Side extensions transform layouts on the typical Merton semi-detached property — gaining a wider kitchen and a useful utility/WC off the back. RCB Design & Build delivers side extension projects right across Merton (SW19) from our Dartford base — typically on site within 48 hours of enquiry, with a fixed written quotation in 5 working days. Our typical side extension project in Merton costs between £36,000 and £96,000 in 2026, with most Victorian villas and Edwardian terraces landing around £66,000. We are FMB, TrustMark, Checkatrade and TrustATrader accredited and operate as a Principal Contractor under CDM 2015.
Wimbledon and Raynes Park drive demand for premium rear extensions and full refurbishments. Working out of London Borough of Merton's area, we know the planners, the local building inspectors, and the named conservation officers — that personal relationship cuts weeks off the consenting timeline. Our typical Merton project is on Victorian villas within easy reach of Mitcham or Wimbledon Common, and we work with a small panel of trusted London-area subcontractors we have used for years.
What's included in our side extension price
Foundations, single-storey side build-up to match existing brickwork and roof pitch, steel beam to remove load-bearing wall, glazing, first-fix M&E, plaster, screed. We coordinate party wall surveyor and structural calcs. Our written quotation covers labour, first-fix materials, project management, scaffolding, skips, Building Control fees and statutory inspections. Second-fix materials (kitchens, bathrooms, flooring, tiles, paint, sanitaryware) are supplied by the client as a deliberate cost-control measure — this typically saves homeowners £5,280–£9,900 in trade markup. We exclude planning fees, party-wall surveyor fees and structural engineer drawings, which we coordinate at cost.
Merton planning, party walls and Building Regs
Wimbledon Village conservation area has strict design rules — slate roofs, sash windows, pointing match. Side extensions in conservation areas must match prevailing roof slope, ridge height and bay-window pattern. Local material match is non-negotiable. Note that Merton has multiple conservation areas — design must match prevailing materials, roof shape and fenestration. Party Wall Awards are typically needed for any excavation within 3 metres of a shared wall — we coordinate the surveyor and serve notices. Building Control is lodged at the start, with inspections at foundation, damp-course, pre-plaster and completion stages.
Timeline and process for Merton projects
Side extensions run 6–10 weeks on site. Matching brick and roof pitch is the design challenge — we'll spend time sourcing the right brick before mobilisation. We hold a weekly call with the client throughout, send a written progress report with photos every Friday, and the lead PM is always reachable on WhatsApp. From SW19 postcodes we can have someone on site within 48 hours of a phone call.
Guarantees and after-care
Every project we deliver in Merton is backed by a 12-month workmanship guarantee. Where we install manufacturer-warranted systems (roofing membranes, windows, MVHR), the manufacturer warranty (typically 10–25 years) runs alongside. As an FMB member we can also arrange an FMB-backed 2-year insurance warranty on qualifying Merton schemes — worth considering for any project above £50,000.
Side Extension cost in Merton — 2026 price guide
Project size
From
To
Narrow side (≤10 m²)
£36,000
£47,000
Standard side (10–20 m²)
£43,000
£58,000
Wide side (20–30 m²)
£50,000
£68,000
Full side (30 m²+)
£58,000
£79,000
Indicative Merton (SW19) ranges for 2026. A site visit firms the figure to +/-5%.
Frequently asked questions
How much does a side extension cost in Merton in 2026?
Typical Merton pricing ranges from £36,000 to £96,000 depending on size, finish and site complexity. Merton sits in the upper-tier London market so labour rates run 15–25% above outer-borough averages. A free site visit firms the number up to ±5%.
How long will a side extension take in Merton?
Allow 6–12 weeks of design, planning and procurement before work starts, then a project-dependent build phase. Side extensions run 6–10 weeks on site. Matching brick and roof pitch is the design challenge — we'll spend time sourcing the right brick before mobilisation. We share a Gantt-style schedule with every quotation.
Will a side extension in Merton need a party wall award?
Almost always — excavating within 3 m of a shared wall triggers the Party Wall Act. We coordinate the surveyor and serve notices on the neighbours as part of the package.
Which areas of Merton do you cover?
We cover all of SW19 and the surrounding London postcodes from our Dartford base. Recent projects sit close to Mitcham and Wimbledon Common. Anywhere within 45 minutes of Dartford is comfortably inside our core delivery zone.
Is RCB insured?
Yes — we hold £5 million Public Liability, £10 million Employer's Liability, full Contract Works insurance and Professional Indemnity. Certificates are provided with every quotation.
How do I get a quote?
Use the free instant calculator at /quote for a 60-second ballpark, configure your project in detail at /configure, or call +44 7359 872594. We respond to every enquiry within 4 working hours.