House Extension Costs UK 2025: Single Storey, Double Storey & More
Adding space to your home is one of the most effective ways to improve your quality of life and increase property value — but the range of costs is wide, and what you end up paying depends heavily on the type of extension, where you live, the finishes you choose, and whether you need structural steel, bi-fold doors, or a Party Wall surveyor. This guide covers every main extension type, with realistic cost ranges, planning rules, regulatory requirements, and practical advice on getting the best from your project.
Types of House Extension and What They Cost
There is no single "extension cost". The structure you build, its size, and the complexity of the work are the primary drivers.
| Extension Type | Typical Size | Cost Range |
|---|---|---|
| Single-storey rear | 20m² | £25,000–£45,000 |
| Single-storey rear | 30m² | £35,000–£60,000 |
| Double-storey rear | 40m² total | £40,000–£80,000+ |
| Side return (infill) | 10–15m² | £20,000–£40,000 |
| Over-garage | 20–30m² | £30,000–£55,000 |
| Wrap-around (side + rear) | 30–50m² | £50,000–£100,000+ |
These are project costs including labour, materials, VAT, and a basic kitchen or utility room fit-out where applicable. They do not include architect fees, planning fees, Party Wall surveyor costs, or landscaping.
London and the South East typically add 20–30% to these figures. The North of England and Wales are often at the lower end.
Key Cost Drivers
Structural steel is one of the biggest variables. An open-plan extension with a wide rear opening requires a structural steel beam (RSJ) to carry the load above. A single beam and installation can cost £1,500–£4,000. Larger openings or complex roof configurations can require multiple beams.
Bi-fold and sliding doors are a popular choice for rear extensions but carry a significant premium. Aluminium bi-folds for a 4-metre opening typically cost £3,000–£6,000 supply and fit. A full-width 6-metre set can reach £8,000–£12,000. uPVC alternatives are cheaper but rarely suit a contemporary extension aesthetic.
Roof specification matters more than most people realise. A flat roof with a living sedum or rubber EPDM finish costs less than a lantern rooflight (£2,000–£5,000) or a glazed roof structure. Pitched roofs to match the existing house cost more in labour but often look better and can qualify for more relaxed permitted development rules.
Finishes are where projects diverge most sharply in cost. Basic plaster and paint, laminate flooring, and standard fittings sit at one end. Underfloor heating (£50–£100/m²), polished concrete floors, bespoke cabinetry, and premium tiling sit at the other.
Party Wall Act surveyor costs (covered below) add £700–£3,000 to projects that affect a shared boundary.
Permitted Development Rights: When You Don't Need Planning Permission
Most single-storey rear extensions can be built under permitted development (PD) rights — meaning no planning application is required — subject to specific limits:
- Rear extension: no more than 4 metres deep for a detached house, 3 metres for a semi-detached or terraced property (standard PD)
- Under the Larger Home Extension scheme (prior approval): up to 8 metres deep for detached, 6 metres for semi or terrace, subject to neighbour consultation
- Maximum height of 4 metres; must not exceed the height of the original roof at the eaves
- No extension forward of the principal elevation
- Materials must be similar in appearance to the existing house
PD rights do not apply if you live in a conservation area, a listed building, an Area of Outstanding Natural Beauty (AONB), or if your property is subject to an Article 4 direction that withdraws PD rights. Always check with your local planning authority before assuming PD applies.
Double-storey extensions almost always require full planning permission, as do side extensions that would bring the building close to the boundary, and anything that would affect the principal elevation.
Building Regulations: What Must Be Approved
Planning permission (or a PD determination) is separate from Building Regulations approval. Every extension must comply with Building Regulations regardless of planning status. The key parts are:
Part A (Structure) — the foundations, walls, and roof must be structurally sound. A structural engineer is needed for any steel beam specification.
Part L (Conservation of Fuel and Power) — new walls, floors, and roofs must achieve specified U-values (typically ≤0.28 W/m²K for walls, ≤0.18 W/m²K for floors, ≤0.15 W/m²K for roofs). Glazed areas are limited to avoid excessive heat loss.
Part F (Ventilation) — habitable rooms require adequate ventilation. Kitchens and utility rooms need extract ventilation to outside.
Part B (Fire Safety) — escape routes, fire doors between garage and house, and smoke alarms.
Part P (Electrical Safety) — all new electrical work must be carried out by a registered electrician (NICEIC, NAPIT, or ELECSA registered), or notified to and inspected by Building Control.
Your builder is responsible for obtaining Building Regulations approval (via a full plans application or building notice), but you should confirm this in writing before work starts. On completion, you will receive a completion certificate — keep it, as you will need it when you sell.
The Party Wall Act 1996
If your extension is within 3 metres of a neighbour's property, or involves excavation within 6 metres, the Party Wall etc. Act 1996 applies. You must serve written notice on the adjoining owner(s) at least two months before work starts (one month for excavation).
If the neighbour agrees in writing, no surveyor is needed. If they dissent (or do not respond within 14 days), both parties must appoint surveyors — either two separate surveyors, or an agreed single surveyor. The building owner typically pays both surveyors' fees.
Typical surveyor costs:
- Agreed single surveyor: £700–£1,200
- Two separate surveyors: £700–£1,500 each
The surveyors produce a Party Wall Award, which sets out how the work must be done, records the condition of the adjoining property beforehand, and provides a mechanism for resolving disputes. Never start work until the Award is in place if there has been any dissent.
Value Added by Extension Type
Extensions are generally a sound investment, though the return depends on location, property type, and what the extension creates.
| Extension Type | Typical Value Uplift |
|---|---|
| Single-storey rear (kitchen/living) | 3–8% |
| Side return (kitchen enlargement) | 5–10% |
| Double-storey rear (adds bedroom + living) | 8–15% |
| Over-garage (adds bedroom/study) | 5–10% |
| Wrap-around | 10–18% |
In a £400,000 property, a double-storey extension that adds 8–15% in value could return £32,000–£60,000 in uplift — potentially offsetting much of the build cost. However, extensions can overcapitalise in lower-value areas. A RICS-registered valuer can give you a realistic pre- and post-extension value estimate.
Architect vs Design-and-Build
Architects (look for RIBA members) provide independent design advice, produce detailed drawings for planning and Building Regulations, and can project-manage the build. Fees are typically 5–12% of the project cost for a full service, or £1,500–£5,000 for drawings only.
Design-and-build contractors combine the design and construction into one contract. They are often quicker to mobilise and provide a single point of contact. The trade-off is that their design is usually less bespoke and the specification may not be independently checked.
For a straightforward single-storey extension, a design-and-build firm overseen by your own structural engineer is a common and cost-effective approach. For a complex double-storey or wrap-around, or a listed building, an independent RIBA architect adds real value.
Always get at least three quotes. A good specification (drawn plans, finishes schedule, and materials spec) is essential — without it, quotes are not comparable. Membership of the Federation of Master Builders (FMB) is a useful marker of professionalism, though not a guarantee of quality. Check references and, where possible, visit a completed project.
Red flags: vague quotes without a written spec; requests for large upfront cash payments; no public liability insurance; pressure to sign quickly; no Building Regulations process discussed.
Next Steps
- Decide on the type and approximate size of extension you want and get a ballpark cost from this guide.
- Check whether your project falls under permitted development or needs planning permission — the Planning Portal has a useful interactive tool.
- Appoint an architect or structural engineer for drawings before approaching builders.
- Obtain at least three written quotes against the same specification.
- Serve Party Wall notices early — the two-month notice period can delay your start date if left late.
- Confirm Building Regulations approval will be obtained before work starts, and ask for the completion certificate on handover.
Use the Renovate Me platform to build a personalised renovation roadmap, track your budget, and find accredited tradespeople for your extension project.