains. The same repair rules apply as for heating: fixing, servicing or maintaining a gas supply or boiler on its own is outside CIS.

Supply and fit: the materials part

On a covered job where you supply the boiler, bathroom suite or pipe, the CIS deduction is taken only from the part of the payment that is not the direct cost of materials you paid for. HMRC's manual warns that a "free installation" offer does not make the whole charge materials: if the equipment cost is not shown, the contractor must estimate it. The materials guide explains what counts.

What this means for you

Much plumbing work is outside CIS anyway, because a homeowner paying you direct is not a CIS contractor. The same job done for a builder is a different matter: a bathroom refit through a builder is covered, while a boiler swap for the same builder is not, unless that contract also includes building work. The homeowner guide and the guide to what work is covered by CIS cover the wider rules.

Steps to check a plumbing or heating job

1. Ask who is paying you. A private homeowner paying direct is not a CIS contractor.
2. Ask whether you are installing or replacing a whole system, fitting a kitchen or bathroom, or repairing, servicing or adding to a system.
3. Check whether the contract includes any making good or other building work. If it does, every payment under it is covered.
4. On supply-and-fit jobs, show the materials cost on your invoice and keep the receipts.
5. Keep each monthly payment and deduction statement, and record CIS and non-CIS income separately.

The free MTD check shows when Making Tax Digital starts for you, and the trades section has more guides on CIS.