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Extensions

Single-storey extension estimate.

Single-storey rear and side extensions are among the most common domestic projects in the UK. This guide covers what to price, what builders often miss, and how to produce a reliable estimate for extensions up to 40 m².

Authored by Alexandr Vreme & Maksym Vasylkov
Published July 2026 — Next review July 2027

Who this guide is for

This guide is for builders, contractors, and estimators pricing single-storey rear and side extensions up to around 40 m². The approach also applies to larger single-storey works, with adjustments for scale and structural complexity.

Information you need before pricing

A reliable estimate starts with good information. Gather these before you begin:

• Existing house construction type (cavity wall, solid wall, timber frame)
• Foundation design required (strip, trench fill, piled)
• Roof type — flat (GRP, EPDM, felt) or pitched (tiles, slate)
• Glazing specification — double or triple glazing, size of openings
• Kitchen or bathroom fit-out scope within the extension

Typical work breakdown

• Strip-out and enable — clear site, remove doors and windows, break through existing wall openings
• Dig-out and foundations — excavation, hardcore, concrete foundations and slab
• Cavity walls and insulation — blockwork inner and outer leaf, cavity insulation, wall ties
• Lintels and steelwork — lintels over openings, steel beams for wide spans
• Roof structure and covering — joists, insulation, decking, felt and tiles or flat roof system
• Windows and doors — supply and install frames and glazing
• First fix — plumbing and drainage runs, electrical first fix, data cables
• Plastering — dry lining or wet plaster to walls and ceilings
• Second fix — sockets, switches, sanitaryware, radiators, kitchen fit
• Decoration — painting, tiling, flooring
• External works — making good outside, drainage connections, paving

Commonly missed costs

• Underpinning of adjoining walls — required when new foundations are deeper than existing
• Drainage connections — diverting or extending existing drains, new manholes
• Building control fees — full plans or building notice submission
• Skip hire and waste removal — often underestimated for the full build programme
• Scaffolding — access for roofing, cladding, and window installation
• Party wall surveyor fees — if extending close to or on the boundary
• Additional sockets, data points, and lighting — scope creep from last-minute client requests

Miniature worked example

A 25 m² single-storey rear extension with a flat roof and basic fit-out:

• Substructure (foundations and slab): £14,000
• Superstructure (walls, roof, windows, doors): £38,000
• First fix (plumbing, electrics, drainage): £12,000
• Finishes (plaster, tiling, flooring): £16,000
• External works (making good, paving): £6,000

Total: approximately £86,000. Excludes kitchen joinery, decorating, and external paving. Prices vary by region and specification.

Suitable output documents

• Detailed estimate including preliminaries (scaffold, skip, welfare, supervision)
• Priced bill of quantities for transparency
• Schedule of works showing trades and programme

Questions to ask before pricing

• Flat or pitched roof design?
• What is the existing foundation depth? Will underpinning be needed?
• Are any walls on or near the boundary — party wall issues?
• Will the client want a fitted kitchen or bathroom within the extension?