A practical budget guide before you ask for a quote
Short answer: most London house extension budgets are moved less by the headline square metre figure and more by five practical things: structure, glazing and roof design, drainage/services, access, and finish level. A rear extension with simple access is a different job from a side return or wraparound on a tight terrace street.
If you are asking “how much does a house extension cost in London?”, you are probably not looking for a perfect number. You are trying to work out whether the idea is sensible before you pay for drawings, speak to neighbours, or invite builders round.
The catch is that London extensions do not price neatly. Two kitchens can be the same size on paper and land in very different budgets once steelwork, drainage, party-wall matters, rooflights, glazing, access and the finish specification are included.
Use the budget band estimator first
This is a rough planning tool. It is not a quote and it is not a promise of BCS pricing. It is designed to show why one extension feels “simple” and another quickly becomes a larger design-and-build project.
London extension budget band estimator
The five dials that move a London extension budget
Cost by extension type
Use these as planning bands only. The real number depends on survey, drawings, specification and site conditions. If a quote looks much cheaper than everything else, check what is missing before you get excited.
| Extension type | Often best for | Main budget risks |
|---|---|---|
| Rear extension | More kitchen/dining space and better garden connection. | Opening size, steels, glazing, roof type and finish level. |
| Side return extension | Widening a narrow London terrace kitchen. | Small footprint but high detail: drains, neighbours, rooflights and awkward access. |
| Wraparound extension | A larger ground-floor transformation. | More structure, more roof/glazing decisions, more design coordination. |
| Double-storey extension | Adding ground-floor space and upstairs bedrooms/bathrooms. | Planning, structure, foundations and making the new elevation look right. |
| Loft-style extension | Extra room without taking garden space. | Head height, stairs, fire safety, roof structure and bathroom services. |
Why London is different
London adds friction. Many homes have tight rear access, narrow terrace layouts, parking pressure, neighbours close to the work, and older drains or walls that were never designed for a modern open-plan kitchen. A project in a conservation area or a flat can also need a different permission route.
The Planning Portal explains that some extensions can fall under permitted development, but only if the conditions and limits are met. If you go beyond those limits, or if the property has extra restrictions, a planning application may be needed.
Party-wall matters can also affect both timing and cost. GOV.UK’s party wall guidance explains the framework for works near party walls, boundary walls and excavations.
What a builder needs before a serious quote
- the rough extension type and the problem you want to solve;
- photos of the rear/side access and existing kitchen or living area;
- any drawings, planning notes or structural comments you already have;
- whether drains, manholes, boilers, meters or utilities may be affected;
- your finish expectations: simple, mid-range, high-spec or full design-and-build.
When not to rush the build
If the budget only works when everything goes perfectly, pause. A good London extension needs some contingency. It also needs enough design work to avoid expensive changes once walls are open, neighbours are served notice, or steel is already ordered.
Rear extension proof page. Bayston Road, London
House extension proof page. Greenwich, London
Extension project proof page.
Want a realistic number for your property?
BCS can look at the layout, access, likely structure and finish level before you compare quotes. Start with the house extension service or request a site visit.
