Why the boundary matters
The Construction Industry Scheme requires contractors to deduct money from payments to subcontractors and pass it to HMRC as an advance against the subcontractor's tax and National Insurance. For 2026/27 the deduction rate is 20% for a registered subcontractor, 30% for an unregistered one, and 0% for a subcontractor who holds Gross Payment Status. Those rates apply to the labour element of the payment only, not to materials.
CIS applies to construction operations as defined in Finance Act 2004 s.74, with the operations that are excluded set out in s.74(3). It does not apply to every trade that operates in or around buildings. Getting the boundary wrong creates real problems in both directions. A subcontractor incorrectly treated as inside CIS is subject to deductions and compliance obligations that do not apply to them. A subcontractor incorrectly treated as outside CIS avoids deductions that the law requires, exposing the contractor to penalties and the subcontractor to the 30% unregistered rate once HMRC picks up the error.
Understanding what the Construction Industry Scheme is and how it works is the starting point. This guide focuses specifically on where the boundaries of the scheme sit and which activities fall outside them.
Work that IS within CIS
Before examining the exclusions it is useful to set out what the scheme covers. Construction operations under Finance Act 2004 s.74(2) include: building, altering, repairing, extending, demolishing or dismantling buildings or structures; installing heating, lighting, ventilation, drainage, water supply or fire protection systems; civil engineering works (roads, railways, pipelines); groundwork, excavation, tunnelling, laying foundations and site clearance preparatory to a construction project; and painting or decorating internal or external surfaces. The scope is wide. Most trades working on a construction site are included, and the exclusions below are specific carve-outs rather than a general narrowing of scope.
Work that is NOT within CIS
Professional services
Architects, structural engineers, surveyors, site managers and other construction professionals providing advisory, design or supervisory services are excluded from CIS, provided they are not also personally carrying out construction operations. The exclusion follows the nature of the work performed, not the professional title. An engineer who produces drawings and specifications is outside CIS. An engineer who both designs and then carries out the physical installation under a design-and-build arrangement may find the construction element of the payment falls within scope. The line sits between advisory and physical work.
Materials-only supply
A supplier who delivers materials to a site without carrying out any installation or construction work is outside CIS for that transaction. CIS only bites on the labour and construction-services element of a payment. Once a subcontractor both supplies and installs materials, the labour element of the total payment falls within CIS, but the cost of the materials themselves remains excluded from the deduction base. This distinction between supply-only and supply-and-install is one of the most practically important in the scheme, because it determines both whether CIS applies at all and, if it does, what figure the deduction is calculated on.
Drilling and extracting (oil, gas and minerals)
Operations involving drilling or extracting oil, gas or other minerals are specifically excluded from the definition of construction operations. These trades operate under a separate tax regime. A subcontractor whose work is entirely in oil and gas extraction has no CIS obligations, and a contractor in that sector does not make CIS deductions from payments for drilling and extraction operations.
Security system installation (alarms, CCTV, public address)
The installation of security systems, including burglar alarms, closed circuit television (CCTV) and public address systems, is one of the operations expressly excluded from CIS by Finance Act 2004 s.74(3). Because the exclusion is set out in the statute, it holds even where the installation is carried out alongside a construction or refurbishment project: an express statutory exclusion is not pulled back into scope by the incidental-work rule. A specialist alarm or CCTV installer is therefore outside CIS for that work. The only point to watch is a single contract that bundles excluded security work with genuine construction operations, where only the construction element is within CIS and a clear invoice split keeps the treatment clean.
Manufacturing and prefabrication
The manufacture of building components, including prefabricated structures, off-site fabrication and the production of materials, is outside CIS at the manufacturing stage. Delivering those components to a site is also outside CIS. CIS becomes relevant only when construction operations, including installation of the manufactured components, begin on site. A manufacturer who delivers and installs a prefabricated bathroom pod is in a different position from one who simply delivers it: the delivery is outside CIS, but the installation brings the labour element within scope.
Cleaning (other than construction cleaning)
Ordinary cleaning of buildings and structures is not a construction operation. A commercial cleaning company that cleans finished office premises is not doing construction work. The exclusion does not, however, cover all cleaning. Internal cleaning carried out in the course of construction, alteration, repair, extension or restoration of a building is within CIS. The distinction is between routine operational cleaning and cleaning that forms part of or directly follows a construction project. Post-construction builder's cleans sit in a grey area where the context of the wider project will determine the answer.
New residential builds for the owner-occupier
Where an individual commissions a builder to construct their own home, the homeowner is not a contractor under CIS and has no CIS obligations. The builder is the contractor, must be registered for CIS, must verify any subcontractors used on the project and must make deductions at the correct rate. The homeowner is the end consumer of the construction work, outside the scheme entirely. This rule does not apply to property developers building for sale: a developer building homes for the market is a contractor for CIS purposes.
The grey areas
HMRC guidance is not definitive on every type of work, and a number of common activities sit in genuinely uncertain territory.
Carpet fitting, blinds and furniture assembly
HMRC's view is that carpet fitting, blind installation and furniture assembly are generally not construction operations. They are treated as closer to furnishing or fitting-out than to construction or decoration. However, if they form part of a contract that is itself a construction contract, the incidental-work rule can pull them in. A standalone carpet fitter working independently on a finished building is in a different position from a fitting-out subcontractor engaged under a construction contract where flooring is one line item among many.
Scaffolding
Unlike the examples above, scaffolding is clearly within CIS. Erecting and dismantling scaffolding on a construction site is a construction operation. This surprises some contractors because scaffolding is temporary and does not form part of the permanent building, but Finance Act 2004 s.74 treats it as within scope and HMRC's position has been consistent. A scaffolding subcontractor must be registered, the contractor must verify their CIS status, and deductions must be applied at the correct rate.
Summary table: common activities and CIS status
| Activity | CIS status | Notes |
|---|---|---|
| Bricklaying, carpentry, plastering | Within CIS | Core construction operations |
| Plumbing, electrical, HVAC installation | Within CIS | Building systems installation is included |
| Civil engineering (roads, pipelines) | Within CIS | Within scope under Finance Act 2004 s.74 |
| Groundwork, excavation, demolition | Within CIS | Preparatory and demolition operations included |
| Scaffolding (erection and dismantling) | Within CIS | Commonly misunderstood; clearly within scope |
| Painting and decorating | Within CIS | Internal and external surfaces included |
| Architecture, surveying, engineering consultancy | Outside CIS | Excluded unless the professional also does physical construction work |
| Materials-only delivery (no installation) | Outside CIS | Delivery without installation is excluded |
| Materials supply with installation | Labour element within CIS | Materials cost remains excluded from the deduction base |
| Drilling and mineral extraction (oil/gas) | Outside CIS | Separate tax regime; expressly excluded by Finance Act 2004 s.74(3) |
| Off-site manufacturing and prefabrication | Outside CIS at manufacture stage | Installation on site brings the labour element within scope |
| Ordinary cleaning (not construction-related) | Outside CIS | Cleaning in the course of construction is within scope |
| Carpet fitting and blind installation | Generally outside CIS | Context-dependent if part of a construction contract |
| Alarm, CCTV and public address system installation | Outside CIS | Expressly excluded by Finance Act 2004 s.74(3); the exclusion holds even alongside a construction project |
| Homeowner commissioning a new build | Homeowner outside CIS | Builder is the contractor; homeowner is the end consumer |
The incidental-work rule
The boundary between inside and outside CIS is not drawn solely by reference to what an activity is in isolation. Finance Act 2004 s.74(2) brings within scope work that is integral to, preparatory to, or for rendering complete a main construction operation. If an activity that is not itself listed as excluded is performed as part of and in connection with a construction project, HMRC may treat it as caught by CIS. This is the rule that determines whether a post-construction clean or a carpet fitting falls in or out: the question is not simply "what trade is this?" but "is this work part of a construction contract?" One important limit: operations that are expressly excluded by s.74(3), such as professional services and security system installation, are not pulled back into scope by this rule. The incidental-work rule only operates on activities that are not already carved out by statute. The same subcontractor doing the same non-excluded physical task may be inside or outside CIS depending entirely on whether their engagement is standalone or part of a broader project.
The £3 million deemed-contractor threshold
One other boundary is worth flagging, because it catches businesses that do not think of themselves as being in the construction industry at all. A business that spends £3 million or more per year on construction work is classed as a deemed contractor under CIS, even if construction is not its trade. Large retailers, property developers, and commercial landlords overseeing major refurbishment programmes can fall within this threshold. A deemed contractor has the same CIS obligations as a conventional construction contractor: it must register, verify subcontractors before paying them and file monthly CIS300 returns. Failing to notice the threshold applies is one of the more costly registration errors in the scheme.
For a full picture of the monthly contractor obligations once registration is in place, including the CIS300 filing deadline on the 19th of the following tax month and the reinstated nil-return requirement from 6 April 2026, our guide to contractor monthly responsibilities under CIS covers the ground in detail.
Why registration errors in either direction are costly
Registering for CIS unnecessarily is not harmless. It creates a monthly CIS300 filing obligation, requires subcontractor verification before every payment, and ties the business into the compliance regime. A contractor who incorrectly treats payments to a professional services firm as CIS deductions may also create disputes with the payee and expose itself to claims for the wrongly deducted amounts.
Failing to register when required is more serious. An unregistered subcontractor suffers deductions at the 30% rate rather than 20%, a 10-point gap on every labour payment that accumulates quickly. A contractor who pays subcontractors without registering, verifying or deducting correctly is exposed to penalties from HMRC. From 6 April 2026, under Finance Act 2026, sections 62A and 62B of Finance Act 2004 charge a payer, or return-maker, who knew of a connected party's deliberate CIS failure a penalty of 20% of the payment (or of the sums the return treats as paid), and where a company is involved HMRC can pursue its officers personally under the existing officer-liability rules. If you are uncertain whether your work falls within CIS, taking specialist advice before HMRC raises an enquiry is the sensible step. You can see how we approach CIS compliance and registration on our services page.