How Much Does a House Extension Cost in London? A Practical 2026 Guide

A plain-English guide to what a house extension costs in London in 2026 — indicative ranges for single-storey, double-storey and loft projects, and the factors that move the price.

“How much does an extension cost?” is the first question almost every homeowner asks — and the honest answer is that it depends. Every home, site and specification is different, so the only accurate figure is one based on your actual project. That said, indicative ranges are useful for early planning and budgeting. This guide sets out realistic 2026 cost ranges for the most common types of extension in London, and explains the factors that push a price up or down. All figures are indicative, exclude VAT and professional fees, and should be confirmed with a site-specific estimate.

What affects the cost of an extension?

• Size and number of storeys — cost broadly scales with floor area, but going up a storey adds structural and roofing work.
• Specification and finishes — kitchens, bathrooms, glazing, flooring and joinery can vary enormously in price.
• Structural complexity — steel beams, foundations, ground conditions and removing load-bearing walls all add cost.
• Services — moving or extending heating, plumbing, drainage and electrics.
• Access and location — restricted access, party walls and working in a dense London street affect logistics and price.
• Planning and design — architect, structural engineer, planning and building control fees sit on top of build costs.

Single-storey rear or side extension

A single-storey rear or side-return extension is the most popular way to enlarge a London home, typically to create an open-plan kitchen-diner. As an indicative guide for 2026, build costs commonly fall in the region of £2,500–£3,500 per square metre for a good-quality finish, with higher specifications and complex sites costing more. A kitchen or bathroom fit-out, high-performance glazing or bi-fold doors will push the figure towards — and beyond — the top of that range.

Double-storey and wrap-around extensions

Adding a second storey is often more cost-efficient per square metre than a single-storey extension, because you gain floor area while sharing foundations and roofing across two levels. Wrap-around extensions (combining a rear and side-return) deliver the most space but involve more structure, roofing and internal reconfiguration, so the total project cost is higher even where the rate per square metre is competitive. These projects benefit most from careful design and a fixed, itemised specification before work starts.

Loft conversions

A loft conversion is often the best value way to add a bedroom and en-suite without losing garden space. Costs depend heavily on the type — a simple rooflight (Velux) conversion is the most economical, while dormer and hip-to-gable conversions cost more because they involve altering the roof structure. Head height, staircase position and the need to meet building regulations for fire escape and insulation all influence the final figure.

Do you need planning permission?

Many single-storey extensions and loft conversions can be built under permitted development rights, without a full planning application — but limits apply, and permitted development does not exist in every situation (for example, some flats, conservation areas and listed buildings are excluded). You will usually still need building regulations approval, and where you build up to or near a boundary the Party Wall etc. Act 1996 may require agreements with neighbours. It is always worth checking your specific circumstances before committing to a design.

Getting an accurate estimate

Ranges are only a starting point — the reliable number comes from a proper survey and a detailed specification. RS Construction and Property Services Ltd delivers full refurbishments, extensions and multi-trade building work for homeowners, landlords and developers across East London and London. We provide clear, itemised estimates so you know what is — and is not — included before any work begins. Contact us for a free, no-obligation estimate on your project.