House Extension Cost London: 2026 Price Guide

What Actually Drives House Extension Costs in London in 2026

The single biggest driver of extension cost in London isn't finish quality, it's structural complexity at the point where the new build meets the old house. Almost every Victorian and Edwardian terrace in the capital sits on shallow, tapered brick footings, typically 450-600mm deep, laid long before concrete strip or trench-fill foundations going down to 1-1.5m or more became standard. A new extension foundation dug to modern depth right next to that old shallow footing creates a cold joint, and if that junction isn't detailed and, where necessary, underpinned by a structural engineer, differential settlement shows up as diagonal cracking at the abutment within a year or two of completion. This single detail, more than cladding choice or glazing spec, is why two extensions of identical floor area on the same road can carry very different price tags.

Site access is the second major cost driver specific to London terraced streets. Rear gardens accessed only through a narrow side passage, or via next door's permission, mean muck-away and materials have to be barrowed rather than skipped straight from a lorry, which adds real labour hours that don't show up in a national average cost-per-square-metre figure. Add to that the higher day rates for groundworkers, bricklayers and structural steel erectors in London compared with the rest of the country, and the same specification of single-storey rear box that might cost £1,800/m² in the Midlands routinely lands at £2,800-£4,500/m² once it's built across London zones 2-6, before fit-out.

Single-Storey Rear Extension Cost

A basic-to-mid-spec single-storey rear extension shell in London — groundworks, structure, roof, windows and doors, and a basic first fix — typically costs £2,800-£4,500 per m², putting a common 20-25m² addition at roughly £56,000-£112,500+ before kitchen fit-out, flooring, bespoke joinery or decoration, costs we price separately once the shell is watertight and the second-fix trades can quote against a firm specification. The width of that range comes down to structural scope as much as finish: a flat roof with a single rooflight and standard brick-and-block walls sits toward the bottom, while a vaulted ceiling, full-width bifold or sliding doors, and a structural steel beam opening into the existing kitchen push it toward the top. Where the extension opens the kitchen into the new space, that steel installation — including calculations, padstones and installation — typically adds £1,800-£4,500+ depending on the span and how the beam bears onto the existing wall.

The foundation detail is where a single-storey rear extension quietly becomes more expensive than the headline square metre rate suggests. Because the existing house sits on a shallow footing and the new extension is dug to modern depth, the engineer usually specifies either a stepped foundation that ties the new concrete into the old brick footing at a matching level, or localised underpinning of the section of original wall the extension abuts. Skipping this detail to save cost is the single most common reason single-storey rear extensions develop a crack running from the corner of the old house into the new render within the first two winters, once the ground has been through a full shrink-swell cycle under London's clay subsoil.

Sizing also has a regulatory ceiling worth checking before the design is priced: under permitted development, a single-storey rear extension on a terraced or semi-detached house can extend up to 3m from the original rear wall without planning permission (4m on a detached house), or further under the Larger Home Extension Scheme with prior approval. Going beyond those limits doesn't necessarily add much to the build cost itself, but it does add a planning application, fee and timeline — covered in full in the permitted development section below.

Side-Return Extension Cost

A side-return extension — infilling the narrow strip of side garden alongside a Victorian or Edwardian terrace's kitchen or scullery — typically costs £4,500-£5,500 per m², noticeably more than a straightforward rear box of the same floor area. The premium comes from underpinning: the new extension's foundation runs directly alongside, and usually beneath part of, the neighbour's party wall foundation, which almost always means excavation within 3-6 metres of the adjoining owner's building and triggers a Party Wall etc. Act 1996 notice before groundworks can start. The cost of underpinning a party wall for this kind of extension is rarely itemised as its own line by builders, but it's the main reason side-return pricing sits above straightforward rear-box rates, not the roof or the glazing spec. The structural engineer also has to design the new roof, often with rooflights running the full length of the return, and any steel to tie into the existing party wall at a level that doesn't disturb its bearing, which adds engineering and site time that a free-standing rear extension doesn't need.

Side-return jobs also commonly involve removing the wall between the existing kitchen and the new space, and often the chimney breast alongside it, to create one open-plan room. That means a structural engineer's beam design and installation, which for a standalone opening runs £1,800-£4,500+ including calculations, padstones and installation — broadly consistent with the cost of a knock-through steel beam priced on its own. The padstones that beam bears onto still have to sit on adequate masonry above the original shallow footing, which is exactly where an engineer's inspection of the existing wall build-up earns its fee.

Wraparound Extension Cost

A wraparound extension combines the side-return infill with a rear extension into one continuous L-shaped structure, usually to create a single large kitchen-diner-family room spanning the full width of the house at ground floor. Priced per square metre (per m²), it's more cost-efficient than pricing the two elements separately, since groundworks, scaffolding and roofing are shared across the whole footprint, but it still sits toward the upper end of the £4,500-£5,500/m² side-return range rather than dropping to single-storey rear pricing — most of the floor area still touches the party wall and inherits the same underpinning requirement.

Because a wraparound creates two structural junctions rather than one — the party wall interface down the side-return, and the rear wall opening into the original house — it typically needs two separate structural steel arrangements, or one continuous beam spanning both, and it's the extension type most likely to expose an old inspection chamber or soil stack that has to be relocated, since side-return excavations routinely run into drainage laid along that alley when the house was built. The larger footprint also increases the odds of the extension running within 3 metres of a public sewer that Thames Water requires a build-over agreement for, so a drainage survey at design stage is worth doing before the layout is fixed rather than after.

The two junctions also mean two places where insulation continuity and damp-proof course levels have to be checked against the new floor slab and finished ground levels, rather than one. Missing either junction is how thermal bridging and condensation show up around lintels after the first cold winter, and how rising damp appears in the original wall if the new slab level bridges the existing DPC — both are Part L and Part C requirements, not optional extras, and they're cheaper to detail correctly at design stage than to open up and fix after decoration.

Double-Storey Rear Extension Cost

A double-storey rear extension typically costs £2,800-£4,200 per m² for the additional floor area, and because the ground floor structure, foundations and roof are effectively shared across two storeys of usable space rather than one, the overall value per square metre across the whole project is often better than a single-storey extension of similar ground-floor footprint. The upper floor does add its own costs: additional steel to carry the first-floor structure, a new staircase or staircase alteration if the layout changes, and typically a full re-roof over the extended footprint rather than the simpler flat or lean-to roof often used on single-storey rear boxes.

Planning is the detail that catches people out on double-storey rear extensions. Permitted development rights for two-storey rear extensions are considerably tighter than for single-storey ones: under the GPDO, a two-storey rear extension is generally limited to 3 metres beyond the original rear wall regardless of whether the house is terraced, semi-detached or detached, compared with the 3m/4m single-storey allowance or up to 6m/8m under the Larger Home Extension Scheme with prior approval. Many double-storey rear extension designs homeowners actually want — particularly anything that also affects eaves height within 2 metres of a boundary, or uses different materials to the existing house — fall outside permitted development and need a full householder planning application rather than relying on PD rights.

Permitted Development Rights vs Full Planning Permission

Under the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended, single-storey rear extensions on terraced or semi-detached houses can extend up to 3 metres beyond the original rear wall without planning permission (4 metres for detached houses), rising to 6 metres (terraced/semi) or 8 metres (detached) under the Larger Home Extension Scheme, which requires a prior approval application with neighbour consultation rather than a full planning application. Two-storey rear extensions are capped at 3 metres regardless of house type. These limits apply on top of conditions around materials matching the existing house, eaves and ridge height, and side elevation windows facing a boundary requiring obscure glazing, so a design that technically fits within the depth limit can still fall outside permitted development on a different condition.

A significant number of London boroughs have removed some or all of these permitted development rights through Article 4 Directions, meaning a project that would be PD-compliant anywhere else in the country needs a full householder planning application in that borough or conservation area regardless of size. Camden, Islington, Hackney, Southwark and Kensington & Chelsea are among the boroughs with borough-wide or conservation-area Article 4 restrictions on householder extensions, so checking the specific borough and conservation area status before assuming PD rights apply is a necessary first step, not a formality.

Where a full planning application is required, the current householder application fee in England is £548 per dwelling, plus a Planning Portal service charge of roughly £75.83 including VAT if submitted online. A full application also adds an 8-week statutory determination period as a minimum (longer if the case is called to committee), which needs to be built into the programme alongside the Party Wall Act notice periods running in parallel rather than treated as a separate delay after the build contract is signed.

Building Regulations: Full Plans vs Building Notice

Every extension needs Building Control sign-off regardless of its planning status, and there are two routes to get it. The full plans route means detailed drawings and, for anything structural, the engineer's calculations are submitted and checked by Building Control before work starts on site, which catches problems with the foundation design, steel beam sizing or drainage layout on paper rather than after the concrete is poured. The building notice route lets work start on site sooner, without prior approval of detailed drawings, but the risk of a failed inspection sits with the homeowner: if an inspector isn't satisfied with a foundation depth or a steel bearing once it's built, opening up finished work to fix it costs considerably more than getting the design checked upfront.

Building Control fees for a house extension typically run £400-£1,200 depending on the local authority and the size and complexity of the project, entirely separate from any planning application fee. On a project involving underpinning, structural steel or a basement, the full plans route is worth the slightly longer lead time given the number of inspection points involved: the open foundation trench before concrete is poured, the damp-proof course level, the structural steel and roof structure before it's covered, and a final inspection before the completion certificate is issued. We generally recommend full plans wherever steel beams, underpinning or basement work are involved, and reserve building notice for simpler, non-structural work where the risk of a costly rework is lower.

Party Wall Act 1996 Triggers for Extensions

Most rear, side-return and wraparound extensions on London terraces and semis trigger the Party Wall etc. Act 1996 in one form or another. Section 6 covers excavation within 3-6 metres of a neighbour's foundations, which catches almost any extension with new foundations dug near a shared boundary; Section 1 covers building on the line of junction where a new wall is built up to or astride the boundary; and Section 2 covers work directly to an existing party wall, such as cutting in a new beam or underpinning a shared foundation on a side-return or wraparound job. Starting groundworks before the statutory notice period has run its course, or before an award is agreed where a neighbour dissents, is one of the more common and avoidable delays on London extension projects.

Party wall surveyor fees are paid by the building owner for both sides where each party appoints their own surveyor, and in London this typically runs £900-£3,000+ across the notice, schedule of condition and award stages — higher than the national average given London surveyor rates and the frequency of adjoining owners appointing their own representation rather than agreeing to a single surveyor. A single agreed surveyor acting for both parties, where the neighbour is willing, keeps this toward the lower end. Notices are best served once the design is fixed, in parallel with the Building Control application, rather than after the build contract is signed, so the statutory one or two-month notice periods run alongside the rest of the pre-construction process instead of holding up a crew that's already booked and on the clock.

Foundations, Drainage and Structural Risks in Older London Stock

Old glazed clay drainage pipework running under or near the extension footprint is frequently found to be cracked, root-infiltrated or laid to a poor fall once it's exposed during groundworks. London clay's seasonal shrink-swell cycle has usually already stressed the pipe joints over the decades, so a CCTV drainage survey before digging, with budget set aside for partial pipe replacement and a new inspection chamber if needed, avoids an expensive mid-build surprise once the trench is already open and the programme is running. Rear and side-return extensions also routinely land within 3 metres of a Thames Water public sewer, or 1 metre of a public lateral drain, without the owner realising until the survey flags it — building over or near a sewer needs its own build-over or build-near agreement with Thames Water, a process entirely separate from planning permission and Building Control that can add several weeks to the programme if it isn't started as soon as the design is fixed.

Removing a chimney breast or a load-bearing wall to open the new extension into the existing kitchen is another point where cost and risk concentrate. The cost of a structural engineer's steel beam for a kitchen extension opening depends entirely on this design work: without a proper beam design and an adequately sized padstone bearing onto the original shallow footing, the result is the cracking and sagging lintels commonly found in older stock where this was done on the cheap. Ex-council flats and maisonettes from the 1930s-60s add a further wrinkle: the freeholder or management company's lease frequently requires separate written consent for structural alteration or roof-level work in addition to Building Control sign-off, and this is commonly missed until the freeholder objects partway through the project, at which point work has to stop while consent is sought retrospectively.

2026 London House Extension Cost Guide by Project Type

The table below summarises typical 2026 London price ranges by extension type, plus the structural and professional costs most projects need to budget for alongside the build itself. Build costs are shell figures covering structure, roof, windows and basic first fix, and exclude kitchen fit-out, bespoke joinery, decoration and professional fees — all of that gets its own quote once the shell is watertight and the exact specification is agreed. Treat the lower end of each range as achievable with a straightforward structural scope, standard-depth foundations and no drainage diversion, and the upper end as realistic once underpinning, a complex steel arrangement or basement waterproofing is involved — most real jobs sit somewhere between the two once the structural engineer's survey confirms what the existing foundations and drainage actually look like. Figures include labour and materials at London rates; VAT applies to both on the great majority of residential extension work.

How Extension Cost Compares to a Full Refurbishment per m²

Homeowners weighing up an extension against a full internal refurbishment — renovating the house as it stands, rather than adding to it — are often comparing two different things dressed up as the same decision. An extension creates new floor area from scratch, with its own foundations, external walls, roof and services connections, so the £2,800-£5,500/m² range set out above reflects genuinely new construction. A full house refurbishment works within the existing envelope, reusing the foundations, external walls and roof structure that are already there, which is why whole-house refurbishment typically prices lower per m² than new-build extension work even though it can still involve significant re-wiring, re-plumbing and structural alteration internally.

The comparison isn't quite like-for-like: refurbishment cost buys improved existing space — rewiring, replastering, new bathrooms, a reconfigured layout — while extension cost buys genuinely new floor area. Where a house is structurally sound and the layout just needs reconfiguring, a refurbishment or internal reconfiguration will usually deliver more usable improvement per pound spent than adding floor area that isn't strictly needed. Where the house is genuinely too small for the household regardless of layout, an extension is the only route to more square footage, and the per-m² premium over refurbishment reflects the cost of new foundations, structural openings and full weatherproofing that refurbishment work doesn't have to build from scratch.

In practice, the two are frequently done together rather than as alternatives: a rear or side-return extension adds usable space, while a parallel refurbishment of the existing rooms brings the rest of the house up to the same standard as the new addition, so the finished result reads as one house rather than a new box bolted onto a dated original. Running both under a single accountable contractor rather than a separate extension builder and a separate refurbishment team avoids the handover gaps — particularly around where new services tie into the existing supply and where the new floor level meets the old — that tend to surface as snagging issues when two unconnected teams have each assumed the other covered a detail.

How We Manage an Extension Project From Design to Completion

An extension touches more disciplines than most homeowners expect — groundworks, drainage, structural steel, roofing, first and second fix — each with its own Building Control inspection point, plus planning, Party Wall Act and, where relevant, Thames Water processes running alongside. We run all of it under one contract and one site team rather than a homeowner coordinating separate self-employed trades who each disclaim responsibility when something at a handover point, such as the steel bearing onto the old footing or the drainage tie-in, doesn't line up with what the trade before them left behind.

In practice that means instructing the structural engineer early enough that foundation and steel design account for the actual condition of the existing footings and walls, choosing the full plans or building notice route based on how structurally involved the job genuinely is, and serving Party Wall Act notices as soon as the design is fixed so the statutory notice periods aren't what's holding up the start date. We're based in Kingston upon Thames and run projects across all 32 London boroughs and the City of London, plus Surrey, Kent, Essex, Hertfordshire and Middlesex — a Victorian terrace in Wandsworth, an ex-council maisonette in Southwark and a conservation-area house in Richmond each bring a different set of structural and planning questions, which is exactly why the figures in this guide are given as ranges rather than a single number. The 26 five-star reviews on our Google Business Profile reflect completed projects and word-of-mouth referrals rather than paid advertising, which is also why we'd rather set out realistic cost ranges here than quote a figure that only holds up in the best-case scenario.