The employer's representative under a design and build contract. Because the contractor controls the design in D&B procurement, the contract assigns the employer's administrative functions — instructions, payment notices, change control, completion certification — to an employer's agent, who performs them and protects the employer's interests from brief through to final account.
Procurement route decides it. On a design and build contract the employer's agent is the recognised role, exercising the powers the contract gives the employer. On traditional procurement, a project manager (with a contract administrator) fills the equivalent space. The skill set overlaps heavily — which is why the same RICS surveyors provide both, matched to how your project is contracted.
Before the building contract is signed — ideally at brief stage, so the employer's requirements are drafted by the person who will later police them. The requirements document determines what the contractor must deliver; appointing after it is written means inheriting whatever gaps it contains, and those gaps are where D&B projects disappoint.