Double Storey Extension in Westminster | RCB Design & Build
Double-storey extensions deliver the most bedrooms-and-bathrooms per pound on a Westminster project — typically two new rooms over a single new footprint. RCB Design & Build delivers double storey extension projects right across Westminster (W1) from our Dartford base — typically on site within 48 hours of enquiry, with a fixed written quotation in 5 working days. Our typical double storey extension project in Westminster costs between £102,000 and £261,000 in 2026, with most mansion blocks and stucco-fronted Victorian terraces landing around £181,500. We are FMB, TrustMark, Checkatrade and TrustATrader accredited and operate as a Principal Contractor under CDM 2015.
Heritage refurbishment, structural alterations and amenity work. Almost every property is listed or in a CA. Working out of Westminster City Council'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 Westminster project is on mansion blocks within easy reach of Marylebone or Pimlico, and we work with a small panel of trusted London-area subcontractors we have used for years.
What's included in our double storey extension price
Foundations sized for two storeys, blockwork or timber-frame superstructure, pitched roof tied into existing, first-fix M&E, plaster, glazing, intermediate floor. New first-floor bathroom and bedroom layout is the typical scope. 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 £14,520–£27,225 in trade markup. We exclude planning fees, party-wall surveyor fees and structural engineer drawings, which we coordinate at cost.
Westminster planning, party walls and Building Regs
Westminster City Council enforces the strictest planning regime — most buildings need LBC for any external change. Double-storey requires careful daylight analysis — neighbours will object on Rights to Light and 25/45 grounds. We run a sketch BRE check before submitting planning. Note that Westminster 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 Westminster projects
Double-storey extensions run 14–20 weeks on site. Scaffolding, weatherproofing the gable and lifting steels safely add time vs. single-storey work. 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 W1 postcodes we can have someone on site within 48 hours of a phone call.
Guarantees and after-care
Every project we deliver in Westminster 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 Westminster schemes — worth considering for any project above £50,000.
Double Storey Extension cost in Westminster — 2026 price guide
Project size
From
To
Compact (≤25 m² each floor)
£102,000
£133,000
Standard (25–40 m² each)
£122,000
£163,000
Large (40–55 m² each)
£143,000
£194,000
XL (55 m² + each)
£163,000
£224,000
Indicative Westminster (W1) ranges for 2026. A site visit firms the figure to +/-5%.
Frequently asked questions
How much does a double storey extension cost in Westminster in 2026?
Typical Westminster pricing ranges from £102,000 to £261,000 depending on size, finish and site complexity. Westminster 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 double storey extension take in Westminster?
Allow 6–12 weeks of design, planning and procurement before work starts, then a project-dependent build phase. Double-storey extensions run 14–20 weeks on site. Scaffolding, weatherproofing the gable and lifting steels safely add time vs. single-storey work. We share a Gantt-style schedule with every quotation.
Do I need planning for a double-storey extension in Westminster?
Yes — first-floor extensions sit outside PD in nearly every situation. Allow 8–12 weeks for planning determination including a neighbour consultation period.
Which areas of Westminster do you cover?
We cover all of W1 and the surrounding London postcodes from our Dartford base. Recent projects sit close to Marylebone and Pimlico. 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.