What does an extension cost calculator actually do?
An online extension cost calculator takes a small number of inputs — usually floor area, extension type, and location — and multiplies them by a cost-per-square-metre rate to produce an estimate. Most use figures in the range of £1,500–£3,500/m², sometimes with a regional multiplier for London or the South East.
That is the extent of what they do. They do not account for:
- The complexity of your specific structure
- Whether groundworks are straightforward or involve contaminated material, high water table, or tree roots
- Whether you need a structural steel frame
- What level of internal finishes you want
- Whether you need planning permission, a Party Wall Agreement, or building over a sewer
- How accessible your site is
- What your existing services (electrics, heating, plumbing) look like and whether they need extending
How extension cost calculators can mislead you
The biggest danger is anchoring. Once you have seen a figure — even a rough one — it becomes your reference point. If a contractor quotes you significantly more, your instinct is to assume they are overpriced, rather than to consider that the calculator was oversimplified.
In practice, a calculator might show you £45,000 for a 25m² rear extension. A properly qualified contractor who has visited your property, reviewed your drawings, and understood your specification might quote £68,000. That difference is not markup — it is real scope the calculator could not see.
Cost-per-m² ranges in 2026
| Extension type | UK-wide range | London premium |
|---|---|---|
| Single-storey rear | £1,800–£2,800/m² | £2,400–£3,800/m² |
| Single-storey side | £1,800–£2,800/m² | £2,400–£3,800/m² |
| Double-storey rear | £1,600–£2,500/m² | £2,200–£3,400/m² |
| Wrap-around | £1,900–£3,000/m² | £2,600–£4,000/m² |
| Kitchen extension (high spec) | £2,500–£4,000/m² | £3,200–£5,000/m² |
These figures include structural work, roofing, insulation, glazing, plastering, first-fix electrics and plumbing, and basic finishes. They exclude: kitchen or bathroom fixtures, flooring, decoration, external landscaping, professional fees, and VAT.
What a calculator cannot tell you
Groundworks
Groundworks are one of the highest-risk cost items in any extension project. Clay soils require deeper strip foundations or engineered piles. Made ground from previous development can require significant over-excavation. Tree roots from protected trees require specialist root management. Water tables in some areas require waterproofing membranes. None of this is visible on a calculator form.
Structural steels
Most extensions that create a large opening into the existing house require a structural steel beam (RSJ or UC section). Depending on span, loading, and what is above, this can range from £800 to £8,000+ installed. Some projects require multiple beams. A calculator has no way of knowing this.
Services and utilities
If your extension includes a kitchen or utility room, you will need gas, hot and cold water, drainage, and electrical circuits extended into the new space. If your boiler is at capacity, you may need a new one. If your consumer unit is outdated, it may need upgrading to support additional circuits. These are real costs not captured by a floor-area calculation.
Planning and professional fees
Planning applications cost £258 per house in England as of 2026 (or are free under Permitted Development if you notify via Prior Approval). But architect fees, structural engineer fees, CDM coordinator fees, party wall surveyor fees, and Building Control fees can add £3,000–£12,000 to a project. A cost calculator does not include these.
How to use a calculator sensibly
1. Use it as a broad filter only. If the calculator says £35,000 and your budget is £50,000, you may have headroom. If the calculator says £95,000 and your budget is £50,000, the project is very unlikely to be viable.
2. Use multiple calculators. Take the range rather than a single figure.
3. Add 15–25% for contingency to any figure you get, before you do anything else.
4. Do not compare calculator outputs with contractor quotes. They are measuring different things.
5. Get a proper feasibility estimate. A qualified quantity surveyor or design-and-build contractor can give you a structured ballpark estimate based on your drawings and a site visit. This is far more useful than any online tool.
The right way to budget a UK extension in 2026
The most reliable budget process for a UK extension is:
1. Brief an architect or designer to produce planning drawings
2. Obtain a pre-planning feasibility estimate from a contractor or QS
3. Submit for planning if required, or confirm Permitted Development compliance
4. Commission technical drawings (structural, M&E, building regulations)
5. Get detailed contractor quotes based on full information
6. Confirm budget, agree contract, and proceed
A cost calculator sits at step zero — it tells you whether it is even worth pursuing the conversation. After that, it should be set aside.
Get a structured estimate
Dwellinger connects homeowners with Builder Score-verified contractors who can provide structured ballpark estimates based on your specific project and drawings — not a generic price per square metre.