Free House Extension Price Estimator – Updated 2026 Supply & Fit Prices
Enter your requirements below to estimate the cost of a house extension in the UK in 2026. All prices include standard structural works, insulation, plastering, windows, doors, roofing, and first-fix services. Kitchen, bathroom, and internal finishing costs are listed separately.
A house extension is one of the most significant home improvement projects a UK homeowner can undertake — and in 2026, it remains one of the best ways to add space, value, and functionality to a property. Extension costs in the UK range enormously: from approximately £20,000–£35,000 for a small single-storey rear extension (around 15–20 m²) at budget specification, to £150,000–£300,000+ for a large, architect-designed double-storey extension with structural glazing, a high-specification kitchen, and premium finishes throughout. The average single-storey rear extension of approximately 20 m² at standard specification costs approximately £45,000–£75,000 in South East England in 2026, or £35,000–£58,000 in North England.
The single most important driver of extension cost is the floor area — every additional square metre adds materially to the cost of foundations, walls, roof, glazing, and finishes. After floor area, the specification level has the most significant impact: a budget-specification extension might cost £1,800–£2,200/m², a standard specification £2,300–£3,000/m², a high specification £3,000–£4,000/m², and a luxury architect-led extension £4,000–£6,000+/m². The choice of roof type (pitched vs flat), glazing (standard doors vs large bi-fold or sliding doors), and wall construction method also significantly affect cost, as does the UK region — London and the South East command a 20–35% premium over the national average.
It is important to understand what is and is not included in an extension cost estimate. Most builder quotations cover the structural shell (foundations, walls, roof, windows, doors), first-fix services (electrical and plumbing routing), and internal finishing (plastering, flooring, decoration). They typically do not include professional fees (architect, structural engineer), planning application fees, party wall surveyor costs, kitchen or bathroom fitting (unless specifically requested), or external works (paving, landscaping). These additional costs can add 15–30% to the base build cost and must be budgeted for separately.
A standard house extension supply-and-fit price from a UK building contractor in 2026 should include:
House extension resources UK 2026: Planning Portal | Checkatrade – Find a Builder | TrustATrader | GOV.UK Building Regulations | RIBA – Find an Architect
The table below provides a comprehensive price guide for house extensions in the UK in 2026, broken down by extension type, floor area, and specification level. All prices are mid-range South East England estimates for a fully completed extension including structure, services, plastering, and standard finishing. VAT, professional fees, and kitchen/bathroom costs are not included.
| Extension Type | Floor Area | Budget Spec | Standard Spec | High Spec | Luxury / Architect |
|---|---|---|---|---|---|
| Single Storey Rear | 15 m² | £28,000–£38,000 | £38,000–£52,000 | £52,000–£72,000 | £72,000–£110,000+ |
| Single Storey Rear | 20 m² | £36,000–£48,000 | £48,000–£68,000 | £68,000–£95,000 | £95,000–£150,000+ |
| Single Storey Rear | 30 m² | £52,000–£70,000 | £70,000–£98,000 | £98,000–£138,000 | £138,000–£210,000+ |
| Single Storey Side | 15 m² | £26,000–£35,000 | £35,000–£50,000 | £50,000–£68,000 | £68,000–£100,000+ |
| Single Storey Side | 20 m² | £34,000–£46,000 | £46,000–£65,000 | £65,000–£90,000 | £90,000–£140,000+ |
| Wrap-Around (side + rear) | 30 m² | £58,000–£78,000 | £78,000–£110,000 | £110,000–£155,000 | £155,000–£240,000+ |
| Wrap-Around (side + rear) | 40 m² | £76,000–£100,000 | £100,000–£142,000 | £142,000–£200,000 | £200,000–£300,000+ |
| Double Storey Rear | 20 m² (40 m² total) | £65,000–£88,000 | £88,000–£122,000 | £122,000–£170,000 | £170,000–£260,000+ |
| Double Storey Rear | 25 m² (50 m² total) | £80,000–£108,000 | £108,000–£152,000 | £152,000–£210,000 | £210,000–£320,000+ |
| Double Storey Side | 15 m² (30 m² total) | £55,000–£74,000 | £74,000–£104,000 | £104,000–£145,000 | £145,000–£220,000+ |
| Extension Over Garage | 15–20 m² | £28,000–£40,000 | £40,000–£58,000 | £58,000–£80,000 | £80,000–£120,000+ |
| Basement / New Basement | 20 m² | £60,000–£90,000 | £90,000–£130,000 | £130,000–£180,000 | £180,000–£300,000+ |
All prices are South East England mid-range 2026 estimates, supply and fit, excluding VAT, professional fees, and kitchen/bathroom fitting. Deduct approximately 12–20% for North England, Wales, Scotland, and Northern Ireland. Add approximately 15–30% for Greater London. Prices assume standard ground conditions; complex ground (filled ground, high water table, proximity to trees) will increase costs.
The following additional costs are commonly encountered in UK house extension projects in 2026 and are typically not included in a builder's standard extension quotation.
| Item | Typical Cost Range (2026) | Notes |
|---|---|---|
| Architect / architectural designer fees | 8–15% of build cost; or £2,000–£8,000 fixed fee for drawings | Full service (design through to completion) typically 10–15%; drawings only from £1,500–£4,000 |
| Structural engineer fees | £500–£2,500 | Required for all extensions; calculates beam sizes, foundation loads, roof structure |
| Planning application fee (England) | £258–£578 | Householder application fee; not required for permitted development extensions |
| Building Regulations application fee | £200–£900 | Full Plans or Building Notice; always required for extensions regardless of planning status |
| Party wall surveyor fees | £700–£2,500 per surveyor | Required where extension works affect a shared wall or boundary; both parties may need separate surveyors |
| Topographical / boundary survey | £400–£1,200 | Often required for planning applications and to confirm site dimensions |
| Budget kitchen (supply and fit) | £5,000–£12,000 | Flat-pack or semi-bespoke units, basic appliances, worktops |
| Standard kitchen (supply and fit) | £12,000–£30,000 | Rigid units, integrated appliances, stone or composite worktop |
| High-spec / bespoke kitchen (supply and fit) | £30,000–£100,000+ | Bespoke cabinetry, premium appliances, quartz or granite worktops |
| Standard bathroom / shower room | £4,000–£9,000 | Suite, tiles, plumbing, electrics, extraction |
| High-spec bathroom / en-suite | £9,000–£25,000+ | Freestanding bath, premium tiles, bespoke joinery, underfloor heating |
| Bi-fold doors (3-panel aluminium) | £3,500–£6,500 | Supply and fit; thermally broken aluminium; double or triple glazed |
| Bi-fold doors (5–6 panel aluminium) | £6,000–£12,000 | Large opening; may require structural beam above |
| Roof lantern / flat roof rooflight | £2,500–£8,000 | Aluminium framed; adds natural light to flat-roofed extensions |
| Underfloor heating (electric, per m²) | £60–£120/m² | Electric mat under screed; popular for kitchen extensions |
| Underfloor heating (water / wet, per m²) | £80–£160/m² | Connected to boiler; more efficient long-term; requires screed |
| External paving / landscaping | £60–£200/m² | Replacing/extending patio or garden after construction; porcelain, block paving, or natural stone |
| Scaffolding (if not included) | £800–£3,000 | Depends on height and duration; usually included in builder's quote for extensions |
The average cost of a single-storey rear extension in the UK in 2026 is approximately £48,000–£68,000 for a 20 m² standard specification build in South East England. In North England and Scotland, the equivalent project typically costs £38,000–£54,000. London extensions command a significant premium of 20–30% above South East prices.
The most useful benchmark for comparing extension quotes is the cost per m² of total floor area. In 2026, typical ranges are: budget £1,800–£2,300/m²; standard £2,300–£3,200/m²; high specification £3,200–£4,500/m²; luxury/architect-led £4,500–£6,500+/m². Quotes significantly below £1,800/m² in the South East should be treated with caution.
A well-designed and well-built extension typically adds 10–20% to a property's value and in many cases adds more value than the build cost — particularly in high-demand areas. Kitchen extensions and additional bedrooms with en-suites consistently deliver the highest return on investment. Poor-quality or badly designed extensions can actually reduce saleability.
Many single-storey rear extensions in England are permitted development — no planning permission required. Under current PD rules, a single-storey rear extension can be built up to 4m deep for detached houses or 3m deep for semi-detached and terraced houses without planning permission (subject to height, boundary, and other restrictions). Larger extensions under the Neighbour Consultation Scheme may extend to 8m/6m. Always check with your Local Planning Authority.
Building Regulations approval is always required for a house extension, regardless of whether planning permission is needed. Building Regulations cover structural safety, fire safety, thermal insulation, ventilation, drainage, and means of escape. A Completion Certificate issued by Building Control is essential — without it, the extension may cause problems when you come to sell the property.
A typical single-storey rear extension of 20 m² takes approximately 10–16 weeks from groundwork start to completion. A double-storey extension takes 16–24 weeks. A large or complex wrap-around or basement extension may take 6–12 months. Planning and design typically add 3–6 months before build work begins. Allow 9–18 months from decision to completion for a standard extension project.
A flat roof is typically 10–20% cheaper than a pitched roof for a single-storey extension, primarily due to lower material and labour costs. However, a well-designed pitched roof typically looks better, is more durable, and adds more to property value. Modern flat roof systems (EPDM rubber, fibreglass/GRP) are highly reliable with 25–50 year lifespans. Adding a lantern or large rooflights to a flat roof significantly improves natural light and adds value.
A double-storey extension costs approximately 50–70% more than a single-storey of the same ground footprint — but delivers twice the floor area. This makes double-storey extensions significantly more cost-effective per m² than single-storey. The foundations, scaffold, and planning process are similar for both; the additional cost of the upper storey is primarily structural (floor joists, upper walls) and finishing (additional rooms, staircase).
Professional fees for a UK house extension project in 2026 typically amount to 12–20% of the build cost. This covers: architect or architectural designer (8–15% of build cost); structural engineer (£500–£2,500); planning application fees (£258–£578 in England); Building Regulations fees (£200–£900); and party wall surveyor if required (£700–£2,500). These fees must be budgeted in addition to the build cost.
If your extension involves work on or near a shared wall with a neighbour (e.g. a terraced or semi-detached property), or requires excavation within 3–6m of a neighbour's foundations, the Party Wall etc. Act 1996 applies. You must serve written notice on affected neighbours at least 1–2 months before work begins. If neighbours dissent, party wall surveyors must be appointed — adding cost and time. Factor £700–£2,500+ per surveyor into your budget if the Act applies.
New-build construction and certain conversions qualify for zero-rated or reduced-rate VAT, but most house extension work on existing dwellings is subject to 20% standard rate VAT. The reduced rate of 5% may apply in some circumstances (e.g. where the property has been empty for 2+ years). Always confirm the applicable VAT rate with your contractor before signing a contract — VAT on a £60,000 extension adds £12,000 at the standard rate.
Extension prices vary enormously between contractors — variation of 30–50% is common for equivalent specifications. Always obtain a minimum of three written quotes from established local builders. Ensure all quotes cover the same scope of work, specification, and inclusions. Beware of unusually low quotes — they may indicate underspecification, hidden costs, or an unreliable contractor. Ask for references from recent comparable projects.
| Extension Type | Planning Permission Required? | Building Regulations Required? | Notes |
|---|---|---|---|
| Single-storey rear – detached house, up to 4m deep | No – permitted development (PD) | Yes – always required | Subject to height (max 4m), boundary (min 2m from boundary if within 2m), and other PD conditions |
| Single-storey rear – semi / terraced, up to 3m deep | No – permitted development | Yes | 3m PD limit for semi-detached and terraced; Neighbour Consultation Scheme allows up to 6m |
| Single-storey rear exceeding PD depth limits | Yes – planning required | Yes | Full planning application required; design must comply with local planning policies |
| Double-storey rear extension | Usually Yes – planning required | Yes | Double-storey rear extensions are generally outside PD rights; check with LPA |
| Single-storey side extension | Usually Yes – side extensions restricted under PD | Yes | PD only permits side extensions on detached houses that are less than half the width of the original house and single-storey; terraced/semi side extensions usually need PP |
| Wrap-around (side + rear) extension | Yes – planning required in most cases | Yes | Combination extensions typically exceed PD limits; always apply for planning |
| Extension on listed building | Yes – Listed Building Consent + planning permission | Yes | LBC required for any external or internal works to a listed building |
| Extension in Conservation Area | Usually Yes – PD rights often restricted | Yes | Article 4 directions in Conservation Areas frequently remove PD rights; check with LPA |
| Extension over existing garage | Usually No – PD if within limits | Yes | Check PD height and volume limits; structural assessment of existing garage required |
| Basement / new basement | Depends – check with LPA | Yes | Basement excavation may not require PP but often does; always check; structural engineer essential |
Planning rules above apply to England only and are correct to the best of our knowledge for 2026. Wales, Scotland, and Northern Ireland have different planning rules. Always confirm with your Local Planning Authority and Building Control office before commencing any extension works. PD rights may have been removed from your property by an Article 4 direction or planning condition — check your property's planning history on your local council's planning portal.
House extension costs in the UK in 2026 range from approximately £28,000–£38,000 for a small 15 m² single-storey rear extension at budget specification, up to £200,000–£300,000+ for a large luxury wrap-around or double-storey architect-designed extension. The most common scenario — a 20 m² single-storey rear extension at standard specification in South East England — costs approximately £48,000–£68,000 for the build, plus professional fees of £6,000–£12,000, planning/Building Regulations fees of £500–£1,500, and kitchen fitting if required. In North England and Scotland, the equivalent build costs approximately £38,000–£54,000. Always obtain at least three quotes from established local builders.
In England, many single-storey rear extensions do not require planning permission under Permitted Development Rights. A single-storey rear extension can be built without planning permission if it extends no more than 4m from the original rear wall of a detached house (3m for semi-detached and terraced), does not exceed 4m in height, is not forward of the principal elevation, and does not cover more than 50% of the original garden. Under the Neighbour Consultation Scheme, these limits can be extended to 8m (detached) and 6m (semi/terraced) subject to neighbour notification. Most double-storey, side, and wrap-around extensions require planning permission. Always check with your Local Planning Authority before assuming PD applies to your specific property.
A 4m × 5m single-storey rear extension (20 m²) costs approximately £48,000–£68,000 at standard specification in South East England in 2026, or £38,000–£54,000 in North England. At high specification with bi-fold doors, underfloor heating, and a quality finish, the same footprint might cost £68,000–£95,000. Adding a kitchen to this extension typically adds £12,000–£30,000 for a standard fitted kitchen, bringing the total to approximately £60,000–£125,000 depending on specification. These figures exclude VAT (add 20%), professional fees (add £6,000–£15,000), and external works.
In most cases, yes — a double-storey extension offers significantly better value per m² than a single-storey. A double-storey rear extension on a 20 m² footprint delivers 40 m² of additional floor space for approximately 55–70% more than the cost of a single-storey on the same footprint. The planning, professional fees, scaffold, and foundation costs are broadly similar for both; the upper storey adds relatively modest incremental cost for the large floor area gained. If you need both a kitchen extension and an additional bedroom, a double-storey extension is almost always more cost-effective than building two separate single-storey extensions.
The construction phase of a typical single-storey rear extension (20 m²) takes 10–16 weeks from groundwork start to practical completion. A double-storey extension typically takes 16–24 weeks. Complex or large projects (wrap-around extensions, basements) may take 6–12 months. However, the total elapsed time from deciding to extend to moving back into the finished space is typically 12–18 months, accounting for: design and planning application preparation (2–4 months); planning determination (8–10 weeks); Building Regulations application (3–5 weeks for Full Plans); tendering and contractor selection (4–8 weeks); and construction. Starting the design and planning process as early as possible is the best way to reduce the total project duration.
The cost per m² for a house extension in the UK in 2026 varies significantly by specification and region. As a guide: budget specification: £1,800–£2,300/m²; standard specification: £2,300–£3,200/m²; high specification: £3,200–£4,500/m²; luxury / architect-led: £4,500–£6,500+/m². These figures are for South East England; deduct 12–20% for North England, Scotland, and Northern Ireland, and add 15–30% for Greater London. The cost per m² tends to decrease slightly for larger extensions as fixed costs (site set-up, scaffolding, planning) are spread over a larger area. Double-storey extensions also typically achieve a lower cost per m² than single-storey due to the efficient use of foundations and external envelope.
Yes — a well-designed and well-built house extension typically adds 10–20% to a property's value, and in many cases the uplift in value exceeds the cost of the extension. The type of extension matters significantly: kitchen-diners and open-plan living spaces added to the rear of a house consistently deliver the highest returns, as do extensions that create or upgrade the bedroom count (particularly with an en-suite). In high-demand property markets (London and South East), extensions can add £80,000–£200,000+ to a property's value. However, over-development of a property beyond the ceiling value for its street means extensions can reach a point of diminishing returns — always research comparable sold prices in your area before committing to major extension costs.
The Party Wall etc. Act 1996 is UK legislation that governs works to shared walls and certain excavations near neighbouring buildings. It applies to your extension if you plan to: build on or up to the boundary with a neighbour; cut into a party wall or party fence wall; excavate within 3–6m of a neighbouring building's foundations (depending on depth); or insert beams or steels into a party wall. You must serve formal Party Wall Notices on all affected neighbours at least 1–2 months before work begins. If neighbours consent, you can proceed. If they dissent (or don't respond), party wall surveyors must be appointed to prepare a Party Wall Award — costing £700–£2,500 per surveyor. Budget for this early in your project timeline as it can cause significant delays if not managed proactively.
You are not legally required to use a registered architect (RIBA Chartered Architect) for a house extension, but using a qualified designer of some kind is strongly recommended. Options range from a RIBA Chartered Architect (most comprehensive service, highest fees — 10–15% of build cost for full service) to an architectural designer or technician (lower fees, typically 5–10% for drawings and planning package) to a design-and-build contractor who handles design in-house. For complex, high-value, or architecturally sensitive projects, a RIBA architect is worth the investment. For straightforward single-storey extensions, a competent architectural designer or RIBA associate can produce the necessary drawings and handle the planning application for £1,500–£4,000. Always use someone with professional indemnity insurance.
The most cost-effective types of house extension in the UK in 2026 are: extension over an existing garage (the structure already exists; work mainly involves insulation, windows, internal finishing — typically £35,000–£65,000 for a 15–20 m² room); and small single-storey rear extension using a flat roof (lower specification flat roof significantly reduces cost vs pitched). The key to minimising extension costs without compromising quality is: keep the footprint as simple (rectangular) as possible; use a flat roof rather than pitched where appropriate; avoid structural steelwork by designing within timber span limits; minimise the number of corners, hips, and valleys in the design; and use standard cavity brick/block rather than specialist frame systems. Using a design-and-build contractor rather than separate architect + builder can also reduce fees and simplify project management.
Finding a reliable builder for a house extension requires thorough due diligence. Key steps: obtain recommendations from neighbours and friends who have had similar work completed recently; check reviews on Checkatrade, TrustATrader, or Google — look for consistent recent reviews specifically mentioning extension projects; ask to visit recently completed extension projects and speak directly with the homeowners; obtain a minimum of three detailed written quotations ensuring all are based on the same drawings and specification; confirm the builder carries adequate public liability insurance (minimum £2 million, ideally £5 million for larger projects); check they will manage the Building Regulations inspection process and provide a Completion Certificate; agree a clear payment schedule tied to stage completions rather than paying large advance sums; and use a formal contract (JCT Minor Works or similar) rather than relying on verbal agreements. Be cautious of any builder who cannot provide recent extension references or who requests more than 10–15% deposit upfront.
The key differences are in structure, thermal performance, planning classification, and cost. A house extension is constructed with solid insulated walls, a fully insulated roof, and Building Regulations-compliant thermal performance — it is a permanent structural addition to the dwelling that can be used year-round and is treated as part of the house for all purposes. A conservatory has predominantly glazed walls and roof, and traditionally has been exempt from Building Regulations' energy efficiency requirements because it is thermally separated from the house. Conservatories are typically 40–60% cheaper per m² than brick extensions, but provide a lower quality of year-round space. Modern orangeries and glass garden rooms bridge the gap — providing the light and garden connection of a conservatory with improved insulation approaching a solid extension. For a year-round fully functional living space, a brick or timber-frame extension is always the superior option; for a budget garden room with decorative appeal, a well-specified conservatory or orangery may offer better value.
Additional house extension resources 2026: Planning Portal | Checkatrade – Find a Builder | RIBA – Find an Architect | GOV.UK Building Regulations | TrustATrader