Under The Building Regulations 2010 (as amended), a domestic client is a client where the project is being carried out for non-business reasons.
A contractor is someone who carries out the work, and can be a client, but specifically cannot be a domestic client.
If a domestic client is carrying out the work themselves (i.e. DIY), then there would be no contractors. 11C of the regulations states:
(2) If a domestic client fails to make the appointments required by regulation 11D (principal designer and principal contractor)—
(a) the designer in control of the design phase of the project is the principal designer;
(b) the contractor in control of the construction phase of the project is the principal contractor.
What would happen in this case? There are no contractors so would there just be no principal contractor?