Short answer. The Construction (Design and Management) Regulations 2015 apply to every construction project in Great Britain, however small: clients, designers and contractors all have basic duties from day one. Only the duty to notify the HSE, and to appoint a principal designer and principal contractor, depends on the size or structure of the project.
What CDM is for
Construction is hazardous. People work at height, structures collapse, people are struck by equipment, excavations cave in, ground turns out to be contaminated. The Construction (Design and Management) Regulations 2015 (CDM 2015), supported by HSE's guidance L153, manage health and safety through the whole life of a project, from initial design through planning, the construction work itself, and eventual demolition.
CDM's central idea is to spread responsibility beyond the people doing the physical work. It puts duties on designers and clients as well as contractors, on the basis that hazards managed earlier mean fewer people get hurt later.
Does CDM apply to every project?
Yes. CDM 2015 applies to all construction work, commercial or domestic, regardless of how long it takes or how many people are on site. A single builder replacing a roof on a small extension is still doing work covered by CDM, and the client, designer and contractor all have basic duties, such as planning the work and managing risks sensibly.
Two things are genuinely threshold-based, and it is worth being precise about which is which. The client must appoint a principal designer and a principal contractor, in writing, whenever more than one contractor is, or is expected to be, involved in the project, whatever its size. Separately, the client must notify the HSE only where the project is expected to last longer than 30 working days and have more than 20 workers on site at any one time, or where it will involve more than 500 person-days of construction work. A project can be well below the notification threshold and still need a principal designer and principal contractor, because that duty turns on the number of contractors, not on the size of the job.
The key roles
The client
The client is whoever commissions the project. For a company building a warehouse, the client is the company; for a homeowner having an extension built, the homeowner is the client, though most of their duties pass to the contractor or principal contractor by default (see below).
The client's role is to notify the HSE where the project meets the threshold, appoint a principal contractor and principal designer where more than one contractor is involved, check that the people being appointed are competent, and confirm a health and safety file is created and kept. The client does not need to be a health and safety expert, but does need to take these steps.
The principal designer
The principal designer manages health and safety during the design phase, and is usually an architect or engineer. Their role is to manage the design process, remove or reduce hazards through design choices, check that the design can be built safely, and pass information about remaining hazards to contractors.
For example, if a design specifies working at height, the principal designer should consider whether the design could avoid that height work altogether. Where height work cannot be avoided, the design should make it as safe as reasonably practicable, and the residual hazards should be flagged to the people who will build it.
The principal contractor
The principal contractor manages health and safety during the construction phase, and is usually the main contractor hired to run the work. Their role is to plan the work, manage the other contractors and workers on site, run the site itself, and supervise how the work is actually carried out.
Other designers and contractors
Any other contractor or designer on the project, such as a specialist subcontractor, must cooperate with the principal designer and principal contractor, manage their own work safely, and share information about the hazards their work creates.
Domestic clients
Where the client is a homeowner rather than a business, most client duties transfer automatically: to the contractor where there is only one contractor, or to the principal contractor where there is more than one. The domestic client can choose in writing to keep the duties themselves, but in practice most do not, and the professionals on the job carry the responsibility instead.
The health and safety file
One of CDM's key outputs is the health and safety file: a document gathering information about the construction, the design, the hazards found, and how they were managed. Once construction finishes, the file is handed to the client and kept for future maintenance, refurbishment or demolition.
For example, a maintenance worker who later needs to drill into a wall should be able to check the file for what lies behind it, such as electrical cables, water pipes or structural elements, so they do not drill into something dangerous. Many organisations produce a bare-minimum file simply because they have to. A good one is genuinely useful to whoever works on the building next.
Common misunderstandings
"We're just doing a small office fit-out; CDM doesn't apply."
CDM applies to the fit-out regardless of size. If more than one contractor is involved, such as a principal contractor subcontracting to electricians, plumbers and decorators, the client must appoint a principal designer and principal contractor, whatever the overall scale of the job.
"We're using the same contractor we've used for years; we don't need a principal contractor."
If someone is appointed to manage the construction and more than one contractor is on site, they are the principal contractor, however long you have worked with them. Their competence, and whether they have been properly appointed, is what matters.
"CDM is the contractor's responsibility; we're just the client."
The client has real duties: appointing competent people, checking notification happens where it applies, and confirming a health and safety file is created. A client cannot hand off all responsibility simply by hiring a contractor.
"We don't need a principal designer; the architects are doing the design."
Architects are involved in design, but that does not automatically make them the principal designer. The role needs to be explicitly assigned to someone who then manages health and safety across the whole design process.
"The principal contractor will sort out health and safety."
The principal contractor manages health and safety during construction, but they are only one part of the picture. The principal designer manages it during design, and the client is responsible for appointing the right people. It is a shared responsibility, not one person's job.
Competence, the linchpin
A recurring theme in CDM is competence: people appointed to key roles must have the knowledge, experience and training to do the job, not simply a qualification, though a qualification helps. HSE's guidance on competence points to practical questions:
- Has the person done this kind of work before?
- Do they understand the current regulations and current best practice?
- Are they organised, with documented systems?
- Can they identify and manage the hazards on this type of project?
Many organisations appoint people without seriously weighing competence. A structural engineer might be technically excellent but have never run design-phase CDM before. A large contractor might be highly competent at general building work but not at a specialist task. Competence is specific to the role and the project type, not a general reputation.
Where organisations go wrong
- Tick-box compliance: producing documents and checklists to show compliance, without genuinely managing hazards.
- Unclear appointments: never formally appointing a principal designer and principal contractor, so no one feels responsible.
- Poor communication: the principal designer and principal contractor not talking to each other, so design information never reaches site.
- A weak health and safety file: a bare document produced as an afterthought, rather than built up with useful information as the project goes on.
- Assuming small means exempt: treating a small project as outside CDM, when the basic duties apply regardless of size.
Getting the fundamentals right
CDM is detailed because construction is genuinely hazardous, but the underlying structure is straightforward: clear responsibilities for the client, the principal designer and the principal contractor, competent people in each role, hazards identified and managed as early as possible, and information shared rather than held back. Get those fundamentals right and the specifics become manageable. For businesses working through their duties on a specific project, York Green's health and safety consultancy service can help clients, designers and contractors work through CDM from notification and appointment to completion of the health and safety file.