CDM (NI)2016 which comes into force on 1st August 2016 makes a distinction between CDMNI2016 domestic clients and commercial clients.
A domestic client is any individual who has construction work carried out on their home, or the home of a family member. Such work must not be done for or as part of any business. While the CDM(NI)2016 Regulations places client duties on commercial clients in full, such duties for domestic clients normally pass to the contractor (if it is a single contractor project), or the principal contractor for projects with more than one contractor. If therefore, the domestic client has not appointed a principal contractor, the client duties must be carried out by the contractor in control of the construction work.
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.