Double Storey Extension in St John's Wood NW8

Why St John's Wood Properties Choose Us

Ultra-premium, detached villas, embassies, international residents. Abbey Road Studios, Lord's Cricket Ground. Our team works exclusively in NW London, bringing detailed knowledge of Westminster planning requirements and the specific challenges of Detached villas & mansion flats stock. As a designated conservation area, every project in this postcode requires specialist handling — knowledge our team has built over years of delivering projects here.

Planning Authority

Westminster LBC

Average Property Value

£2,700,000

Conservation Area

Yes — planning applies

Typical Project Cost

£120,000–£280,000

How a Double Storey Extension Works — Step by Step

1. 1

   Survey, Feasibility, and Massing Assessment

   A double storey rear extension is a significant change to the property's massing. We assess the rear garden depth available, the shadow impact on neighbours, and the relationship to the principal rear elevation. We also confirm the structural implications for the existing first floor — new joists, lintels, and load paths are assessed before design commences.

2. 2

   Planning Strategy and Pre-Application Consultation

   Double storey extensions require full planning permission in all NW London conservation areas — they are never permitted development. Pre-application advice from the LPA is essential to understand the acceptable scale, massing, and materials before architectural drawings are committed to. In Hampstead Garden Suburb, HGS Trust pre-application consultation is additionally required.

3. 3

   RIBA Architectural Design

   The architect produces full planning and building regulations drawings including section drawings showing the two-storey massing in the context of the existing house and garden. The design must demonstrate that the extension is subservient to the host property and does not dominate the rear elevation. Material selection is driven by the planning context.

4. 4

   Structural Engineering and Party Wall

   A structural engineer designs the two-storey structure including foundations, ground floor slab, first floor structure, and roof. Party wall notices are served — the first floor of the extension will typically be adjacent to the neighbour's first floor bedrooms, requiring careful detailing of sound insulation and party wall construction.

5. 5

   Demolition, Groundworks, and Foundations

   The rear garden is excavated for the deeper foundations required for a two-storey structure (typically 1.2–1.8m in London Clay). Drainage is diverted, and the reinforced concrete strip or raft foundations are poured and inspected. Underfloor heating and ground floor services are roughed in before the slab is poured.

6. 6

   Ground Floor and First Floor Structure

   The ground floor walls are built in masonry or timber frame, the first floor joists are installed, and the first floor walls are constructed. The roof structure is built and made weathertight before any internal works commence. External cladding, windows, and render are applied to the planning-approved specification.

7. 7

   Rear Wall Openings — Ground and First Floor

   With the extension weathertight, the rear wall of the existing house is opened at ground floor level for the kitchen-dining connection and at first floor level for the new bedroom. Structural steels carry the loads above each opening. The temporary propping is removed after the steels are poured in.

8. 8

   Internal Fit-Out, Services, and Completion

   Ground floor kitchen-dining fit-out and first floor bedroom or bathroom installation proceed in parallel. Building control inspects at each key stage. On completion, the extension adds two large usable rooms and significantly increases the property value — double storey extensions are the highest-value extension type in the NW London market.

Double Storey Extension in St John's Wood — Planning and Local Considerations

Westminster City Council applies its conservation area policies to all double storey extensions in St John's Wood NW8. Large detached Edwardian villas have the greatest scope for substantial double storey additions — some of the highest-value extension projects in the London market occur here. Westminster's heritage officers require detailed justification for any extension that materially alters the massing character of a conservation area property. Gated estate consent must precede planning submission.

Frequently Asked Questions