Realistic starting ranges for substantial residential projects in London and Hertfordshire. Every figure depends on design, structure, access, specification and existing building condition.
Use the build cost calculatorA single-storey rear extension in London or Hertfordshire commonly sits within this range including VAT. The upper end usually reflects larger structural openings, significant drainage changes, extensive glazing, a new kitchen and detailed internal finishes.
Side return projects can look compact but often require careful structural work, party wall coordination, drainage changes and roof glazing. Combining a side return with a full ground-floor remodel generally moves the project towards the top of the range.
A straightforward dormer conversion may sit near the lower end. Mansard roofs, lowered ceilings, complex steelwork, conservation constraints, multiple bathrooms or detailed joinery increase the overall construction budget.
This range covers coordinated structural, mechanical, electrical and finishing work across the property. The final figure depends heavily on floor area, the condition of the existing house, layout changes, heating systems, windows, kitchens, bathrooms and bespoke elements.
What the ranges mean: these are initial construction allowances including VAT, not quotations. Professional fees, planning charges, party wall surveyors, temporary accommodation, finance and major client-supplied items may sit outside the building contract.
Select a project type, location and specification level. The result is a planning allowance only; a fixed price requires drawings, specification and a site assessment.
Share your drawings, planning status and intended finish so the main cost drivers can be reviewed.