Project type
Home extensions: the decisions that shape the cost
An extension is not one decision. It is roughly forty of them, and about six of those set most of the final figure. Making the expensive ones early, with information, is what separates a project that lands near budget from one that does not.
The decisions that move the number most
- Footprint and whether the roof structure and foundations become more complex.
- Glazing: rooflights, sliding doors and structural openings carry both product and structural cost.
- Groundworks: trees, drains, ground conditions and existing foundations are the classic overspend.
- Whether the existing house is reconfigured, which usually costs more per square metre than the new part.
- Kitchen, bathroom and finishes specification, which is where allowances hide.
- Service diversions: boilers, meters, soil stacks and manholes.
The order that keeps you in control
- Agree a realistic budget range and a contingency before design gets detailed.
- Resolve planning or confirm permitted development.
- Get structural and technical information before asking builders to price.
- Tender on the same written scope so quotes can be compared like for like.
- Agree a contract, a payment schedule tied to progress, and a change process in writing.
What extension quotes commonly leave out
- Provisional sums for kitchens, sanitaryware, flooring and landscaping.
- Building control and party wall fees.
- Scaffolding duration, welfare and skips beyond an assumed period.
- Decoration, making good and the boundary between new and existing.
- Anything below ground that has not been investigated.
Case-style example
Three quotes, three different projects
A homeowner receives extension quotes of £96,000, £128,000 and £152,000 and assumes the middle one is safest.
Read side by side, the cheapest excludes the kitchen and any groundworks beyond a nominal depth; the most expensive includes both, plus a longer scaffold period and a stated contingency.
They were never the same project. The comparison only becomes meaningful once each price is mapped against the same list of works.
How The Construction Clinic helps with this decision
We help homeowners compare quotes properly, understand what an allowance really means, and see which decisions are still open. The Quote Clarity Check maps quotes against a common scope so the gaps are visible before you commit.
Common questions
Questions homeowners ask about this
What contingency should I hold on an extension?
Many homeowners hold around ten per cent, and more where groundworks are uncertain or the existing house is old. The purpose is unforeseen work, not upgrades.
Is a fixed price really fixed?
It is fixed against a defined scope. Change the scope, discover something unforeseen, or firm up a provisional sum and the price moves legitimately.
Should I appoint the designer or the builder first?
In most cases the designer, so builders price the same information. Design-and-build can work well when the scope is clearly written down.
How long does an extension take?
It depends on size and complexity, and the programme in the quote should say. What matters more is whether the programme is written down and reviewed as work proceeds.