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Supported projects → Two-storey extension
A two-storey side extension under scaffolding, bricklayer on the lift and a labourer loading blocks
extension · provisional

Two-storey extension: how it is priced

A ground floor plus a bedroom or bathroom above. It needs its own foundations sized for two storeys, scaffold on every lift, a floor between, and fire and sound separation the single-storey job never sees.

FICTIONAL WORKED EXAMPLE · 4m × 5m (external) · 32.6 m² internal · finish: complete
£80,073.60
inc. VAT at 20% · £66,728.00 before VAT · about £2,049.39/m² · roughly 8 weeks on site
Materials (71 lines, catalogue allowances dated 2026-09-24)£17,959.30Labour (830 person-hours at a blended £30/hr)£24,886.05Provisional allowances (5)£6,780.00Plant£830.00Overheads 10% + contingency 5%£7,568.31Profit (15% markup)£8,703.55
WHERE THE MONEY GOES (AT COST)
External walls£10,965.90Roof£6,034.95Structural elements£5,492.80Internal finishes£3,524.75Windows & external doors£3,190.00Insulation & internal linings£3,052.30Kitchen & bathroom£2,960.00Floors£2,238.40

Open the full line-by-line example →

Default answers, a blended £30/hr labour rate and dated catalogue allowances. Your job, your rates and your merchant prices will differ — that is the point of the tool. Templates marked provisional are being validated against measured projects; every estimate says so.

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 from the house, Width — width along the house, Wall height per storey
  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. First-floor windows and doors — Openings, Internal doors upstairs
  8. Site — Anything to take down first?, Drainage, Access for deliveries and plant
  9. How far do you take it? — Stage of completion, Specification level
  10. Electrics, heating and plumbing — Sockets and switches, Light fittings, Heating, Plumbing
  11. Inside finishes — Plastering, Flooring, Decoration, Internal doors…
  12. Inclusions, exclusions and notes — Exclusions, Notes for the estimate

Assumptions this example carries

  • The new first floor is tied into the existing house wall; a doorway is formed through the existing first-floor wall (non load-bearing assumed).
  • 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.
  • External walls: facing brick outer skin, 100mm full-fill insulated cavity, 100mm block inner skin.
  • 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.
  • Heating: 2 radiators added to the existing system; the existing boiler is assumed to have capacity.

Priced work packages

Preliminaries & site setup · Site preparation · Demolition & clearance · Excavation & foundations · Floors · External walls · Roof · Windows & external doors · Drainage · Plumbing & heating · Electrical · Insulation & internal linings · Internal finishes · Kitchen & bathroom · Scaffold & access · Plant hire · Waste removal · Deliveries · Testing & certification · Fees & approvals · Structural elements

Provisional sums

  • Structural design for openings and the first floor where spans exceed 4.5m
  • Roof trusses
  • Scaffolding (two lifts)
  • Building control fees
  • Kitchen and bathroom supply

Not included

  • Planning application, drawings and party wall matters
  • Furniture, appliances and loose fittings
  • Landscaping and reinstatement beyond the working area
  • Anything found on opening up that could not be seen at survey
  • Alterations to the existing first floor beyond forming a doorway
Related build types
A builder measuring the new blockwork of a single-storey rear extension on a 1930s semi

Single-storey extension

Rear or side extension on one level — kitchen-diners, living rooms, utility rooms.

A roofer fixing tiles on the pitched roof of a new extension, suburban rooftops behind

Wraparound extension

Rear extension plus a side return, joined into one L-shaped space.

Price your own two-storey extension

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