Basement
conversions
in Clapham, SW4
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A Clapham basement answers to Lambeth's Policy Q27 or Wandsworth's LP6, depending on which side of the common the house stands, and the two set different limits.
Below ground the drivers are the familiar ones of a closely built SW4 street: terrace structure, party-wall awards, drainage and levels to the rear, tree and boundary constraints, matching brickwork or stucco, and access through tight streets. That translates into careful underpinning, a waterproofing strategy suited to the rear levels, and a sequence agreed with the neighbours who share the party walls. The lightwells and any front-area change are kept subordinate to the frontages facing the green.
On rates, converting existing space and a new dig with underpinning are priced differently, and both exclude fit-out.
Lambeth's Q27 and the Wandsworth side's LP6
Clapham is split between two councils. Wandsworth's appraisal notes that Clapham Common straddles North Battersea to the west and Clapham to the east, with half of the common in Lambeth, so a basement is judged under Lambeth's Policy Q27 or Wandsworth's LP6 depending on the address.
In Lambeth, Policy Q27 F limits a residential basement to one floor below ground and does not support a new basement beneath an existing basement or semi-basement, and the Design Guide asks that floor-to-ceiling heights in a new basement not exceed 3m.
Beyond the house's footprint, Q27 G allows a basement only at the rear, no larger than the footprint of the house itself, with at least 70% of the rear garden left free of basement and at its original level.
At the front, Q27 H keeps lightwells and basement areas to the minimum size their use requires, the Design Guide does not support glazing over a lightwell, and where existing basement railings are of interest they should be kept.
Many of Clapham's older houses already sit on a semi-basement. In the Clapham Park and Northbourne Road appraisal, render is usually limited to the semi-basement storey to suggest a classical plinth, and Q27 F protects existing basement areas from inappropriate alteration.
Where brick is exposed by a new lightwell, the Design Guide expects matching materials, which in Clapham usually means London stock brick, the predominant walling material the 2023 appraisal records.
The engineering behind a Clapham basement, from the underpinning sequence to the neighbour notices the Party Wall Act requires, is covered in more depth on our Clapham structural works page.
Outside Clapham: Fitzjohns Avenue, NW3, a house on Fitzjohn's Avenue in Hampstead that gained a basement and an extension.
We set out the 70% garden calculation on a measured plan of the plot early, because Lambeth asks designers to show it in the application and it often decides the basement's size.
We confirm which council covers the address before anything else, because Lambeth's 70% garden rule and Wandsworth's half-garden rule produce different basement sizes on the same plot.
Sources: Wandsworth: Clapham Common conservation area appraisal and management strategy; Lambeth Design Guide SPD Part 5: Basements; Lambeth: Clapham Park and Northbourne Road conservation area character appraisal (November 2023).
Basement Conversions in Clapham: Questions
Does a Clapham lightwell need matching brick?
Usually. Lambeth's basement guidance says matching materials will generally be required, so on a stock-brick house the walls exposed by a lightwell are faced in matching brick in conservation areas.
How deep can a new Clapham basement be?
In Lambeth, one floor: Policy Q27 F allows a single storey of basement accommodation, and the Design Guide says the floor-to-ceiling height of a new residential basement should not exceed 3m, to keep the excavation down.
Can a Clapham basement have a glass roof over the lightwell?
Not in Lambeth: its basement Design Guide says glazing over light wells is not supported, and Policy Q27 H keeps front and side lightwells to the minimum size necessary, so walk-on grilles and planting are the usual answer.
Does Clapham's conservation area cover basement works?
It can, and the council matters as much. In Lambeth, Policy Q27 limits basements to one floor and keeps 70% of the rear garden free; on the Wandsworth side LP6 applies. Conservation area status adds the protection of existing basement areas and railings.
Do the common-facing Georgian houses suit a basement?
Some have one already: houses such as those on Clapham Common North Side sit above basements, and Lambeth does not support a new basement beneath an existing basement or semi-basement, so the existing space is usually improved instead.
What structural issues arise digging under a Clapham terrace?
Underpinning the party walls, temporary support of the house above and keeping water out. GOV.UK says you must tell your neighbour before digging below and near the level of their foundations, so the notices go out early.
What does a basement conversion cost in Clapham?
Improving an existing basement is priced from the area, the waterproofing and the light it needs; a new basement adds excavation, underpinning and drainage. Both are quoted after a survey, before VAT.
Which Clapham houses suit below-ground work?
Houses with an existing semi-basement or a large enough rear garden: in Lambeth a basement beyond the footprint goes only at the rear and must leave 70% of the garden free and at its original level.
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