1. Measure and agree what you're pricing
Length, width, eaves height, number of storeys, and whether your dimensions are internal or external. Agree the finish stage with the customer: shell, watertight, or fully finished. Most disputes start with that question unanswered.
2. Below ground
Excavation and muck-away, foundation type and depth (strip or trench fill), concrete, blockwork to DPC, and the ground floor build-up: hardcore, blinding, DPM, insulation, slab or beam and block, screed. If you don't know the ground, carry a provisional sum and say so.
3. The shell
Cavity walls (bricks, blocks, insulation, ties, lintels, cavity closers), the roof structure and covering, fascias, soffits and rainwater goods. Count every opening: windows, doors, bifolds, rooflights — and the knock-through into the house with its steel, padstones and making good.
4. Services and finishes
Drainage (new runs, connections, gullies), heating, electrics, plumbing, then plastering, flooring, decoration, and kitchen or bathroom allowances if they're yours to supply.
5. Preliminaries — the bit that gets forgotten
- Skips and muck-away
- Scaffold (every lift on a two-storey)
- Deliveries and crane or hiab
- Welfare and site set-up
- Protection to the existing house
- Building control fee
- Party wall surveys if needed (usually provisional)
6. Labour
Price labour from hours, not from a percentage of materials: how long does a bricklayer and labourer take per square metre of cavity wall, a roofer per square metre of tiles? Use your real productivity, at cost, then charge-out.
7. Overheads, profit, VAT
Add overheads (van, insurance, office, unpaid quoting time) and profit separately, so you can see them. Then VAT at the right rate. Finally sense-check the total against a cost per square metre for similar jobs you've built.
Do it in 15 minutes instead
BuildLegend asks these questions in this order, works out the quantities, labour and preliminaries, applies your margins and VAT, and gives you a quotation ready to send.
Questions builders ask
How much should a builder charge per square metre for an extension?
It varies too much by specification, region and finish to use as a price — use it only as a check after pricing the job properly. Our worked examples show a full breakdown for each build type at default rates.
What is most often missed when pricing an extension?
Preliminaries (skips, scaffold, deliveries, welfare), the steel and making good at the knock-through, drainage connections, building control, and the difference between watertight and fully finished.
Should I give an estimate or a quotation?
Give a budget estimate while things are uncertain, then a fixed quotation once the open items are resolved. Make clear which one the customer has.