If you’re having work done on your own home or the home of a family member, and it’s not in connection with a business, you’ll be a domestic client. The only responsibility a domestic client has under CDM 2015 is to appoint a principal designer, and a principal contractor when there is more than one contractor.
However, if you don’t do this, (as is common practice) your duties as a domestic client are automatically transferred to the contractor or principal contractor. If you already have a relationship with your designer before the work starts, the designer can take on your duties, provided there is a written agreement between you and the designer to do so.
For builders, here is a useful link to an app for your smartphone to help you to comply with your requirements: