Building Extension Costs and Value — What Drives the Price and What a Realistic Return Looks Like

House extension costs span a wide range — from £25,000 for a modest single-storey rear extension to over £100,000 for a complex two-storey or wrap-around project with high specification. This range is not arbitrary: it reflects genuine differences in floor area created, structural complexity, specification quality, and regional labour and material costs. Understanding what drives the variation is the foundation of a sensible buying decision and a basis for getting quotes that are genuinely comparable.

Cost by Extension Type

The cost hierarchy broadly follows structural complexity and floor area. A single-storey rear extension of 15–20 m² runs £25,000–£45,000. A larger single-storey extension of 25–35 m² costs £40,000–£70,000. A side return infill (8–12 m²) adds £25,000–£45,000 to a property's footprint. A wrap-around combining rear and side return is £55,000–£90,000. A two-storey extension starts at £50,000 and reaches £100,000 or more for a complex project with full planning. These figures include structural work, roofing, insulation, external walls, internal plastering, and electrics — but typically exclude kitchen or bathroom fitting, decoration, floor finishes, and furniture.

Property Value Uplift

A well-executed single-storey rear extension that creates an open-plan kitchen-dining-living space typically adds 10–20% to a UK property's value, though this varies significantly by location, current market conditions, and whether the extension creates a formal additional bedroom. The value added is highest when: a bedroom is added (particularly in high-value markets); the property is below its street ceiling before the extension; and the work is fully compliant with a completion certificate. The value added is lower when the property is already at the top of its local range, or when the extension adds floor area that is not in demand in that market.

Getting Comparable Quotes

The most important practice when soliciting extension quotes is ensuring that every quote covers the same scope. Specify in writing: the extension footprint and height; the roofing type (flat, mono-pitch, pitched) and finish; the glazing system and specification (material, thermal performance, opening type); the insulation specification for roof, walls, and floor (target U-values); the structural engineer's design (included or separate); building regulations application (included or separate); and any internal works (wall removal, beam installation). Quotes that omit line items will appear cheaper but are not comparable — the difference surfaces during the build as a variation order.