One question decides it
Under CDM 2015 a client must appoint a principal designer and a principal contractor when a project involves more than one contractor. That is the trigger. There is not a second one.
Not the value of the work. Not the floor area. Not how long it runs. Not whether it is notifiable to HSE. Those are all things people reach for, and all of them are the wrong test.
Contractor means more than you think
A contractor is anyone who manages or carries out construction work. A one-person business with no employees is a contractor. So is the electrician who comes for a day.
Which is how ordinary jobs cross the line without anybody noticing. A kitchen fitter and a separate electrician are two contractors. That project needs a principal designer and a principal contractor, and nobody on it would call it a construction project or think the regulations were in play.
Domestic work does not escape it. The duties move.
The comfortable belief is that CDM is for commercial sites and a house extension is outside it. It is not. What changes for a domestic client is who carries the client duties, and the answer is somebody else, automatically.
- More than one contractor: the client duties pass to the principal contractor, on top of their own.
- One contractor: they pass to that contractor.
- Unless a designer has agreed in writing with the client to take them on, which is the only way they land anywhere else.
That last point is the one that catches architects and designers. Manage the project for a domestic client without a written agreement and the duties do not sit with you. Have the agreement and they do. Either way somebody is carrying them, usually without having been told.
What the two roles are actually for
The principal designer takes control of the pre-construction phase: planning, managing and monitoring it, and coordinating health and safety while the design is still moving. The whole idea is to design risk out before anybody is on site, which is the only point at which it is cheap.
The principal contractor does the same job for the construction phase: planning, managing and monitoring the work, and coordinating the contractors doing it.
One organisation can hold both where it genuinely has the capability for each. On a small design-and-build job that is often sensible. Where design carries on into the build, it is worth asking whether both roles are being done or neither is.
The plan, whoever ends up writing it
Separate from all of this, a construction phase plan is needed on every project, including the small ones and including domestic work. The construction phase plan builder covers what has to be in it and why the small-job version is not a cut-down version of the big one.
The task-level documents sit underneath it. The risk assessment and the method statement cover the individual jobs, and what a risk assessment actually is covers the difference between the two, which is asked constantly.
Notification to HSE is a different question again, with its own thresholds about duration and worker numbers. Needing a principal designer does not mean the job is notifiable, and a notifiable job is not automatically one with two contractors. Check that test separately.
Common questions
What actually triggers needing a principal designer?
More than one contractor on the project. That is the whole test. If there is only one contractor you need neither a principal designer nor a principal contractor. The moment a second one is involved, the client must appoint both, in writing. It is not triggered by the value of the work, the floor area, the duration or whether the job is notifiable, which are the things people assume and then get wrong in both directions.
Does CDM apply to work on someone’s house?
Yes, and the common belief that it does not is the most expensive misunderstanding in the regulations. What changes for a domestic client is not whether CDM applies but who carries the client duties. They pass to somebody else automatically: to the principal contractor where there is more than one contractor, or to the contractor where there is only one. A designer can take them on instead, and only where there is a written agreement with the client saying so.
Am I a contractor if I am a sole trader?
Yes. A contractor is anyone who manages or carries out construction work, and that includes a one-person business with no employees. This matters because it is how small jobs cross the line without anybody noticing: a kitchen fitter and a separate electrician are two contractors, so the project needs a principal designer and a principal contractor even though nobody involved would describe it as a construction project.
Can the same person be the principal designer and the principal contractor?
They can, provided they genuinely have the skills, knowledge, experience and organisational capability for both, and the two roles cover different phases. The principal designer plans and coordinates the pre-construction phase, the principal contractor the construction phase. On a small job with a design-and-build firm that is often the sensible answer. On anything where design continues into the build it is worth asking whether one organisation is really doing both jobs or quietly doing neither.
What does a principal designer actually do?
They take control of the pre-construction phase: planning, managing and monitoring it, and coordinating health and safety while the design is being worked out. The point is to design risk out before anybody is on site, because that is when it is cheap and possible. In practice the most visible output is pre-construction information going to the people who will build it, and the health and safety file at the end.
Is this the same as notifying HSE?
No, and mixing the two is common. Whether a project is notifiable to HSE on form F10 is a separate test with its own thresholds about duration and the number of workers. A project can need a principal designer and a principal contractor without being notifiable, and the reverse is possible too. Check the notification thresholds on the HSE site rather than assuming one answer settles the other.