Glossary
Basement extensions beneath a London terraced house
A subterranean addition under the existing footprint or rear garden. Heavily regulated in central London.
A subterranean addition under the existing footprint or rear garden. Heavily regulated in central London.
A basement extension is new habitable space created below ground, either by converting and lowering an existing cellar or by excavating fresh volume under the house or garden. Converting an existing cellar without altering the external appearance is usually permitted development; excavating new volume almost always needs planning permission in London.
Changing a cellar you already have into habitable space, with no change to the external appearance and no new lightwell, generally falls within permitted development as an alteration. Cut a new lightwell, extend the footprint beyond the original house, or lower the floor and you are into a full application.
Many London boroughs also operate specific basement policies that bite before the national rules do — limits on the proportion of garden that can be built under, bans on double-storey basements outside commercial areas, and mandatory basement impact assessments.
A basement is a structural and hydrological project. The package normally includes a ground investigation, a structural scheme for underpinning or piling, a groundwater and ground-movement assessment, a waterproofing design to BS 8102 and a construction method statement covering sequencing and temporary works.
Waterproofing to BS 8102 is specified by grade. Grade 3 — a dry environment suitable for habitable rooms — normally requires two independent protection types: a cavity drain membrane plus a drained sump, or structurally integral waterproof concrete plus a membrane.
Excavating within 3 m of an adjoining structure, and to a depth below its foundations, is notifiable under section 6 of the Party Wall etc. Act 1996. The notice period is two months, and the drawings and sections accompanying it have to show the excavation depth against the neighbour's foundation.
Basement extension in a Hammersmith and Fulham context: London Borough of Hammersmith and Fulham (Hammersmith & Fulham Council) runs 45 conservation areas including the Hammersmith Grove Conservation Area and the Brackenbury Village Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how basement extension is handled: £548 and 8 weeks for a Hammersmith and Fulham householder application, or £274 for the Lawful Development Certificate that records basement extension as lawful. The law behind basement extension sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Waterproofing standard | BS 8102, Grade 3 for habitable space |
|---|---|
| Party wall notice (excavation) | Two months, section 6 |
| Excavation trigger distance | 3 m (section 6(1)) / 6 m (section 6(2)) |
| Typical head height sought | 2.4 m finished floor to ceiling |
| Planning fee (full householder) | £548 |
| Our basement drawing package from |
Often yes, if the external appearance is unchanged and no lightwell is added. Local basement policies can override that.
Most boroughs restrict habitable basements to a single storey below ground outside commercial areas.
If you excavate within 3 m of a neighbouring structure and below its foundations, yes.
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