CDM 2015 makes a distinction between domestic clients and commercial clients, who commission construction work as part of their business.
A domestic client is any individual who has construction work carried out on their home, or the home of a family member, that is not done as part of any business. While CDM 2015 places client duties on commercial clients in full, such duties for domestic clients normally pass to:
If a domestic client has appointed an architect (or other designer) on a project involving more than one contractor, they can ask them to manage the project and take on the client duties instead of the principal contractor. The designer then takes on the responsibilities of principal designer and must have a written agreement with the domestic client, confirming they have agreed (as principal designer) to take on the client duties as well as their own responsibilities.
Any designer in charge of coordinating and managing a project is assumed to be the principal designer. However, if they do not have a written agreement with the domestic client to confirm they are taking on the client duties, those duties automatically pass to the principal contractor.