A useful extension budget starts with what you want built, what happens to the existing house and what the builder includes. A square-metre figure alone cannot answer all three. A small kitchen extension with a large structural opening can cost more per square metre than a larger, simpler room.
The figures here are illustrative budgeting assumptions, not a survey of current London quotes or a price promise. Planning application records do not establish construction costs. Replace the assumptions with project-specific estimates before choosing a design or signing a building contract.
What does a cost per square metre tell you?
It gives you a way to test different budgets at an early stage. It only becomes comparable when each estimate uses the same floor-area measurement and specification. Ask whether the area is internal or external and whether work to the existing house is included.
For example, testing a 25m² single-storey extension at assumed construction rates of £3,200, £3,900 and £4,600 per m² produces the following figures. These are sensitivity calculations, not three verified market specification bands.
| Assumed rate | Area | Construction allowance before VAT |
|---|---|---|
| £3,200/m² | 25m² | £80,000 |
| £3,900/m² | 25m² | £97,500 |
| £4,600/m² | 25m² | £115,000 |
For this comparison, construction means the new structure, ordinary external glazing, basic internal finishes and services within the extension. It excludes kitchen supply, major alterations elsewhere, professional fees, statutory charges, landscaping and contingency. A builder may define the scope differently, so request an itemised description rather than assuming these inclusions.
Compare extension types by the work involved
| Project | Main budget questions |
|---|---|
| Single-storey rear | How much existing wall is removed? What foundations, drainage changes and roof glazing are required? |
| Side return | How will materials reach the rear? How much structural work is needed for a relatively small area gain? |
| Wrap-around | Does the design require a larger structural opening, more roof junctions or extensive ground-floor refurbishment? |
| Two-storey | What supports the additional storey, and how do stairs, bathrooms, roof connections and existing rooms change? |
| Loft conversion | What roof alterations, floor strengthening, stair arrangement and fire-safety work are required? |
| Basement | What excavation, temporary works, waterproofing, access and specialist design are required? |
There is no reliable universal rule that a two-storey extension is the best value or that a loft avoids all foundation-related work. Compare feasible designs with professional input. A basement needs a separate specialist estimate; do not price it by multiplying an ordinary rear-extension rate.
For more focused scope questions, see our side-extension planning guide and terraced house extension guide.
What belongs outside the building quote?
Keep the total project budget separate from the builder's contract price. Ask each provider whether VAT is included, excluded or not applicable to their own quotation.
- Design and technical work: measured survey, planning drawings, structural design, building regulations information and any tender or construction-stage services.
- Applications and approvals: council and submission charges, building control and site-specific reports or agreements where required.
- Owner purchases: kitchen, appliances, fittings and finishes supplied outside the builder's package.
- Work beyond the extension: utility upgrades, repairs, drainage alterations, garden reinstatement and decoration of adjoining rooms.
- Living arrangements: storage, temporary kitchen facilities and accommodation if needed.
- Contingency: money retained for defined uncertainties, separate from optional upgrades.
Our planning application cost guide covers application charges. The architect fee guide explains professional service scopes. Neither is a substitute for a construction estimate.
A worked total-project example
Suppose a homeowner receives a £90,000 construction quote including VAT for an agreed 25m² extension. The following separate allowances demonstrate how a headline quote can become a larger project budget. They are invented assumptions for the calculation, not typical fees or an actual customer project.
| Budget item | Example allowance including any applicable VAT |
|---|---|
| Agreed construction quote | £90,000 |
| Design, engineering and application-related costs | £8,000 |
| Kitchen and appliances excluded from construction | £15,000 |
| Landscaping and owner costs excluded above | £5,000 |
| Subtotal | £118,000 |
| Contingency assumed at 10% of subtotal | £11,800 |
| Total available budget | £129,800 |
Avoid counting the same work twice. If the builder includes kitchen installation or the design quote includes structural work, adjust the corresponding allowance. Ten per cent is simply this example's contingency assumption; unresolved ground conditions, incomplete design or substantial existing-building work may justify a different reserve.
Why the London address matters
Price the practical constraints instead of applying a fixed borough surcharge. Restricted access can affect deliveries, waste removal and labour. Parking arrangements, scaffolding, storage and working space should appear in the builder's proposed method and price.
Existing construction also matters. A period property may need more investigation or carefully designed junctions, but age does not determine a fixed premium. Read extension costs by house age for the questions to investigate before tendering.
How to obtain comparable quotes
Give builders the same drawings, specification and schedule of work. Ask them to identify exclusions, provisional sums, owner-supplied items, programme assumptions and how variations will be priced. Check quote validity and availability directly instead of relying on a general market lead time.
Resolve large unknowns before comparing totals. A low quote with substantial unpriced structural work is not equivalent to a more complete quotation. Record accepted changes in writing and keep a running forecast of the final cost throughout construction.
Start with our planning permission check to explore the planning route, then review current drawing packages. The check is not a builder's quotation. Agree the design and technical scope before seeking a construction price you can commit to.
Frequently asked questions
What should I do if every builder's quote exceeds my budget?
Ask the designer and builders to identify the main cost drivers before changing the specification. Compare a smaller footprint, fewer structural alterations or simpler finishes against the rooms you need. Agree a revised scope and obtain updated prices; removing contingency to make an unaffordable quote fit leaves the underlying uncertainty unresolved.
Can I rely on a quote while waiting for planning permission?
Check the quote's expiry date and the assumptions about drawings, specification and start date. Ask what would need repricing if approval changes the design or the build is delayed. A quotation is not necessarily a reservation of the builder's time; establish availability and booking terms separately before paying a deposit.