Wraparound extension: how it is priced

What the wizard asks

1. Where it joins the house — The side return runs along which side of the house?, Total width of openings into the house
2. Dimensions — Are these internal or external measurements?, Length — rear part — depth from the house, Width — rear part — width along the house, Side return — length along the house…
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

• The rear and side parts share one internal junction; no wall is priced between them and the roof is treated as continuous.
• 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.
• Flat roof: warm-deck construction with 120mm PIR, 47×195 C24 joists at 400 centres with firrings to fall, epdm rubber roof membrane with adhesives & trims.
• 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.