Costs · Extensions
What Affects the Cost of a House Extension in London
Extension costs vary widely with the property and the brief. This guide explains the factors that drive the price, so you can budget realistically — without relying on a single headline figure.
Why a single figure is misleading
It is tempting to look for a cost-per-square-metre rule of thumb, but extension costs in prime London vary so much with the property, the brief and the specification that a single figure is more likely to mislead than to help. The same floor area can cost very different amounts depending on the factors below. The reliable way to a real number is a complete design and a competitive tender.
Our fixed-fee packages
Design fees, inc. VAT — separate from the council fees above
| Package | Fixed fee |
|---|---|
| Planning Route ReviewA paid route check before drawings or applications. | £395 inc. VAT |
| Standard Planning Drawings PackageFor straightforward householder schemes. | from £1,450–£1,950 inc. VAT |
| Full Householder Planning PackageDrawings plus application preparation and submission. | from £2,250–£3,250 inc. VAT |
| Licence to Alter / Flat Refurbishment Drawing PackLeasehold flat alterations and freeholder consent drawings. | from £1,500–£2,750 inc. VAT |
| Building Regulations / Technical Drawing PackageTechnical drawings once the planning route is clear. | from £2,750–£4,950 inc. VAT |
| Listed / Conservation Planning PackHeritage-led drawings and consent strategy. | Quoted after review |
Fixed fees quoted before you commit, credited from the £395 route review if you proceed. Larger, listed or complex projects are quoted after a paid review rather than sold at a fixed price. See all packages or find your package.
Size, type and structure
A single-storey rear extension, a side-return infill, a wraparound and a two-storey addition are different undertakings, and the structural work — new foundations, beams and openings — is one of the biggest cost variables. Older London properties can also hide structural, damp or drainage issues that add cost once uncovered.
Glazing, finishes and services
Specification drives cost as much as size. Large areas of glazing, rooflights, high-quality finishes, bespoke joinery, underfloor heating and a relocated or upgraded kitchen all add up. A clear specification at tender means the quality you expect is priced from the start rather than discovered as extra later.
Planning, party walls and consultants
Beyond construction, a project carries professional and statutory costs: architectural fees, structural engineering, planning application fees, building control fees, and — where the Party Wall etc. Act applies — party wall surveyors. These are normal parts of a London project and should be budgeted for from the outset.
Access, logistics and the unforeseen
Tight London sites, limited access, restricted working hours and the need to protect neighbours can all affect a builder’s price. A sensible contingency for the unforeseen — particularly in period homes — protects against the surprises that even a careful survey cannot rule out.
Getting to a real number
The most dependable cost certainty comes from a complete tender package priced competitively by suitable contractors. That is why we prepare coordinated drawings and an outline specification before tender: it makes quotes comparable and reduces the variations that inflate the final account.
Frequently asked questions
Can you tell me what my extension will cost?
What should I allow for beyond the build?
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Need drawings or planning advice for your property?
Tell us the property address, type and what you are considering. We will advise the likely route — planning, building regulations, or both — and what information will be needed.