Double Storey Extension in Swiss Cottage NW3

Why Swiss Cottage Properties Choose Us

Mix of mansion flats and Victorian terraces, good transport links. Modern developments alongside period stock. Our team works exclusively in NW London, bringing detailed knowledge of Camden planning requirements and the specific challenges of Mansion flats & terraces stock.

Planning Authority

Camden LBC

Average Property Value

£1,200,000

Conservation Area

No

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 Swiss Cottage — Planning and Local Considerations

Swiss Cottage NW3 requires full Camden LBC planning permission for all double storey rear extensions. The Victorian terrace stock along the Adelaide Road corridor and surrounding streets has good structural candidates. Camden conservation officers assess two-storey proposals for their massing impact on the rear character of the conservation area. Pre-application advice is strongly recommended — officer feedback at this stage substantially reduces the risk of a refusal on massing grounds.

Frequently Asked Questions