Detailed plans for builders working across London homes

Why can a builder not price from planning drawings?

Because planning drawings deliberately leave out almost everything that costs money. A proposed elevation shows a window opening and annotates the material; it does not say what the lintel is, how the cavity is closed, what the U-value target is, or where the DPC tray sits. A proposed plan shows a wall; it does not say whether it is load-bearing, what it is built of, or what supports it once an opening is formed.

Handed a planning set, a builder does one of two things. The careful ones price high to cover the unknowns. The rest price low and recover the difference through variations once work has started, when you have no competitive leverage at all. Neither outcome is the builder’s fault — it is a consequence of pricing a drawing that was never meant to be priced.

What does a detailed construction set contain?

• Dimensioned setting-out plans — every wall positioned from a fixed datum, so the builder is not scaling off a drawing to find where a partition goes.
• Construction sections — each floor, wall and roof build-up layer by layer with insulation thicknesses, membranes, cavity closers and U-values.
• Junction details at 1:5 or 1:10 — eaves, verge, threshold, cill, lintel, ridge, and the junction between new and existing construction where cold bridging concentrates.
• Structural layouts — every beam and lintel identified and cross-referenced to the engineer’s calculation sheet, with padstone sizes and bearing lengths shown.
• Drainage layout — falls, invert levels, connections and any build-over agreement position.
• Electrical and services layout — socket, switch and light positions agreed on paper rather than decided by a spark on a Tuesday.
• Written specification — the materials and workmanship standards each trade is pricing against, which is what makes three tenders genuinely comparable.

On domestic work the construction set and the building regulations set overlap heavily, which is why we produce them together. The approver needs the same sections, structure and specification the builder needs, and drawing them once for both purposes is the efficient route.

How do detailed plans change what you pay a builder?

They make competition possible. Three builders pricing the same complete set are answering the same question, so the returns are comparable and the differences are real. Three builders pricing a planning drawing are each pricing a different project, and the cheapest is usually the one who has assumed the least.

The second effect is on variations. Every decision made on a drawing is a decision not made on site, and site decisions are the expensive kind — agreed under time pressure, without alternatives, by someone whose incentive is to keep going. A detailed set moves those decisions into a phase where you can compare options and change your mind for nothing.

The third is evidence. When something is built differently from the drawing, a dimensioned, revision-controlled set establishes what was agreed. Without it, the argument is one person’s recollection against another’s.

Construction drawing questions