CDM 2015, explained: who's responsible for what on a construction site

The Construction (Design and Management) Regulations 2015 (CDM 2015) set out health and safety duties for construction projects in Great Britain, and structure those duties around five named roles: client, principal designer, principal contractor, designers, and contractors. Which one you are — sometimes more than one at once — determines what you're actually required to do.

The roles, briefly

• –Client — commissions the work, and has to make sure suitable arrangements are in place for managing the project
• –Principal designer — coordinates health and safety during the pre-construction phase
• –Principal contractor — coordinates health and safety during the construction phase itself, see our full breakdown of principal contractor duties
• –Designers and contractors — responsible for managing risks arising from their own design or work

Client duties, unpacked

The client's duties are easy to underestimate because they sound administrative — "make suitable arrangements" — but they carry real weight. A client has to make sure the project is set up so it can be carried out safely: allowing sufficient time and resources at every phase, providing relevant pre-construction information to the designers and contractors bidding on the work, and making sure welfare facilities will be provided for the duration of construction. On any project needing a principal designer and principal contractor, the client has to appoint both in writing, and appoint them early enough that they can actually influence how the project is planned — not bolted on once design decisions are already locked in.

Principal designer duties, unpacked

The principal designer's job is to plan, manage, monitor and coordinate health and safety during the pre-construction phase — which in practice means during design, before anyone's on site with tools. That includes identifying, eliminating or reducing foreseeable risks that the design itself creates, making sure designers cooperate with each other and share relevant information, and putting together the health and safety file that gets handed over at the end of the project for anyone maintaining or altering the building later. A principal designer who treats this as a paperwork exercise, rather than genuinely reviewing what the design is asking site teams to build and how, is missing the point of the role.

Designers and contractors, unpacked

Designers — architects, engineers, anyone preparing drawings or specifications — have to eliminate foreseeable risks where they reasonably can, and where they can't, reduce and then manage what's left, giving the principal designer the information needed to coordinate that across the whole project. Contractors, meanwhile, have to plan, manage and monitor their own work so it's carried out without risk to health and safety, provide their workforce with the information, instruction and training they need, and — for anyone running a crew of their own subcontractors — make sure those subcontractors are complying with the regulations too. It's worth noting these duties apply to sole traders and self-employed people on site as well, not just registered businesses.

Domestic clients — a different default

Domestic clients — someone having work done on their own home, not as part of a business — are treated differently. Their client duties transfer by default to the contractor (on a single-contractor project) or the principal contractor (on a multi-contractor project), unless the domestic client agrees in writing to take those duties on themselves, which is unusual and generally not advisable unless they're genuinely equipped to hold them. This is a common area of confusion on smaller residential jobs where everyone assumes someone else is holding the client duties, when by default the responsibility has actually landed on the contractor.

Where confusion actually happens

The most common source of confusion isn't the roles themselves — it's projects involving more than one contractor, where CDM 2015 requires a principal designer and principal contractor to be appointed, and a construction phase plan to be in place before work starts. Smaller projects, or those with only one contractor, have lighter requirements — but "only one contractor" is a narrower category than it might sound once subcontractors are involved. A single main contractor who brings in three specialist subcontractors for different trades is, for CDM purposes, very likely running a multi-contractor project, even if the client thinks of it as "one contractor doing the whole job."

What this looks like when it goes wrong

In practice, duty confusion tends to surface at the worst possible moment — after an HSE inspector has already turned up, or after an incident, when someone needs to establish who was actually responsible for a given control measure. A site with no principal contractor formally appointed, but with three trades working concurrently and nobody coordinating between them, is exactly the scenario CDM 2015 exists to prevent. It's also the scenario most likely to produce an improvement or prohibition notice, because the absence of coordination is visible the moment an inspector starts asking who's in charge of what.

Why this matters beyond the site itself

Duty confusion under CDM 2015 doesn't stay contained to a health and safety conversation — it tends to surface in contracts and insurance too. A professional indemnity or public liability insurer will generally want to know who held which CDM duty when something goes wrong, and a contract that's silent or vague on who's acting as principal designer or principal contractor leaves that question open at exactly the point it matters most. Getting the appointments right in writing, early, isn't just a regulatory box to tick — it's what makes the rest of the project's risk allocation actually hold together if something does go wrong later.

It's also worth remembering that appointments can change hands mid-project — a principal contractor going into administration, or a client switching principal designer partway through design development. When that happens, the gap between one appointment ending and the next one starting is a genuine risk window, and it's worth having a clear process for closing it quickly rather than assuming duties are somehow still being covered informally.

Common mistakes

• –Assuming a project has "one contractor" when subcontractors make it multi-contractor in substance
• –Appointing a principal designer or principal contractor late, after key design or planning decisions are already made
• –Domestic clients not realising their duties have defaulted to the contractor, and nobody picking them up as a result
• –Treating the health and safety file as an end-of-project formality rather than a document someone will actually rely on later
• –Confusing "designer" under CDM with the architect specifically — the duty applies more broadly than that
• –Leaving a gap unaddressed when a duty-holder changes mid-project, rather than confirming the handover explicitly