How Much Does a Rear Extension Cost in London 2026?

Rear extension cost ranges in London 2026

Rear extensions remain the most popular home improvement in London, and for good reason - they transform how a property lives without altering its street-facing appearance. In 2026 the market has settled after several years of volatile material pricing, but skilled labour remains in high demand across the capital. Realistic cost ranges for a single-storey rear extension in London look like this: 15 m² from GBP 42,000; 20 m² from GBP 56,000; 25 m² from GBP 70,000; 30 m² from GBP 84,000. These figures assume a mid-specification finish and include structural shell, roofing, insulation, internal plastering, basic electrics and plumbing, and a standard floor finish.

These are starting points, not fixed prices. The final cost depends on your specification level, the structural complexity of the project, site access, and the finishes you choose. A rear extension with bi-fold doors, a roof lantern, underfloor heating and a high-end kitchen fit-out will cost significantly more than the same footprint with standard patio doors and a painted plaster ceiling.

At RCB Design & Build, we quote rear extensions with a detailed cost breakdown separating structural shell from finishes, so you can see exactly where the money goes and make informed decisions about specification levels. As FMB and TrustMark accredited contractors, we stand behind every figure we present.

Cost per square metre by specification level

The per-square-metre rate for a rear extension in London in 2026 varies significantly by specification. Basic specification - standard aluminium patio doors, plasterboard ceiling, vinyl or laminate flooring, basic electrics and no structural glazing - runs GBP 2,400-GBP 2,800 per m². This is a clean, functional finish but without premium features.

Mid specification - bi-fold or sliding doors (aluminium or composite), a single roof lantern, good quality laminate or engineered wood flooring, recessed LED downlights, and a properly finished plastered or painted ceiling - runs GBP 2,800-GBP 3,500 per m². This is the most common specification level our London clients choose and delivers an excellent balance of quality and value.

High specification - slim-framed minimal glazing systems, large-format roof lanterns, polished concrete or natural stone flooring, bespoke joinery, integrated smart lighting, underfloor heating and premium kitchen cabinetry - runs GBP 3,500-GBP 4,500+ per m². At this level the extension becomes a design statement and the finishes account for a larger share of the total cost than the structural shell.

These rates assume standard London site conditions. Properties with restricted access, basement-level complications, significant structural alterations to the existing house, or conservation area requirements will sit above these ranges.

What is included at each price level

At every specification level, the build cost should include: groundworks and foundations (strip or trench fill), structural shell including blockwork and steelwork, flat or pitched roof construction with insulation to current Part L standards, damp-proof course continuation, external drainage connections, internal plastering, first-fix electrics and plumbing, and second-fix to a defined specification. Building Regulations inspections and a completion certificate should also be covered.

What varies by specification is the finish layer: door and window systems, flooring, ceiling treatments, lighting, heating approach (radiators vs underfloor), kitchen fit-out, and decorative finishes. We always present these as clearly separated line items so you can upgrade or downgrade individual elements without losing clarity on the overall budget.

Items commonly excluded from headline build costs - and which catch homeowners off guard - include: architectural design fees, structural engineering calculations, planning application fees, Building Regulations submission charges, party wall surveyor fees, utility disconnections and reconnections, kitchen appliances, and any work to the existing house beyond the immediate connection point. Always ask what is excluded before comparing quotes.

Hidden costs and regional variation across London

Several costs catch first-time extenders by surprise. Thames Water build-over agreements (if you are building over or near a public sewer) can add GBP 400-GBP 800 in fees and 6-8 weeks to the programme. Party wall agreements with neighbours typically cost GBP 900-GBP 1,500 per neighbour when a surveyor is needed. Temporary works - propping, scaffold, skip permits and parking suspensions - commonly add GBP 3,000-GBP 6,000 on tight London terraces.

Regional variation within London is meaningful. Central and West London (W, SW, NW postcodes) typically sits 10-20% above the ranges quoted here, driven by labour competition, parking logistics and higher skip and permit costs. East and South-East London (E, SE, DA postcodes) tracks closer to the ranges above. North London (N postcodes) sits somewhere between. The Kent commuter belt (BR, TN, ME) generally runs 5-10% below outer London pricing.

At RCB Design & Build we present clients with a full all-in project cost at the review stage, not just the headline construction figure. Our Checkatrade reviews consistently highlight this transparency as a key differentiator.

Professional fees breakdown

Beyond the build cost, budget for the professional team that makes the project possible. Architectural drawings for a rear extension typically run GBP 2,500-GBP 4,500 depending on complexity and whether a full planning application is needed. Structural engineering calculations - essential for any project involving steelwork or alteration to load-bearing walls - cost GBP 900-GBP 1,800. Planning application fees are currently GBP 258 for a householder application, or GBP 129 for a Lawful Development Certificate if permitted development applies.

Building Regulations charges run GBP 600-GBP 1,200 depending on project value and whether you use local authority Building Control or an approved private inspector. Party wall surveyor fees, if applicable, are GBP 900-GBP 1,500 per neighbour. A CDM (Construction Design and Management) principal designer appointment - legally required on most domestic projects since 2015 - is often included in the architect or contractor fee but should be confirmed.

For a typical GBP 70,000 rear extension build in London, professional fees and statutory costs add GBP 8,000-GBP 14,000 on top. We recommend holding a further 5-10% contingency on the build cost itself to cover unforeseen conditions discovered during construction. A properly budgeted project with contingency is the single best protection against the stress that comes from running out of money before the project is finished.

Side-return and wraparound combinations

Many London terraces - particularly Victorian and Edwardian properties - have a narrow side return passage between the house and the boundary wall. Incorporating this side return into a rear extension creates a significantly wider open-plan space and is one of the most effective ways to maximise useable floor area without extending further into the garden.

A side-return rear extension typically adds 10-20% to the cost of a standard rear extension of the same depth, reflecting the additional structural steel needed to support the wider span and the more complex roofing and drainage arrangements. For a 20 m² rear extension that becomes 25 m² with the side return incorporated, the uplift might be GBP 8,000-GBP 15,000 - excellent value for the extra space gained.

Wraparound extensions - combining a full rear extension with a side extension - are the most structurally complex single-storey option and typically cost GBP 3,200-GBP 4,200 per m² in London. They require careful structural design at the corner junction and planning is less likely to fall within permitted development. However, for properties where the layout allows it, a wraparound delivers a transformative amount of ground-floor space. Contact RCB Design & Build for a free project review to discuss which configuration works best for your property.