The number a calculator gives you is usually a fair reflection of what it measures. The problem is what it measures.
Most tools price the structural shell and basic finishes. On a typical extension the shell accounts for somewhere around 60 to 70 percent of the total, which means the gap between the calculator and the final invoice is structural rather than a rounding error. Experienced architects london homeowners work with tend to use calculators as a sense check on builders’ quotes rather than as a budget.
Quick answer: House extension cost calculators typically price foundations, walls, roof, windows, doors and first fix services, which is roughly 60 to 70 percent of total project cost. They usually exclude VAT at 20 percent, professional fees of £6,000 to £15,000, kitchen fit out of £10,000 to £25,000, planning and Building Control fees, party wall costs and making good the existing house. Add 40 to 60 percent to a shell figure for a realistic total.
What Does a House Extension Cost Calculator Actually Include?
Most calculators price the structural shell plus a level of internal finish, and the better ones say so explicitly.
Commonly included:
- Foundations, external walls, roof structure and covering
- Windows and external doors
- First fix electrics and plumbing routed in
- Plastering and basic finishes
- Labour, materials, preliminaries and standard waste removal
- Typical site set up
Some tools stop at shell only, which comes in roughly 30 to 40 percent below a full build figure. Worth reading the small print before comparing two calculators against each other.
Which Costs Do Cost Calculators Leave Out?
The exclusions are consistent across almost every tool, and they add up to a substantial second budget.
| Excluded item | Typical 2026 cost |
| VAT on most extension work | 20 percent |
| Professional fees combined | £6,000 to £15,000 |
| Mid range fitted kitchen | £10,000 to £25,000 |
| Bespoke kitchen | £25,000 to £60,000 and above |
| Bathroom refit | £4,000 to £15,000 |
| Householder planning application | £548, plus around £91 portal charge |
| Building Control fees and inspections | £800 to £1,500 |
| Structural engineer’s calculations | £300 to £3,000 depending on scope |
| Party wall surveyor | £900 to £1,800 per neighbour |
| Moving drains | £1,500 to £3,500 |
| Boiler upgrade, where needed | £2,000 to £3,500 |
| Contingency | 10 to 15 percent, or 15 to 20 percent on complex structures |
Beyond that list sit flooring, lighting, decoration, premium appliances, structural glass, bespoke joinery, external works and landscaping. None of those are unusual on a kitchen extension, and none appear in a per square metre rate.
What Is the Most Commonly Forgotten Cost?
Making good the existing house. It appears in almost no calculator and is frequently absent from builders’ quotes too.
When an old rear wall comes out, the room in front of it needs replastering where the wall was, and the decoration in the affected rooms rarely survives the process. First and second fix work across a 25 square metre extension commonly runs £4,000 to £10,000, and tying into the existing consumer unit is a separate item that some quotes treat as an extra.
Insist on making good as a named line rather than an assumption. It’s the item most likely to arrive as a variation.
Why Do Small Extensions Cost More Per Square Metre?
Because the fixed costs don’t shrink with the floor area. Scaffolding, groundworks mobilisation, design fees, Building Control and forming the structural opening into the house all cost broadly the same whether the extension is 10 square metres or 30.
Spread over less area, those fixed costs push the per square metre rate up sharply. A 10 square metre extension is meaningfully more expensive per square metre than a 30 square metre one.
The practical implication is worth sitting with. If you’re going through the disruption and the planning process anyway, a very small extension rarely represents good value. The same logic explains why a two storey extension usually prices lower per square metre than a single storey one, since one roof and one set of foundations serve two floors.
How Much Do the Published Rates Actually Vary?
More than most guides admit, and the spread itself tells you something.
Published 2026 figures for a single storey rear extension range from around £1,800 to £3,200 per square metre for the UK, with London and the South East commonly quoted at a 20 to 40 percent premium, pushing figures to £2,800 to £4,500. Some tools publish considerably lower London ranges, and some publish figures including VAT while others exclude it.
That variation isn’t sloppiness. It reflects genuinely different scopes being priced under the same label. Before comparing any two numbers, establish whether each includes VAT, professional fees and fit out, because those three account for most of the difference.
A extension cost calculator is most useful once you know which of those three it covers.
What Swings an Extension Budget Most?
Seven factors move the total by a wide margin, and only two of them relate to size.
- Ground conditions. London clay or made ground can require deeper pads, often adding £3,000 to £8,000. Standard strip foundations for a 25 square metre extension run £3,000 to £6,000, while engineered solutions such as mini piles, a raft or deeper trench fill reach £8,000 to £15,000 and beyond
- Trees within around five metres, which can force engineered foundations on their own
- Access. Rear only access through a narrow side passage adds meaningfully to labour, with one source putting it at 8 to 12 percent
- Planning route. Permitted development is fastest, while conservation area or Article 4 constraints add design work and time
- Finish specification. Premium aluminium glazing can run several times the cost of standard alternatives
- Glazing percentage. Exceeding 25 percent of the extension floor area in glazing generally triggers Part L compensation measures or a SAP assessment
- Changes after ordering, which are priced without competition and are consistently the most expensive way to alter a scheme
How Should You Compare Builders’ Quotes?
Get three, then look at the middle one rather than the lowest.
Quotes for genuinely similar work commonly vary by 20 to 40 percent. That variance usually reveals differences in what has been priced rather than differences in quality, and the lowest quote is frequently the one missing scope. Party wall costs, scaffolding, structural calculations, skip permits and decoration are the items most often absent.
Before requesting quotes, have four things ready:
- Architect’s drawings
- Structural calculations
- Planning consent, or written confirmation of permitted development
- A written specification
Without those, you’re not comparing prices. You’re comparing assumptions, and each builder will have made different ones.
How Do You Turn a Calculator Figure Into a Real Budget?
Work upwards from the shell in a fixed sequence, so nothing gets absorbed into an assumption.
Start with the calculator output and confirm whether it includes VAT. Add professional fees at 8 to 15 percent of build cost, then the council and Building Control fees, then structural engineering. Add kitchen and bathroom separately at their own rates. Add flooring, lighting and decoration. Add making good the existing house. Then apply a contingency of 10 to 15 percent, or 15 to 20 percent if the property is old or the ground is uncertain.
Done properly, that exercise commonly lands 40 to 60 percent above the headline construction figure. That isn’t the calculator being wrong. It’s the calculator answering a narrower question than the one you were actually asking.
