House Extension Cost London 2025 — Single and Double Storey Guide

House Extension Costs in London 2025

A well-executed house extension in London typically adds between £30,000 and £80,000 of market value, while costing between £40,000 and £120,000+ to build. This strong return-on-investment makes extensions one of the most popular projects in the capital.

Single-Storey Rear Extension: £40,000–£70,000

The most requested extension type — a single-storey room added to the rear of the property, typically creating an enlarged kitchen-diner or additional reception room.

This range covers a 20–30m² quality extension with a structural roof (flat or pitched), bifold or sliding doors to the garden, tiled floor, plaster and decoration, electrics and lighting. Kitchen fit-out is additional (£12,000–£30,000).

Double-Storey Rear Extension: £75,000–£120,000

A full double-storey rear extension adding a ground floor room plus a first-floor bedroom and en-suite. More cost-effective per m² than a single-storey extension — you are adding roughly twice the space for roughly 50–60% more in cost.

Side Return Extension: £35,000–£65,000

Popular on Victorian and Edwardian terraces with a narrow side return passage. The extension infills the side return — widening the ground floor and transforming kitchen layout. Often combined with a rear extension to create an L-shaped footprint.

Wrap-Around Extension (Rear + Side): £70,000–£120,000

Combines rear and side return into a single continuous extension. Creates the maximum possible ground floor area within permitted development limits. A transformative change to any terrace.

What Drives Extension Costs

Foundation type: Mini-piled foundations in London clay add cost but may be required. Standard strip foundations are cheaper.

Ground conditions: Any underground services (drainage, inspection chambers) that need diverting add cost. Confirm with your builder early.

Glazing specification: Bifold doors (£3,000–£8,000), structural glazed roofs (£8,000–£20,000), and large skylights are significant variables.

Roof type: A flat GRP roof is cheapest. A zinc or copper standing seam roof is premium but long-lasting. A tiled pitched roof is mid-range.

The kitchen: A new kitchen inside the extension is the single biggest variable — from £12,000 (standard) to £40,000 (fully bespoke).

Call SuperBuilder on 0800 530 0853 for a free site visit and fully itemised quotation.