House Extensions London
Extensions Done Right — From Planning to Building Control Sign-Off.
Single storey rear extensions. Double storey additions. Wrap-around extensions. Side returns. RCB Design & Build manages design, planning, structure, build, and compliance under one contract — so you deal with one company, not four.
Why House Extensions Go Wrong in London
Extensions look simple on paper. Dig a foundation, build some walls, add a roof. But in practice, the London market is full of contractors who underquote to win the job, skip structural drawings, do not serve party wall notices, and hand over a damp, unfinished shell two months late. The homeowner then spends months chasing and thousands of pounds fixing what should have been done right the first time. RCB exists to be the exception.
RCB takes structural accountability, manages planning and compliance, and delivers a completion certificate — not just a space that looks like an extension.
Every Type of Extension — Managed End to End
Whether you want to open up the ground floor, add a bedroom above, or completely reconfigure your home's footprint — RCB has built it before and can build it for you.
Rear Extensions — Kitchen, Living & Dining
- Single-storey rear extensions from 3m to 8m depth under permitted development or planning permission
- Open-plan kitchen-diner configurations with structural steel beams and bifold or sliding doors
- Roof lanterns, flat roofs with GRP or warm roof build-up, and pitched roof extensions
- Full drainage, heating, and electrical extension — not a shell handed to a second contractor
Double Storey & Side Extensions
- Double storey rear and side extensions to add bedroom and bathroom capacity
- Side return extensions making use of the often-wasted alley beside Victorian terraces
- Structural design coordinated with our structural engineer from feasibility stage
- Party wall agreement management included — one less thing for you to organise
Wrap-Around Extensions
- Combined rear and side extensions that transform ground-floor layouts entirely
- Design-led approach — we consider how rooms connect before any structure is proposed
- Full planning drawings, structural calculations, and Building Control submission managed in-house
- Existing utility diversions and drainage rerouting coordinated and costed before work begins
Full Design-and-Build Service
- Feasibility review, planning assessment, and initial layout options — before you commit
- Architectural drawings, structural calculations, and Building Regs compliance all under one roof
- Principal contractor appointment — one point of accountability from foundation to decoration
- Fixed-price contract once scope is agreed — no bill creep, no scope surprises
What Does a House Extension Cost in London?
Extension costs in Greater London typically range from £1,800 to £3,500 per square metre for a quality build, depending on specification, structural complexity, glazing, and finish level. The figures below are indicative only.
Typical 3m–6m depth. Includes structure, roof, glazing, and first fix.
Adds bedroom and bathroom capacity above the rear or side extension.
Combined rear and side. Transforms ground floor layout entirely.
Ballpark figures only — not formal quotations. Actual costs depend on design, specification, structural requirements, and site conditions. A detailed estimate requires a site visit and drawings.
How Your Extension Happens
Five stages. One principal contractor. You know exactly where you are and what is coming next from day one to completion certificate.
Feasibility & Design Brief
We visit your property, review your planning position, and discuss what the extension needs to achieve. We tell you honestly what is achievable under permitted development and what requires a planning application. No guesswork, no over-promising.
Planning & Structural Design
Architectural drawings produced. Planning application submitted where required. Structural engineer engaged for beam and foundation design. Party wall notices served on your behalf. Building Control application submitted before work begins.
Fixed Price & Programme
Full itemised cost plan issued. Payment schedule tied to completion stages. Programme agreed with key milestones. You know the end date before we break ground.
On-Site Delivery
Foundation excavation and structure. Waterproofing and roofing to a specified standard. First-fix plumbing and electrics. Plastering and second-fix carpentry. Your project manager on site daily.
Completion & Building Control Sign-Off
Building Control final inspection arranged and completion certificate issued. O&M documentation included. Snagging resolved before handover — not a list emailed to you with an apology.
Why Homeowners Choose RCB for Their Extensions
114 independently verified reviews. A Checkatrade rating of 9.96/10. Accreditations that prove compliance, not just competence.
Principal Contractor — Not a Gang of Subbies.
RCB takes full principal contractor responsibility for your extension — one contract, one insurance policy, one point of accountability. We coordinate every trade. You deal with us, not with six different contractors arguing about whose fault the leak is.
Often Combined With Extensions
Loft Conversions
Add a bedroom or bathroom above — combine with a rear extension to maximise your home's potential.
Full Refurbishments
Many clients refurbish the existing house while the extension is built — one programme, one disruption.
Structural Alterations
Open up internal walls, install steel beams, and reconfigure layouts alongside your extension.
Extension FAQs
Do I need planning permission for a rear extension?
Many single-storey rear extensions are built under Permitted Development rights without a full planning application. The permitted depth is 4m for detached houses and 3m for semi-detached and terraced properties, extendable to 8m or 6m under the Larger Home Extension Scheme with neighbour consultation. We assess your permitted development position at the feasibility stage and advise whether a Lawful Development Certificate is recommended for your sale or mortgage records.
How long does a rear extension take to build?
A single-storey rear extension typically takes 10 to 16 weeks on site, depending on size and specification. A double-storey extension runs 16 to 24 weeks. These timescales assume drawings and Building Control approval are in place before we start. We issue a detailed programme at the point of contract so you know the end date before the first spade goes in.
What is included in your price?
Our price covers structural works, roofing to specification, external brickwork or render to match, all glazing and external doors, first-fix plumbing and electrics, insulation to Building Regs standards, plastering, and internal decoration of the extension. Second-fix fixtures and fittings — taps, sockets, light fittings — are client-supplied. We are transparent about what is in and out from the outset.
Do you manage the party wall process?
Yes. We identify which neighbours need to be served under the Party Wall etc. Act 1996, prepare the notices, and manage the process. If a neighbour appoints their own surveyor, we coordinate with them. Party wall is not a reason to delay a project — it is a process, and we manage it as part of ours.
Can you match the extension brickwork to our existing house?
Usually yes. We source facing bricks that match your existing stock as closely as possible, using reclaimed bricks where a close match is critical. For rendered or clad extensions, the finish is specified at design stage. Brickwork matching is something we take seriously — an extension that looks like an afterthought reduces property value rather than adding it.
Ready to Start Your Extension?
Tell us about your property, what you want to achieve, and your budget. We will tell you honestly what is possible and what it costs.
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