Extensions, properly managed.
Extensions overrun on budget and programme for one reason: nobody is genuinely in charge. Groundworkers wait on the structural engineer, the electrician arrives before the plasterer, and each blames the last. We manage the entire project, so there is one party to ask and one party accountable.
Indicative cost
Typically £2,000–£3,000 per m² in London. A 20m² rear extension usually falls between £45,000 and £65,000.
Glazing and groundworks move the figure most. Bi-fold doors and a lantern rooflight can add £10,000 alone, while difficult foundations or a protected tree can add more. We would rather state an honest range now than win the project with a low figure and find the money later.
From the drawings to the doorstop.
The value a contractor brings to an extension is not the bricklaying — it is the sequencing. Every week saved on the programme is a week you are not living in a house without a rear wall.
Permitted development vs planning
Many rear extensions require no planning permission at all. Under permitted development you can typically extend 3m to the rear on a terrace or semi, or 4m on a detached property, and further under the prior approval route. It depends on your property and whether it has been extended previously. We will confirm which route applies at the first visit, free of charge.
Eight to twelve weeks on site.
Plus the approvals beforehand. Here is the honest shape of it — including the period when your kitchen temporarily lives in the sitting room.
Site visit & feasibility
We assess the space, the access, the drains and the boundary, and advise what is realistic. Access is frequently overlooked — a side alley too narrow for a digger changes everything.
Drawings & approvals
Architect's drawings, structural engineer, planning or prior approval, Building Control and party wall notices. Allow 8–12 weeks. Begin now.
Groundworks & shell
Foundations, drainage, brickwork, steels, roof and glazing. The garden becomes a building site and the rear wall is removed. This is the difficult stage.
Fit-out & handover
Plastering, electrics, heating, flooring and decoration. Building Control carries out its final inspection and the site fencing is removed.
The questions people don't ask out loud.
So we'll answer them here instead.
Everyone says extensions overrun. Will ours?
Not if the quotation is honest from the outset. Most overruns arise from a contractor pricing low to win the project, then charging for 'unforeseen' work that was entirely foreseeable. Our quotations include a stated provisional sum for what genuinely cannot be known until excavation — and if it is not spent, you do not pay it.
What about our neighbours?
If you are building near or on the boundary, a party wall agreement is required, and it is our responsibility to ensure notice is served correctly and on time. Handled properly it is paperwork. Handled late it becomes an injunction that stops the project.
Can we live in the property during the build?
Yes, and most clients do. The most difficult period is when the rear wall is opened — typically one to two weeks. We board and seal it as quickly as possible and provide a date range in advance so you can plan around it.
“They gave us a twelve-week programme and finished in eleven. What impressed me more was that when the groundworks uncovered an old drain nobody knew about, we received a phone call and photographs, and the cost before anything was touched. That is all we wanted from a contractor.”
Michael A. · Rear extension, Bromley
Extensions — straight answers.
Budget £2,000–£3,000 per square metre as a starting point. A typical 20m² single-storey rear extension usually falls between £45,000 and £65,000. Glazing is the largest single variable — bi-folds and a lantern rooflight can add £10,000 or more alone.
Frequently not. Many single-storey rear extensions fall within permitted development, and larger ones can proceed via the prior approval route. It depends on property type, previous extensions, and whether you are in a conservation area. We will confirm your position at the first visit.
Eight to twelve weeks on site for a single-storey rear extension. The drawings, approvals and party wall stage beforehand takes a further 8–12 weeks, so realistically expect five to six months from first enquiry to completed room.
If you are building within 3m of a neighbour's structure, or on the boundary, notice must be served under the Party Wall Act. It is a legal requirement rather than a courtesy. We manage the notices as part of the project — getting it wrong can halt the build with an injunction.
Certainly. If you already hold drawings and approvals, send them over and we will price the build from them. If not, we can arrange the architect and structural engineer as part of the package.
Get a realistic price for your extension.
A free site visit, an honest view on planning, and a written quotation with a realistic timescale.