Cost guide
House extension cost: how to read a price before you commit
Most people searching for extension prices are really asking two questions. Can I afford this, and is the quote in front of me a fair one. This page answers the second question properly, because once you can read a quote the first one gets a lot easier.
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Why a per square metre figure misleads people
Rate per square metre is a useful shorthand for a whole house of similar rooms. An extension is not that. It is a disproportionate amount of the expensive stuff, foundations, structure, roof, glazing and a new junction with an existing building, wrapped around a fairly small floor area. The rate therefore rises as the extension gets smaller, which is the opposite of what most people expect.
It also ignores the existing house. Cutting a large hole in a wall that is carrying a floor above it is a structural job with a cost of its own, and it has nothing to do with how many square metres you gained.
The five cost bands inside every extension quote
| Band | What sits in it | Typical share of the job |
|---|---|---|
| Below ground | Site clearance, dig, drainage diversions, foundations, slab | Large and highly variable |
| Structure | Walls, steels, roof structure, weathertight envelope | Large and fairly predictable |
| Openings | Windows, doors, rooflights, the structural opening into the house | Very variable by specification |
| Services | Electrics, heating, plumbing, ventilation | Moderate |
| Finishes | Plaster, floors, joinery, decoration | Moderate and easiest to phase |
When two quotes differ sharply, the gap is almost always in below ground work or openings.
Questions that expose a weak quote
- What foundation depth have you allowed, and what happens if building control asks for deeper?
- Is the structural engineer's fee inside this price?
- Which windows and doors, by manufacturer and specification?
- Who applies for and manages building regulations approval?
- Is scaffolding, skip hire and welfare included?
- What is your process for variations, and will they be priced in writing before the work happens?
- What guarantee do you give on the workmanship once it is finished?
Design decisions with the biggest price consequences
How much of the rear wall you remove. A structural opening the full width of the house is a different engineering problem from a wide doorway, and it usually needs a heavier steel and more support beneath it.
Whether the roof is flat or pitched. A pitched roof tying into an existing one looks settled and costs more. A warm flat roof with rooflights is cheaper and lets more light into the middle of the plan.
Whether you move the kitchen. Extending and relocating the kitchen at the same time is efficient in disruption terms and expensive in one hit. Extending around the existing kitchen position is the cheaper path.
Free site visit, then a fixed price in writing
We survey the house, look at ground conditions and the existing structure, then issue a fixed price written quote with the items above named individually. Forty plus years building in Kent, our own team on site, and a 5 year workmanship guarantee.
FAQs
Common questions
Is the cheapest extension quote ever the right one?
Only when it covers the same work as the others. Most cheap quotes are cheap because something is missing, and the missing item arrives later as a variation.
Should I use an architect first?
For anything structurally involved, yes, and we work with drawings from any architect. For a straightforward rear extension we can handle design and building regulations drawings ourselves.
Do you price variations before doing them?
Always in writing, and only once you have approved them. Nobody should learn about extra cost from the final invoice.
What does the 5 year workmanship guarantee cover?
The quality of the work we carried out. Manufactured items such as windows, boilers and appliances carry their own manufacturer warranties, which we pass to you.
Can I stay in the house during the build?
Most clients do. We seal off the work area, agree working hours in advance and plan the break through into the existing house for a date you have signed off.
How far ahead do you book?
It varies by season and by project size. Ask us at the survey and you will get a straight answer rather than an optimistic one.
Ready to price your house extension cost: how to read a price before you commit in Kent?
Free site visit, fixed-price written quote and a 5 year workmanship guarantee — backed by 40+ years on the tools and our workshop in Biddenden.
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Free site visit, fixed price written quote, 5 year workmanship guarantee.