Cost Guide · Updated July 2026
How Much Does a Concrete Extension Cost in 2026 (UK)?
A “concrete extension” can mean three very different builds — a traditional brick-and-block extension on concrete foundations, an insulated concrete formwork (ICF) extension, or a precast concrete sectional structure. In 2026, most homeowners pay £1,800–£3,000 per m² for a finished traditional extension, with ICF typically adding a 10–20% premium on the shell. This guide breaks down every cost driver — foundations, slab, groundworks, walls and finishes — with real 2026 UK ranges and total costs for 15, 25 and 40 m² builds.
Quick answer: concrete extension cost in 2026
For a traditional brick-and-block extension with concrete strip foundations and a concrete slab floor, budget £1,800–£3,000 per m² finished. That puts a typical single-storey 25 m² rear extension at roughly £45,000–£70,000, and a double-storey at around £1,900–£2,600 per m² because you gain floor area without doubling foundation and roof costs. Choosing ICF (insulated concrete formwork) for the shell adds roughly £100–£250 per m² but cuts running costs through excellent airtightness and thermal mass. Foundations alone usually run £120–£220 per m² of footprint (more on reactive clay, sloping or tree-lined plots). Prices vary widely by region, ground conditions and finish, so get 3 itemised quotes before you budget.
“Concrete extension” means three different things
Before you compare quotes, it’s worth being clear on which build type you actually want — the word “concrete” gets used loosely, and the cost gap between these options is significant.
1. Traditional (brick-and-block) extension with concrete foundations & floor
This is what most UK homeowners mean. The structure is masonry — an outer skin of brick or render-on-block and an inner blockwork leaf — but it sits on concrete strip or trench-fill foundations and typically has a ground-bearing concrete slab floor (or beam-and-block). Concrete is the hidden backbone, not the visible wall. This is the cheapest, most widely quoted route and the one nearly every general builder can deliver.
2. Insulated concrete formwork (ICF) extension
ICF uses hollow interlocking blocks (usually EPS insulation) that are stacked, reinforced and filled with poured, ready-mix concrete on site. The result is a solid reinforced-concrete wall sandwiched between permanent insulation. ICF gives you continuous insulation, superb airtightness and high thermal mass in one system. It costs more up-front and needs a builder experienced with the system, but running costs and comfort are excellent — a strong choice for extensions with large glazing or open-plan living.
3. Precast / sectional concrete structures
At the budget end, precast concrete sectional buildings (the “concrete garage” style) are craned or bolted together from factory-made panels. These are cheap and fast but are generally used for detached garages, workshops and outbuildings — not habitable home extensions, because they rarely meet Part L insulation and Part C damp standards without heavy upgrading. If you want a warm, connected living space, this is usually the wrong product. We focus the cost tables below on options 1 and 2.
Rule of thumb: if you want a habitable room that matches your house, you’re choosing between traditional masonry (lowest cost, most builders) and ICF (higher performance, higher price). Sectional concrete is for garages and outbuildings, not living space.
Concrete extension cost per square metre in 2026
Cost per m² is the most useful way to budget an extension because it scales with floor area. The figures below are finished costs — structure, roof, windows, doors, plastering, electrics, basic plumbing and standard finishes — but exclude kitchens, bathrooms, expensive glazing (lantern rooflights, sliding doors) and VAT complications. Location is the single biggest swing factor: London and the South East sit near the top of every range.
| Build type | Cost per m² (2026) | What it includes |
|---|---|---|
| Traditional single-storey (basic finish) | £1,800–£2,300 | Strip/trench-fill foundations, concrete slab, cavity masonry walls, warm-roof or pitched roof, standard windows/doors, plaster, basic electrics |
| Traditional single-storey (mid/high finish) | £2,300–£3,000 | As above plus better glazing, upgraded insulation, more sockets/lighting, quality flooring substrate |
| Traditional double-storey | £1,900–£2,600 | Extra floor area over shared foundations & roof lowers the per-m² rate |
| ICF single-storey | £2,100–£3,200 | Insulated formwork shell (poured reinforced concrete), continuous insulation, high airtightness |
| ICF double-storey | £2,000–£2,900 | ICF efficiency plus double-storey economy of scale |
| Premium glazed / structural-opening builds | £3,000–£4,000+ | Large lanterns, sliding/bi-fold doors, steel beams, bespoke design |
The ICF premium over an equivalent traditional shell is usually £100–£250 per m². On a 25 m² extension that’s roughly £2,500–£6,250 extra on the structure — often partly offset by faster erection, no separate insulation trade and lower lifetime energy bills. For a like-for-like comparison, see our ICF vs timber frame notes below and always ask each builder to quote the same specification.
Typical total costs by extension size
Applying the per-m² ranges to three common footprints gives realistic 2026 budgets. These totals assume a standard finish, straightforward access and normal ground conditions. Add a 10–15% contingency — foundations are the most common source of overspend because you can’t fully price them until the ground is open.
| Size | Typical use | Traditional (single-storey) | ICF (single-storey) |
|---|---|---|---|
| 15 m² (approx 3m × 5m) | Small kitchen or utility extension, snug | £28,000–£44,000 | £32,000–£48,000 |
| 25 m² (approx 5m × 5m) | Open-plan kitchen-diner, family room | £45,000–£70,000 | £52,000–£78,000 |
| 40 m² (approx 5m × 8m) | Large kitchen-living space, wrap-around | £72,000–£108,000 | £82,000–£118,000 |
Two-storey versions add roughly 60–80% to the single-storey total (not 100%) because the foundations, roof and much of the groundwork are shared. A 25 m²-footprint double-storey — giving you around 50 m² of new floor space — commonly lands at £90,000–£135,000. Because these swings are so wide, the only reliable budget is a site-specific quote: compare 3 free quotes here.
Foundation cost: strip, trench-fill and raft
Foundations are the concrete that matters most on any extension — and the item most likely to blow a budget, because ground conditions are only fully known once you dig. Building Control will inspect the open trench and can demand deeper or wider foundations than planned. Expect foundations and below-ground work to be £120–£220 per m² of footprint on a straightforward plot, and more where clay, trees, drains or slopes are involved.
| Foundation type | Best for | Indicative cost | Notes |
|---|---|---|---|
| Strip foundation | Good, stable ground; standard depth (≈1m) | £120–£170/m² | Cheapest; a shallow concrete strip under load-bearing walls |
| Trench-fill foundation | Most common today; clay & near trees | £150–£230/m² | Trench filled almost to top with concrete — faster to build off, more concrete |
| Raft foundation | Poor/made-up ground, high water table | £180–£300/m² | A reinforced slab spreading load across the whole footprint |
| Piled foundation | Very poor ground, close to large trees | £300–£600+/m² | Piles plus ground beams; needs engineered design |
Reactive clay soils near mature trees are the classic cost trap: to avoid heave and subsidence, foundations may need to go 1.5–3m deep, sometimes with compressible board to one face. Old drains crossing the footprint can force lintels or diversions. This is exactly the kind of judgement a structural engineer earns their fee on — and why a site visit beats any online estimate.
Concrete slab floor cost
The ground floor of most extensions is a ground-bearing concrete slab: hardcore sub-base, sand blinding, damp-proof membrane, insulation, then poured concrete. For larger spans, poor ground, or where you want to avoid disturbing tree roots, a suspended beam-and-block floor is used instead.
| Floor build-up | Indicative cost | When it’s used |
|---|---|---|
| Ground-bearing concrete slab (incl. sub-base, DPM, insulation) | £90–£150/m² | Standard choice on good ground |
| Beam-and-block suspended floor | £110–£180/m² | Poor ground, sloping sites, near trees |
| Power-floated / polished concrete finish | +£40–£90/m² | Where the slab is the finished floor (exposed concrete look) |
| Wet underfloor heating within slab | +£60–£110/m² | Popular in open-plan concrete extensions |
Concrete floors pair naturally with underfloor heating because the slab’s thermal mass stores and releases warmth gently — a real comfort win in an extension with big glazing. If you want a polished-concrete finished floor, tell your builder early: the slab has to be laid and cured to a higher standard, and services must be set out before the pour.
Groundworks: the cost you can’t see
Groundworks cover everything from breaking ground to damp-proof course level: excavation, muck-away (removing spoil), foundations, drainage, the slab and below-ground drainage connections. On a typical single-storey extension, groundworks are commonly 15–25% of the total build cost — and the phase where nasty surprises appear.
- Muck-away / spoil removal: £150–£300 per grab lorry load; heavy clay and restricted access push this up.
- Drainage alterations: diverting or building over a public sewer needs a build-over agreement with the water authority (typically a few hundred pounds plus survey and works).
- Existing manholes/inspection chambers: may need moving or converting to double-sealed covers.
- Access & site setup: no side access for a mini-digger means barrowing spoil by hand — slower and more expensive.
- Trees & roots: protected trees (TPOs) or root protection areas can dictate foundation type and add engineering.
Because groundworks are hard to price blind, a good builder will either build in a realistic allowance or flag the risk items for a contingency. Treat any quote that skips a site visit with caution.
Traditional masonry vs ICF vs timber frame
For a habitable extension you’re really choosing between three structural systems. All three sit on the same concrete foundations — the difference is the walls and how they perform.
| System | Relative cost | Strengths | Watch-outs |
|---|---|---|---|
| Traditional brick-and-block | Baseline (lowest) | Every builder can do it; easy to match existing house; robust; simple to detail | Weather-dependent; slower; insulation is a separate trade to get right |
| ICF (insulated concrete formwork) | +10–20% on shell | Continuous insulation, excellent airtightness, high thermal mass, strong & quiet | Needs an experienced installer; brick/render slip finish adds cost; fewer local specialists |
| Timber frame | Similar to masonry, sometimes less | Fast, factory precision, good insulation, dry trades once up | Lower thermal mass; needs careful moisture/airtightness detailing; cladding choices affect matching |
In practice, most UK extensions are traditional masonry because it’s familiar, easy to match to the existing house and quoted by nearly every builder. ICF wins when comfort, energy efficiency and acoustic performance matter most — often in large open-plan or heavily glazed spaces. Timber frame is a strong middle path where speed matters. Whatever you choose, get quotes on a single, identical specification so you’re comparing like for like.
Single-storey vs double-storey: the economics
Adding a second storey is nearly always cheaper per square metre than building two separate single-storey extensions, because you pay for the expensive bits — foundations, roof, scaffolding, groundworks — once and reuse them for the upper floor.
- Single-storey (25 m²): £45,000–£70,000. Simplest to build and often within permitted development.
- Double-storey (25 m² footprint, ≈50 m² new space): £90,000–£135,000 — roughly £1,900–£2,600/m², i.e. more space for less per metre.
- Foundation upgrade: a two-storey load may need slightly wider/deeper foundations, but rarely enough to erase the per-m² saving.
- Planning: two-storey and rear extensions beyond permitted-development limits usually need a full planning application (see below).
If your goal is maximum space per pound and the plot and planning allow it, double-storey is the value play. If you simply need a bigger kitchen and want the least disruption, single-storey is the pragmatic choice. Our rear extension cost guide digs into the single-storey numbers in more detail.
Why choose concrete or ICF for an extension?
Beyond being the standard for foundations and floors, concrete-based construction — especially ICF — brings performance benefits that matter in a modern, well-glazed extension:
Thermal mass
A concrete slab and ICF walls store heat and release it slowly, smoothing out temperature swings. In a south-facing extension with big glazing, that means less overheating in summer and steadier warmth in winter — particularly effective when paired with underfloor heating.
Airtightness & energy efficiency
ICF forms a continuous, monolithic insulated shell with very few gaps, so uncontrolled draughts are minimal. Combined with good glazing and ventilation, that translates into lower heating bills and easier compliance with Part L of the Building Regulations. It also helps future-proof the space for a heat pump.
Durability, acoustics & fire
Concrete structures are robust, rot-free, and offer excellent sound insulation and inherent fire resistance — useful next to a busy road or where you want a genuinely quiet room. Maintenance over the building’s life is typically low.
Is ICF worth the premium? If you plan to stay long-term, value comfort and low running costs, or are building a large glazed space, the 10–20% shell premium often pays back through energy savings and comfort. For a small, simple extension on a tight budget, traditional masonry usually makes more financial sense.
Planning permission, building regs & party wall
The construction method (concrete, ICF or masonry) doesn’t change the planning rules — size, height and position do. Here’s what applies to a 2026 UK extension:
Planning permission & permitted development
Many single-storey rear extensions fall under permitted development (PD) in England, within limits (for a detached house, up to 4m deep single-storey; up to 8m under the larger-home “prior approval” route, subject to conditions). Terraced and semi-detached homes have tighter limits, and PD is removed in conservation areas, on listed buildings, and where an Article 4 direction applies. Double-storey extensions usually need a full householder planning application (typically a few hundred pounds in fees). Always confirm with your local planning authority before committing.
Building Regulations
Every habitable extension needs Building Regulations approval regardless of planning status. Building Control will inspect foundations, DPC, drainage, insulation (Part L), structure (Part A), fire (Part B) and more. Budget for the local authority or approved inspector fee (commonly £500–£1,000+ depending on project value) — separate from planning.
Party Wall etc. Act 1996
If you build on or near a shared boundary, excavate close to a neighbour’s foundations, or cut into a party wall, you must serve notice under the Party Wall etc. Act. Deep concrete/trench-fill foundations near a boundary frequently trigger this. If neighbours don’t consent, you’ll each appoint a surveyor (or share one) and agree an award — commonly £800–£2,000+ per neighbour. Factor this in early.
None of these are optional extras: skipping Building Regs or party wall notice can mean enforcement, an indemnity headache when you sell, or a costly dispute. A good builder or your engineer will steer you through them.
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Concrete extension cost: frequently asked questions
Often, yes — if you value comfort, low running costs and acoustic performance, and especially on larger or heavily glazed extensions. ICF adds roughly £100–£250 per m² to the shell but delivers continuous insulation, excellent airtightness and high thermal mass, which cut heating bills and help meet Part L. For a small, simple, budget-driven extension, traditional masonry usually gives better value. Get both quoted on the same spec and weigh the up-front premium against your long-term energy savings.
Traditional masonry (on concrete foundations) is usually the cheapest and most widely quoted, because every builder can do it and it’s easy to match your existing house. Timber frame is often similar or slightly less and builds faster. ICF typically costs 10–20% more on the shell but performs best on energy and comfort. The “cheapest” answer depends on your ground conditions, finish and how much you value running costs — compare all three on an identical specification.
Foundations typically cost £120–£220 per m² of footprint on a straightforward plot. Strip foundations are cheapest, trench-fill (the most common today) a little more, and raft or piled foundations are needed on poor ground or near large trees — piling can exceed £300–£600/m². Reactive clay and mature trees can force foundations 1.5–3m deep, which is the most common cause of budget overruns. Building Control inspects the open trench, so the final depth isn’t confirmed until you dig.
It depends on size, height and position, not the material. Many single-storey rear extensions fall under permitted development within size limits, but double-storey extensions, larger builds, and anything in a conservation area or on a listed building usually need a full householder planning application. Every habitable extension also needs Building Regulations approval separately. Always confirm with your local planning authority before you start.
A typical single-storey extension takes around 10–16 weeks on site once you’ve got approvals — roughly 2–4 weeks for groundworks and foundations, then the shell, roof, and fit-out. ICF shells often go up faster than masonry once the formwork is being placed, though brick or render finishing adds time. Double-storey builds add several weeks. Weather, ground surprises and the party wall process can all extend the programme, so build in some float.
A well-built extension — particularly one that adds a kitchen-diner, extra bedroom or open-plan living space — commonly adds value, often more in higher-priced areas. The energy efficiency of ICF can also be a selling point as buyers focus on running costs. That said, value added varies by location and quality of finish, and you can over-extend for your street’s ceiling price. Build for how you’ll live in the home, and treat the uplift as a bonus rather than the whole justification.
Generally no, not as habitable living space. Precast concrete sectional buildings (the “concrete garage” style) are cheap and quick but rarely meet Part L insulation and Part C damp-proofing standards without heavy upgrading, and they’re hard to connect and match to your house. They’re fine for detached garages, workshops and storage. For a warm, connected room you want traditional masonry or ICF on proper foundations.
Usually yes. A structural engineer designs the foundations for your specific ground conditions, sizes any steel beams for openings between old and new, and produces the calculations Building Control require. Their fee is modest relative to the risk they remove — getting foundations wrong is far more expensive. See our structural engineer cost guide for typical fees.
Written by the BestBuilders Editorial Team · Reviewed by a chartered structural engineer with experience in domestic extensions and foundation design · Last updated: July 2026.
How we produced this guide: Cost ranges are compiled from 2026 UK builder and groundworker quotes and cross-checked against published guidance from the Planning Portal, the Building Regulations (Approved Documents A, C and L) and the Party Wall etc. Act 1996. Figures are indicative and exclude VAT complications and premium finishes — always obtain site-specific quotes before budgeting.
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