What a quantity surveyor actually does
On a traditional procurement route (architect design, separate main contractor competitively tendered), the QS provides: pre-tender cost advice during design, preparation of the bill of quantities or schedule of works, formal tender preparation and evaluation, contract administration support including JCT contract drafting, monthly valuation and interim payment certificates, change order valuation and management, loss-and-expense claim assessment, and the final account negotiation at completion. The QS is the client's commercial guardian — separate from both architect and contractor — and provides independent cost intelligence throughout the project. On large London projects (£500k+ full refurbishments, basement excavations, listed-building works) a QS is essentially mandatory because the cost risk and scope volatility are too high for the homeowner to manage alone.
Traditional procurement versus design-and-build
On a traditional contract the contractor builds to the QS's bill of quantities and is paid for what they deliver — any variation (change in design or scope) is valued by the QS and added to the contract sum. This route gives maximum cost transparency but the homeowner carries the cost risk: if quantities are wrong or work is uncovered, the client pays the increase. On a design-and-build contract (the Builderr model), the contractor delivers a defined scope for a fixed price; cost overruns within the agreed scope are the contractor's risk. The trade-off is reduced cost transparency line-by-line but full cost certainty for the client. A QS adds limited value to a well-defined design-and-build contract because the cost protection mechanism is the fixed price itself, not the QS's certification process. Most London renovations under £400k are now procured design-and-build for this reason.
When to engage a QS even on a design-and-build
Some scenarios warrant engaging a QS even on a design-and-build contract: very large projects above £600k where cost intelligence on the contractor's pricing is valuable; heritage or basement projects with high scope uncertainty where the design-and-build price may carry large contingencies the client wants to challenge; projects financed by complex lender structures (bridging finance, development loans) where the lender requires independent QS sign-off on valuations; and projects where the homeowner wants to retain a separate commercial advisor for psychological cost discipline. Fees in those scenarios are typically £4,000–£12,000 fixed (not percentage-based) for a defined commercial advisor scope rather than full contract administration.
