Full-service appointments are typically charged as a percentage of construction cost — commonly in the 5–10% range depending on project size and complexity — while defined stages (feasibility, tender, inspections only) are quoted as fixed fees. Every fee is agreed before appointment, and on most projects a good project manager recovers their fee through tighter tenders and controlled variations.
The honest answer depends on scale and your own availability. Straightforward single-trade work rarely needs one. For structural alterations, multi-trade projects or anything you cannot supervise yourself, professional management pays for itself at two moments: tendering (a complete specification gets comparable, keener prices) and variations (independent valuation stops costs drifting). A key-stage inspection service is the lighter-touch alternative.
The project manager runs the whole process — feasibility, design co-ordination, statutory approvals, tendering and programme — acting for you throughout. The contract administrator runs the building contract specifically, and acts impartially when certifying payments and completion. On most residential and small commercial projects the same surveyor performs both roles under one appointment.