Gross development value is checked against comparable sales more or less continuously, because
that data is public. Build cost is usually a rate carried across from the last scheme, and it
carries no date.
That asymmetry decides more than it looks. Residual land value sits at the bottom of the appraisal
and absorbs the error in every line above it, so an eight per cent movement in build cost can take
a third of what you can pay for a site. The same rate that loses you good sites in a rising market
lets bad ones through in a falling one.
The second problem is timing. By the time there are drawings worth measuring, the structural
solution, the specification and the procurement route are all chosen, and each of those moved more
cost than the measurement ever will. A surveyor appointed to price a finished design is doing
arithmetic on decisions taken months earlier.
None of this requires anybody to have behaved badly. It is what happens when the only party
without a stake in the outcome is not in the room.