A basement can be designed beneath a house, its garden, or both, but the acceptable footprint depends on local policy and site conditions. An underground extension still affects structures, groundwater and landscaping, so keeping it out of sight does not remove the need for assessment. [1] [2]
Beneath the house
This option places the new accommodation below the existing building. The engineer must establish how the house will remain supported during excavation and how the finished basement will carry its loads. Existing walls, foundations and neighbouring buildings need consideration before a construction sequence is chosen. [3]
It is useful to test the stairs early. Ask the designer to show the complete connection between floors, not just a basement room plan. Stair openings and landings occupy space, and the headroom needs to work throughout the route. [4]
Beneath the garden
Extending underground can leave a garden above, but that garden sits over a roof structure. The design needs to coordinate the retained landscape, structural loading and the waterproofing of the enclosure. Kensington & Chelsea's basement guidance also addresses garden coverage, planting and the treatment of the visible parts of basement schemes. [5]
A practical brief should therefore identify which garden features matter most: usable planting, existing trees, outdoor seating or a particular relationship with the house. Ask for a section showing how these requirements fit above the proposed rooms. Do not agree the basement footprint before the landscape and structural designers have tested the same arrangement. [5] [6]
Beneath both
A combined scheme needs a coordinated design across the junction between the existing building and the garden extension. Waterproofing details must deal with changes in construction and penetrations for services; specifying one product name does not resolve those junctions. [7] [8]
Also test the usable room arrangement rather than treating all excavated space as equally useful. Daylight, outlook, circulation and ventilation can constrain where accommodation should go. A lightwell may improve a room but also changes the amount and character of the garden at ground level. [1]
Local limits are not a national allowance
Camden's adopted basement guidance applies Policy A5, including restrictions on storeys and garden coverage. Meeting a percentage limit is only one part of the assessment: effects on neighbouring buildings, trees, drainage and the character of the site still matter. Do not transfer Camden's policy to another borough. [1]
Camden is examining a replacement Local Plan. Draft Policy D6 and its proposed modifications are not an adopted replacement for A5 as checked on 2 October 2026. [9] [10]
What to establish before fixing the footprint
Request a coordinated plan and section identifying the excavation boundary, retained foundations, trees, lightwells, drainage and the proposed route out. Where a sewer is close to the works, discuss the arrangement with Thames Water rather than assuming planning approval resolves the sewer issue. [11]
Finally, check whether the basement could obstruct groundwater or create flood risk elsewhere. The planning assessment must consider the effects of the proposal, not just whether the finished rooms can be waterproofed. [12]