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CASE STUDY · APPROVED

Basement enlargement and first-floor rear extension to a semi-detached house

Worple Road, Wimbledon · London Borough of Merton

A full planning application to enlarge the existing basement of a semi-detached house into habitable rooms with front and rear lightwells, replace the rear conservatory, add a first-floor rear extension and make internal alterations. We prepared the design and access statement, basement impact assessment and daylight and sunlight report alongside the drawings.

Existing rear elevation and garden (site photographs)
Existing rear elevation and garden (site photographs)

The site

The property is a two-storey semi-detached house with an existing basement and loft on Worple Road, a key route between Wimbledon town centre and Raynes Park. It is not listed or in a conservation area, lies in Flood Zone 1 but within a Critical Drainage Area, and the nearest listed structure is about 210 m away. A single-storey rear extension was approved in 2019 (19/P1134).

The challenge

Merton's basement guidance limits extended basement footprints to less than 50% of the original dwelling's footprint, requires one storey only and seeks to protect neighbouring structures and drainage. The excavation would be within 1.5 m of the neighbouring house and within 6 m of the public pavement. The rear extensions had to avoid harm to daylight and outlook for the attached and detached neighbours.

What was submitted

  1. Design and Access StatementShowed the basement extension kept within 50% of the original footprint (checked against the 1919 historic map), floor-to-floor heights of at least 2.5 m, a front lightwell and a planted stair to a sunken rear garden, while retaining 131.18 sq m of private rear garden and leaving the front elevation unchanged apart from a flush rooflight.
  2. Basement Impact AssessmentBoreholes at a nearby site to 9.45 m and 5.45 m found topsoil and head deposits over London Clay to 9.45 m, with groundwater more than 30 m down. It assessed the basement (finished floor about 2.8-3 m below ground floor) as Geotechnical Category 2 and found that underpinning could keep damage to neighbours to Category 1 (very slight), with monitoring, temporary propping and waterproofing to BS 8102.
  3. Sunlight and Daylight AssessmentApplied the 45-degree and 25-degree tests, VSC and NSL to both neighbouring houses and found BRE compliance. More than 90% of internal and external spaces would receive at least two hours of sun on 21 March, and the overshadowing studies for March and September showed shadows similar to the existing situation.
  4. Proposed section A-A and rear elevationShowed a 3.0 m high glazed rear ground floor with a glazed handrail over the basement garden, new basement windows, a 2.586 m high first-floor hobby room, and render, roof finishes and windows to match the existing house.

Outcome

Planning permission was approved by the London Borough of Merton.

From the reports

Existing and proposed rear elevations (rear elevation drawing)
Existing and proposed rear elevations (rear elevation drawing)
Proposed section through the enlarged basement and rear extensions (proposed section A-A)
Proposed section through the enlarged basement and rear extensions (proposed section A-A)

Images and findings are taken from the documents submitted with the planning application. The full application is available on the council's planning register.

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