Plympton’s housing stock is more varied than it looks from the road. Inside that mix of post-war estates, 1970s infill and Edwardian fringes the build-ups, foundations and existing services range widely. Two rear extensions of the same square metreage can come in 30% apart for reasons that have very little to do with the design.

The variables that consistently move the number: existing foundation depth and adequacy, the route of foul and surface drainage, party wall implications, access for plant, and the realistic finish level the client is aiming for. A take-off that prices those properly is what you want; a per-m² rate isn’t.

If you’re weighing two Plympton quotes that look wildly different, that’s usually where to start looking.