House Extensions in London
More space without the cost of moving. We deliver rear, side, wrap-around and double-storey extensions across London — design coordinated, planning managed, fixed price, fully compliant.
Rear extension · South-East London
Open-plan kitchen-diner. Fixed price. 14 weeks on site.
Types of house extension we build
The right extension type depends on your property, your goals, and what planning allows. We advise at the free survey — no obligation.
Rear extension
From £45,000Extends the footprint of your home into the rear garden. The most common extension type in London — typically used to create a larger kitchen-diner, open-plan living space, or utility room. Single storey rear extensions up to 3m (semi/terrace) or 4m (detached) often fall within permitted development.
Kitchen extension
From £55,000A rear or side-rear extension specifically designed around a kitchen refit. Combines structural works with kitchen design for the most efficient layout. Often includes a structural opening into the existing house, bifold or sliding doors to the garden, and lantern rooflights.
Side extension
From £35,000Uses the gap between your house and the side boundary to add a single-storey room — typically a utility, WC, home office, or additional bedroom. Particularly effective where rear garden space is limited. May need planning permission depending on width relative to the original house.
Wrap-around extension
From £70,000Combines a rear extension with a side extension in an L-shaped footprint to maximise ground floor space. Creates the largest possible ground floor extension. Almost always requires planning permission. Very popular for open-plan kitchen-diner-family room configurations.
Double-storey extension
From £80,000Adds two floors of accommodation at the rear or side of your home — maximising additional space for the cost. The first floor addition typically costs only 40–50% more than a single storey equivalent, making it excellent value where planning allows.
Over-structure extension
From £45,000An extension built above an existing garage, single-storey addition, or bay. Requires careful structural assessment of the existing foundation and load paths. We coordinate the structural engineering as part of the pre-construction package.
Costs are indicative ranges for London 2026. Final price depends on size, specification, site conditions, and drainage. Second-fix materials are client-supplied unless otherwise agreed. RCB does not install carpet.
Do you need planning permission for a house extension in London?
Many extensions in London can be built under permitted development rights — meaning no planning application is needed. Single-storey rear extensions up to 3m (attached houses) or 4m (detached) are usually permitted development, as are single-storey side extensions up to half the width of the original house.
However, properties in Conservation Areas, Article 4 direction zones, and listed buildings have reduced or removed permitted development rights. Double-storey extensions, wrap-arounds, and larger schemes almost always need planning. We assess this at the free survey and confirm the route before any cost is incurred.
How we deliver your extension
Free site survey and feasibility
We assess your property, discuss your goals, and advise on what's achievable — including planning status, likely cost range, and what design approach works best for your property type. No charge, no obligation.
Planning and design coordination
We work with your architect or our design partners to develop drawings that are buildable, cost-effective, and approvable. We advise on PD rights, planning applications, and the most efficient route to approval.
Structural and pre-construction pack
Once design is fixed, we obtain structural engineer input for foundations, beams, and connection details. We also review drainage constraints, party wall requirements, and Building Control strategy before pricing.
Fixed-price contract
You receive a detailed, itemised scope and a fixed price before any work starts. We confirm programme, payment stages, and site logistics. All variations are agreed in writing before any additional work begins.
Delivery — groundworks to completion
We manage the full sequence: foundations, drainage, structure, roofing, windows, insulation, first-fix M&E, plaster, second-fix, and decoration. You have one point of contact from first day to completion.
Building Regulations sign-off and warranty
All extensions require Building Regulations approval. We manage notifications, stage inspections, and obtain your completion certificate. All structural works are covered by our 25-year structural warranty.
Common questions about house extensions
How long does a house extension take to build?
A single-storey rear extension typically takes 12–16 weeks on site. A double-storey extension is usually 16–22 weeks. This follows design and planning, which adds 8–16 weeks for planning applications or 4–8 weeks for permitted development schemes. We provide a full programme at contract stage.
What are the hidden costs of a house extension?
Common additional costs include: architect and structural engineer fees (typically £3,000–£8,000 for a standard extension), planning application fees (£206 at time of writing), Building Control fees (£600–£1,200), party wall surveyor costs if required (£1,000–£3,000), and any kitchen or bathroom fitout over and above the structural shell. We flag all of these at the feasibility stage.
Do I need a party wall agreement for an extension?
If your extension is within 3m (or 6m for certain foundations) of a neighbouring property, or if you are excavating near a party wall, the Party Wall Act 1996 applies and you must serve notice on affected neighbours. We identify this at the survey stage and can refer you to a party wall surveyor.
Can an extension be built while we're living in the house?
Yes — in most cases. Extensions typically have limited impact on the existing living space until the internal connection is made (usually near the end of the project). We discuss the site logistics and any temporary inconvenience periods at the pre-start meeting.
What is a structural opening, and do I need one?
A structural opening is the removal of part or all of a wall between the extension and the existing house to create an open-plan space. It requires a structural engineer to specify the correct steel or timber beam and supports. Almost all kitchen extensions include a structural opening. We manage this as part of the main contract.
House extensions by area
Related services
Useful guides
Rear extension cost in London — realistic figures for 2026
Single-storey and double-storey cost ranges, what's included, and what makes the price move.
Kitchen extension cost in London — what to budget
Realistic cost breakdown for single-storey kitchen extensions in 2026.
Double-storey extension — cost, planning and what to expect
Planning permission requirements, costs, and realistic timelines.
Do you need planning permission for a rear extension?
Permitted development limits, Prior Approval, and when you need a full application.
How long does a rear extension take?
A realistic London timeline from first enquiry to practical completion.
Party Wall Act London — what you need to know
When you need a party wall notice, what it costs, and how to avoid delays on your project.
Permitted development rules for extensions in London
What you can build without planning permission — size limits, conditions, and Article 4 exceptions.
Plan your London extension
Free survey. Fixed price. Planning coordinated. Building Regulations managed. 25-year structural warranty. One team from first call to completion certificate.
Or call us on 07359 872594