Pricing guide
How to price a house extension
An extension asks more of a quote than most jobs: groundworks, structure, roofing and a full internal fit-out, usually while the client still lives in the house around it. Price it in the build sequence and keep every figure that depends on something you cannot yet see clearly marked as provisional.
Short answer
Price a house extension by working through it in build order: groundworks and structure first, then roof, first fix and finishes. Keep foundation depth, steel sizes, party wall costs and anything behind the existing wall as provisional until a structural engineer or the opened-up site confirms them, and add preliminaries, contingency and profit on top of the direct trade costs rather than folding everything into a single rate.
Quick check
- Confirm foundation depth and ground conditions before pricing groundworks firm
- Get the structural engineer's calculations for steels and openings before pricing them firm
- Serve Party Wall Act notices early if the extension sits near a boundary
- Price the trade sequence, not a single rate: groundworks, structure, roof, first fix, finishes
- Separate firm prices from provisional sums for drainage, substrate and the existing structure
- Cost welfare, scaffold, skips and site protection as preliminaries, not overhead
- Build in contingency for what a site visit cannot see below ground or behind old walls
- Show the client the sequence and the provisional items so a cheaper quote is easy to compare
1. Scope the job on site before pricing it
An extension quote is only as good as the information behind it. Capture the following before you start pricing, and record what is still missing rather than guessing it.
- The current drawing revision and any structural calculations already produced
- The age and construction of the existing house, for asbestos risk in pre-2000 properties
- Access for materials, skip and scaffold, including any shared driveway or narrow side passage
- Ground conditions and foundation type where visible, and whether trial holes are needed
- Existing service runs (drains, water, gas, electricity) that cross the extension footprint
- Planning status: permitted development, prior approval or a full application
- Proximity to the boundary and whether the Party Wall Act applies
Do not price steels and openings from a sketch
Structural elements depend on the engineer's final calculations, not a rough idea of what the room needs. Price a provisional allowance and confirm it once the design is signed off.
2. Labour and trade sequence
An extension is built in a fixed order, and pricing it in that order makes it easier to see what drives the cost at each stage.
| Stage | What drives the price |
|---|---|
| Groundworks and drainage | Ground conditions, foundation depth, and any existing drain diversion. |
| Structure and steels | Opening sizes, the engineer's specified steel sections, and temporary support while they go in. |
| Roof | Structure type, covering, and the junction with the existing roof. |
| First fix | How far new services extend and how they tie into the existing systems. |
| Finishes and second fix | Specification level, client-supplied items, and decoration. |
3. Materials and where costs blow out
Steel, brick and roofing lead times move independently of the build programme, and a mismatched brick or roof tile can hold up the whole job while a supplier sources a batch that matches the existing house.
Glazing and insulation specification chosen to meet current building regulations can add cost late in the process if it was priced from an assumption rather than the actual design. Confirm the specification before pricing the fit-out, not after the frames are ordered.
4. What to make provisional, not firm
Some items on an extension cannot be priced firmly until the ground is opened up or a specialist signs off the design. Name them rather than absorbing the risk into a single total.
| Item | Why it stays provisional |
|---|---|
| Foundation depth and type | Only confirmed once trial holes are dug or building control inspects the ground. |
| Party wall costs | Depend on the adjoining owner's surveyor and whether an award is disputed. |
| Steel sizes | Depend on the structural engineer's final calculations, not the initial concept. |
| Existing drainage diversion | Often unknown until excavation exposes the run. |
| Asbestos in the existing structure | Cannot be firm-priced until a survey or removal quote is in. |
Provisional does not mean vague
State what information is missing and how the figure will be adjusted once it is known. An unexplained allowance reads the same as padding to a client comparing quotes.
5. Access, welfare, scaffold, skips and preliminaries
- Scaffold hire duration and the access route it needs
- Skip size, number of exchanges, and a road permit if it sits on the highway
- Welfare facilities for the length of the build
- Protecting the occupied part of the house from dust and weather
- Temporary weatherproofing while the roof is open
- Parking, site security and material storage
6. Contingency and margin
Keep contingency, risk allowance and profit as separate lines rather than one blended figure. Contingency covers what a careful site visit cannot see, such as ground conditions or the state of a wall once it is opened up. Profit is the return for taking on the job, not a cushion for scope you have not yet checked.
Worked example
Size contingency to the risk, not the job value
A rear extension on a surveyed, well-drained plot carries less ground risk than one on clay or near mature trees. Base the contingency percentage on what you actually know about the site, and record the reasoning so it can be defended if the client asks.
7. The underquoting traps specific to extensions
- Treating the join between new and existing roof as a simple tie-in when it needs structural work
- Missing the cost of temporary weatherproofing if the roof is open during wet months
- Pricing steels from a sketch before the engineer's final calculation is issued
- Underestimating groundworks cost on clay or high water table sites
- Forgetting party wall surveyor fees and the delay a dispute can cause
- Not pricing making good to the existing house's decoration and services disturbed by the works
- Assuming new circuits and plumbing runs are a simple extension of the existing system
8. Present the quote so it compares fairly
Break the price into the same stages you priced it in, and keep the scope description next to each one. A client comparing your quote against a cheaper, vaguer one can see that your groundworks price includes a specific foundation type and your steels are based on an engineer's calculation, not a guess.
List provisional sums and their basis separately from the fixed price, so the client understands which figures could move and why. Quoting software can help keep this structure consistent from job to job, but the discipline of separating firm price from provisional allowance is what actually protects the margin.
Common questions
How much does a house extension cost?
There is no reliable single figure, because ground conditions, structural design, specification and access all change the price. Build the figure up from groundworks, structure, roof, first fix and finishes as described above, and treat any per-square-metre number you see elsewhere as a rough steer, not something to quote from directly.
Do I need planning permission to price an extension?
Many single-storey rear extensions fall within permitted development limits set out by the Planning Portal, but the limits depend on the house type, extension depth and whether the site is on designated land. Confirm the planning route before pricing, since a full application changes the programme and the fees involved.
Who pays for the party wall surveyor?
The building owner carrying out the work is normally responsible for the reasonable costs of the party wall process, including the adjoining owner's surveyor if one is appointed. Price this as a provisional sum, since the cost depends on whether the adjoining owner agrees or a dispute needs resolving.
When do I need a structural engineer to quote an extension?
Bring one in whenever the job involves new openings, steel beams, or a foundation design close to an existing structure. Pricing these elements from experience alone on anything beyond a straightforward single-storey extension is where quotes go wrong.
How do I price for asbestos risk on an older extension?
Treat any disturbance of materials in a pre-2000 property as provisional until a survey confirms what is present. If asbestos-containing material is found, licensed removal has its own cost and programme impact that cannot be estimated from the outside.
Sources and further reading
Primary and authoritative sources used to check this guide.
- Party Wall etc. Act 1996legislation.gov.uk
- The Party Wall etc Act 1996: explanatory bookletGOV.UK
- Extensions: planning permissionPlanning Portal
- Building regulations approval: when you need approvalGOV.UK
- AsbestosHealth and Safety Executive
General information for UK residential building work, not legal, tax or structural engineering advice. Party wall, planning and building regulations requirements depend on the specific property and works. Confirm uncertain treatment with a structural engineer, party wall surveyor, planning authority or other relevant professional.