Single-storey extension: how it is priced

What the wizard asks

1. Where it joins the house — Which side of the extension is against the house?, Opening into the house — width
2. Dimensions — Are these internal or external measurements?, Length — depth — how far it comes out from the house, Width — width along the house, Wall height to eaves
3. Roof — Roof type, Pitch, Covering, Rooflights
4. Walls — Wall construction, Outside finish
5. Ground and foundations — Foundation type, Foundation depth, Ground, Floor construction…
6. Windows and doors — Openings
7. Site — Anything to take down first?, Drainage, Access for deliveries and plant
8. How far do you take it? — Stage of completion, Specification level
9. Electrics, heating and plumbing — Sockets and switches, Light fittings, Heating, Plumbing
10. Inside finishes — Plastering, Flooring, Decoration, Internal doors…
11. Inclusions, exclusions and notes — Exclusions, Notes for the estimate

Assumptions this example carries

• Foundations: trench fill 600mm wide × 1.0m deep in GEN1 concrete under all new external walls.
• Ground floor: 100mm concrete slab on 150mm compacted hardcore with 100mm PIR insulation.
• Openings over 2.7m wide may need an engineer-specified lintel or steel; a heavy-duty catalogue lintel has been allowed.
• External walls: facing brick outer skin, 100mm full-fill insulated cavity, 100mm block inner skin.
• Lean-to roof at 22° in concrete interlocking tile, cut on site from C24 timber; rafter length 4.3m.
• Drainage: rainwater and any foul water connected to the existing system within 6m, capacity assumed adequate. Building over or near an existing sewer needs the water company's agreement.
• The existing consumer unit has spare capacity for the new circuits; if not, a new unit is extra.