Rear extensions are the most common project we deliver in Lewisham — single-storey kitchen-diner extensions account for roughly a third of our annual pipeline. RCB Design & Build delivers rear extension projects right across Lewisham (SE13) from our Dartford base — typically on site within 48 hours of enquiry, with a fixed written quotation in 5 working days. Our typical rear extension project in Lewisham costs between £39,000 and £105,000 in 2026, with most Edwardian villas and inter-war semis landing around £72,000. We are FMB, TrustMark, Checkatrade and TrustATrader accredited and operate as a Principal Contractor under CDM 2015.
Strong volume of side-return extensions, mansard lofts and full refurbs across SE13, SE23 and SE4. Working out of London Borough of Lewisham'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 Lewisham project is on Edwardian villas within easy reach of Forest Hill or Telegraph Hill, and we work with a small panel of trusted London-area subcontractors we have used for years.
What's included in our rear extension price
Quotation covers steel beam(s), foundations to Building Control, structural glazing or aluminium sliding doors, flat-roof construction with single-ply or warm-roof build-up, MVHR or trickle vents, first-fix M&E, plaster and screed. Second-fix (kitchen, flooring, paint) is client-supplied for cost control. 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 £5,760–£10,800 in trade markup. We exclude planning fees, party-wall surveyor fees and structural engineer drawings, which we coordinate at cost.
Lewisham planning, party walls and Building Regs
Blackheath and Telegraph Hill conservation areas — match brick course, fenestration and ridge line. Rear extensions live or die on the rear elevation glazing — sliding doors, structural slot glazing or roof lights set the overall feel and most of the lighting strategy. Note that Lewisham 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 Lewisham projects
Single-storey rear extensions run 8–12 weeks on site for a 25–35 m² scheme. Pre-construction (drawings, Building Control, party walls) typically adds 6–10 weeks 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 SE13 postcodes we can have someone on site within 48 hours of a phone call.
Guarantees and after-care
Every project we deliver in Lewisham 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 Lewisham schemes — worth considering for any project above £50,000.
Rear Extension cost in Lewisham — 2026 price guide
Project size
From
To
Small rear (≤20 m²)
£39,000
£51,000
Standard rear (20–30 m²)
£47,000
£59,000
Large rear (30–45 m²)
£55,000
£70,000
Full-width rear (45 m²+)
£62,000
£78,000
Indicative Lewisham (SE13) ranges for 2026. A site visit firms the figure to +/-5%.
Frequently asked questions
How much does a rear extension cost in Lewisham in 2026?
Typical Lewisham pricing ranges from £39,000 to £105,000 depending on size, finish and site complexity. Lewisham sits in the mid-band of our delivery area. A free site visit firms the number up to ±5%.
How long will a rear extension take in Lewisham?
Allow 6–12 weeks of design, planning and procurement before work starts, then a project-dependent build phase. Single-storey rear extensions run 8–12 weeks on site for a 25–35 m² scheme. Pre-construction (drawings, Building Control, party walls) typically adds 6–10 weeks before mobilisation. We share a Gantt-style schedule with every quotation.
Do I need planning permission for a rear extension in Lewisham?
Up to 6 m on a detached and 4 m on a semi/terrace can sit inside Larger Home PD via the Neighbour Consultation Scheme. Conservation areas, Article 4 zones and any rear scheme above 8m³ on a flat will need full planning.
Which areas of Lewisham do you cover?
We cover all of SE13 and the surrounding London postcodes from our Dartford base. Recent projects sit close to Forest Hill and Telegraph Hill. 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.