Ask five people what an extension costs and you'll get five different per-square-metre figures, because the honest answer is that it depends heavily on type, specification and location. This guide sets out realistic UK cost ranges, the professional fees and hidden costs that sit outside the headline build figure, and where extension budgets typically go wrong. For the process of planning and sequencing the work itself, see the Renovation Planning guide.
1. Realistic cost per square metre
A double-storey extension is not simply double the cost of a single-storey one for the same footprint. Adding a second storey shares the same foundations and roof-related savings across two floors, so the cost per square metre for the additional upper floor is often meaningfully lower than for the ground floor alone. This is one reason a double-storey extension can represent better value per square metre if you need the additional space, even though the total project cost is higher.
Costs vary significantly by region, with London and the South East typically at the higher end of the national range and other regions lower, reflecting both labour costs and demand for tradespeople. Getting current, local quotes is essential; national average figures are a starting reference point only.
2. What actually drives the cost
- Groundworks and foundations. Sloping sites, poor ground conditions, or proximity to trees and drains can add significant unplanned cost once a structural engineer assesses the site.
- Roof type and complexity. A flat roof is typically cheaper than a pitched roof matching the existing house, but a mismatched roofline can affect both appearance and resale appeal.
- Glazing. Large glazed openings, bifold or sliding doors, and roof lanterns add cost quickly relative to standard windows, but are often central to why people extend in the first place.
- Structural openings. Removing walls between the extension and the existing house, particularly load-bearing ones, requires steel beams and structural calculations that add cost beyond the extension footprint itself.
- Services. Moving or extending plumbing, drainage, and electrics to serve the new space, particularly for a kitchen extension, is a genuine cost line that's easy to underestimate.
3. Fees and costs beyond the build
| Cost item | What it covers |
|---|---|
| Architect or designer | Drawings, planning application support, and design development |
| Structural engineer | Calculations for beams, foundations, and any structural openings |
| Planning application fee | Payable if the work falls outside permitted development rights |
| Building control fees | Inspection and sign-off of the completed structural work |
| Party wall agreement | Required if work affects a shared wall or boundary with a neighbour, potentially including a surveyor's fee |
| VAT | Standard-rated on most extension work, unlike some new-build construction |
Professional fees, planning and building control costs, and party wall costs are each individually modest compared with the build itself, but added together they can represent a meaningful percentage of the total project cost. Budget for them explicitly as a line item rather than treating the "cost of the extension" as the build quote alone.
4. Extension cost versus added value
An extension can increase a property's value, but the relationship between what you spend and what value it adds is rarely one-to-one, and depends heavily on the local market, the ceiling price for similar properties in the area, and how well the extension integrates with the existing house. In some cases, particularly where a property is already near the top of what the local market supports, a large or high-specification extension may add proportionally less value than it costs, even though it substantially improves your quality of life in the home. If resale value is a genuine consideration, research comparable extended properties in your specific area before committing to scope and specification.
5. Common mistakes
- Budgeting from the build quote alone. Professional fees, planning, building control and party wall costs need to be budgeted separately and explicitly.
- Underestimating groundworks risk. Sloping sites, poor ground conditions, and nearby trees or drains can add cost that isn't visible until a structural engineer assesses the site properly.
- Choosing specification before checking local resale ceilings. A high-specification extension in an area with a lower ceiling price may not return its cost if resale value matters to you.
- Assuming permitted development rights apply without checking. Confirm what your specific extension requires before committing to a design, since planning requirements affect both cost and timeline.
6. Frequently asked questions
Is a double-storey extension twice the cost of a single-storey one?
No. A second storey shares the same foundations and roof-related costs across two floors, so the additional floor is often meaningfully cheaper per square metre than the ground floor extension alone, even though the total project cost is higher.
Do I need a party wall agreement for an extension?
If the work affects a shared wall or boundary with a neighbouring property, a party wall agreement is typically required under the relevant legislation, which can include a surveyor's fee. Confirm whether this applies to your specific project early, since it affects both timeline and budget.
Will an extension definitely add more value than it costs?
Not necessarily. The value added depends on the local market, comparable extended properties in your area, and how well the extension integrates with the existing house. Research comparable local sales before assuming a straightforward return on the spend.
Is VAT charged on a house extension in the UK?
Most extension work on an existing home is standard-rated for VAT. This differs from some new-build construction, which can be zero-rated, so it's worth confirming the VAT treatment with your builder or accountant as part of the overall budget.
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