The principal designer is the designer appointed to control the design phase where more than one contractor is involved. Under the CDM Regulations 2015 and the Building Safety Act the client must make the appointment in writing.
The role plans, manages and monitors the design phase, coordinates design information and the flow of health and safety information, and contributes to the golden thread for higher-risk work. At the end of the appointment the principal designer hands over the information it holds.
The building control dutyholder version of the role sits alongside the CDM version. For smaller firms the point is the same: someone competent has to own the design information and be able to evidence it.