Enter your extension type, floor area, specification level and location to instantly estimate your house extension cost from a compact rear kitchen addition to a full double-storey build, with all key cost components included.
Extension Specs
Example: A 5x6m single-story extension is 30m².
Estimated Breakdown
Core Build Cost£66,000
Architect & Planning (~10%)£6,600
VAT Element (20%)+ £13,200
Predicted Total:
£85,800 - £92,400The higher number includes a 10% safety contingency fund. Always have a contingency when building.
Key Considerations
VAT Applicability
For standard residential extensions, Labour and Materials are generally subject to 20% VAT unless the property has been empty for over 2 years.
Hidden Costs
Don't forget to account for moving manholes, deep foundations for unstable soil, and Party Wall Agreements if building near a neighbor.
How Much Does an Extension Cost in the UK (2024)?
Building an extension is one of the most effective ways to add value and space to your property, but budgeting accurately is critical. A standard 30sqm single-story extension outside of London will typically start from £55,000 to £70,000+ depending on the level of finish.
Single Storey vs Double Storey
A common misconception is that a two-story extension will cost double the price of a single-story. It doesn't.
The most expensive elements of an extension are the foundations and the roof. Because a two-story extension uses the same foundation footprint and the same roof size as a single-story extension, the cost per square metre is usually cheaper on the second floor. Our calculator automatically adjusts the rate per square metre based on the storey you select.
When receiving quotes from builders, strictly clarify whether they are quoting inclusive or exclusive of VAT. 20% is a huge sum on a build project. Our calculator explicitly breaks out the 20% VAT element so you aren't hit with unexpected surprises.
The Contingency Fund
As seen in the calculator results, we always calculate a 10% contingency into your maximum range. Groundworks can reveal unexpected pipes. Soil types might require deeper trenches. You might decide you prefer the more expensive tiles midway through the build. Always secure a contingency fund before breaking ground.
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How Much Does a House Extension Cost in the UK?
House extension costs in the UK range from £1,800 to £3,500 per square metre in 2026, depending on extension type, specification and location. A typical 20m² single-storey rear extension costs between £40,000 and £66,000 at standard specification before VAT. London and the South East add a 20 to 40 percent premium on top of national average rates.
Here is a full overview by extension type at standard build quality:
Extension Type
Cost Per m² (Ex VAT)
Typical Total (20m²)
Build Time
Single storey rear
£1,900 – £3,300
£40,000 – £66,000
10–16 weeks
Double storey
£2,000 – £2,800
£80,000 – £140,000 (40m²)
16–24 weeks
Side return
£2,500 – £3,500
£15,000 – £35,000 (10m²)
8–14 weeks
Wraparound (L-shape)
£2,400 – £3,300
£40,000 – £100,000
14–20 weeks
Shell only
£1,300 – £1,900
£26,000 – £38,000
8–12 weeks
All figures include groundworks, foundations, structure, roofing, windows, doors, electrics, plumbing and plastering. They exclude VAT at 20%, architect and structural engineer fees, kitchen or bathroom fitting, and landscaping.
Extension Cost Per m² by Specification Level
Specification level is one of the most significant variables in any extension quote. Two extensions of identical size can cost very different amounts depending on the materials, roof type and finishes chosen.
Specification
Cost Per m² (Ex VAT)
What It Includes
Budget
£1,500 – £1,900
Flat roof, standard brickwork, basic windows
Standard
£1,900 – £2,800
Pitched roof option, uPVC or aluminium windows, mid-range finishes
Premium
£2,800 – £4,500
Bi-fold or aluminium sliding doors, structural glazing, high-end finishes
Bi-fold doors add £3,000 to £8,000 depending on width and glazing specification. Structural glass or roof lanterns add a further £3,000 to £12,000. A flat roof extension is cheaper structurally but premium glazing above it is often where the cost climbs quickly.
Single Storey vs Double Storey
This is one of the most common questions on Reddit forums and homeowner communities and the answer is not what most people expect.
Double storey extensions cost less per square metre than single storey because foundations, drainage and groundwork are shared across both floors. A second storey typically adds 40 to 60 percent to the cost of the equivalent single-storey build rather than doubling it. For a 20m² footprint, a single storey costs approximately £40,000 to £66,000. A double storey on the same footprint, delivering 40m², costs approximately £80,000 to £140,000, meaning you get twice the floor area for roughly 1.6 times the cost.
The practical considerations that determine the right choice:
Single storey is better when you need more ground-floor space only, want to avoid planning complexity or your property type makes upstairs extension impractical. It is also the standard choice for kitchen-diners, garden rooms and living space extensions.
Double storey is better when you need both a bedroom and ground-floor space, when the per-m² economy matters or when a single storey would consume too much garden. Double storey extensions often deliver stronger returns on investment because they add more functional space that buyers value, a bedroom and an additional living area in a single project.
What Is Included in an Extension Quote
Understanding what a typical builder's quote covers is essential when comparing prices. Most quotes cover the shell and first-fix elements:
Usually included: Groundworks and foundations, brickwork and blockwork, roof structure and covering, windows and external doors, first-fix electrics and plumbing, plastering and basic internal decoration.
Usually excluded:
Kitchen units, worktops and appliances (add £8,000 to £35,000)
Bathroom or shower room fitting (add £3,000 to £8,000)
Flooring and tiling (add £2,000 to £8,000)
External landscaping, patios and garden reinstatement (add £1,000 to £5,000)
Drainage connections and soakaway (add £1,500 to £3,500)
VAT at 20% on all elements
Architect and structural engineer fees
Always request a fully itemised quote with provisional sums for groundwork and steelwork included, not excluded. A quote that omits structural steelwork is not directly comparable to one that includes it.
Additional Costs Every Extension Budget Must Include
These items are frequently absent from early cost estimates and collectively add £10,000 to £25,000 to most mid-sized extension projects.
VAT at 20% applies to all house extension work on existing residential properties. Not all small builders are VAT-registered and therefore charge no VAT. This looks cheaper on paper but means they cannot provide a VAT receipt, important if you ever run a business from home or claim any VAT back. Always clarify VAT registration status before comparing quotes.
Architect and structural engineer fees add 10 to 15 percent of the build cost. A structural engineer alone costs £550 to £1,500 for a residential extension report and drawings.
Planning application fee: the householder planning application fee in England increased to £548 from April 2026. A Lawful Development Certificate (strongly recommended even for permitted development work) costs around £258, typically half the planning fee.
Party wall agreement: required when work affects a shared wall or sits within 3 metres of a neighbour's foundations. Budget £1,000 to £2,500 per affected neighbour for a surveyor-agreed award.
Contingency: always build in 10 to 15 percent of the build cost. A 2026 survey by the HomeOwners Alliance and FMB found that ground conditions, unexpected utilities and structural issues are the most common sources of cost overrun, with 47% of homeowners reporting final costs higher than the original quote.
Planning Permission and Permitted Development
Many single-storey rear extensions fall under permitted development rights and do not require a planning application. Under current rules in England:
Attached houses can extend up to 3 metres from the original rear wall without planning permission
Detached houses can extend up to 4 metres
Under the Prior Approval process (Larger Home Extension scheme), attached houses can extend up to 6 metres and detached up to 8 metres
Planning permission is required if the extension exceeds these limits, covers more than half the garden, extends higher than the existing roof or if your permitted development rights have been removed by an Article 4 direction. Conservation area properties and listed buildings have additional restrictions regardless of size.
Lawful Development Certificates are not legally required but are strongly recommended, as buyers' solicitors regularly seek confirmation during future sales that alterations were carried out lawfully.
How Much Value Does an Extension Add?
A well-executed house extension adds 10 to 20 percent to property value, typically returning £1.20 to £1.50 for every £1 spent. In London, where space is at a premium, well-designed extensions can add as much as £4,000 per square metre of new floor area in added value.
The extensions that consistently deliver the strongest return are kitchen-diners and additional bedrooms, the spaces buyers value most. Extensions that create open-plan living spaces and improve natural light tend to outperform those that simply add square footage without improving the quality of the space.
One important caution: do not over-extend for your street. Building a significantly larger house than comparable properties on your road can reduce your return because the ceiling price for that road limits what buyers will pay. A local estate agent can advise on the realistic ceiling price for your postcode before you commit to a full double-storey build.
