Basement
conversions
in Westminster, SW1
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The Grade II* terraces around Smith Square were built over basements in the 1720s, behind the cast iron railings the audit records, so a Westminster lower-floor project usually begins with a room that is already there.
Around Smith Square and Lord North Street — among the most intact Georgian streets in London — a basement conversion falls within the Westminster Abbey & Parliament conservation areas, some of the most tightly-protected ground in the country, long before any question of finish or floor area arises.
External change here is exceptionally tightly controlled and listed-building consent applies widely, so a basement excavation beneath one of these Georgian townhouses needs consent for what happens inside the building as much as for anything visible from the street — the Crown Estate houses among them add a further heritage layer on top of planning and listed-building consent. Security and access constraints near Parliament shape the practicalities of getting a dig started at all.
Around Great Peter Street and Vincent Square the larger houses and mansion flats bring their own leasehold and acoustic considerations to a basement scheme. A new excavation with underpinning is priced differently from converting cellar space that already exists, with fit-out still to come.
Lower floors in Westminster's Georgian streets
The early eighteenth century houses around Smith Square already stand over basements, the 2005 audit records, and most have fine cast iron railings to the basement lightwell. The existing lower floor is the natural starting point.
Because those houses are listed at Grade II*, the City Plan's requirement that a basement must not upset a listed building's original hierarchy of spaces applies in full.
Planning permission is always needed for a new basement in Westminster since the council's 2016 Article 4 direction, and the City Plan looks for a basement of a single storey, around 2.7 metres floor to ceiling, beneath no more than half of the garden.
The audit describes a mixed commercial and residential enclave bounded to the south by Millbank. Close to the river, a structural methodology statement, required for every basement in Westminster, deals with flood risk as well as with the party walls.
We look first at the lightwell, the under-pavement vaults and the state of the existing basement walls, and price waterproofing and ventilation of the existing floor separately from any new digging.
The audit records most of these terraces with fine cast iron railings around the basement lightwell. Those railings, the stone steps and any vaults under the pavement are recorded before work starts, protected while it runs and repaired in place.
Smith Square's conservation area contains an exceptionally well preserved enclave of early 18th-century terraced town houses, its audit says, and most of the terraces have cast iron railings to the basement lightwell. Basement work there usually means repair and new services rather than digging.
One related project, though not in Westminster, is Belsize Park Gardens, NW3, a whole-house refurbishment in Belsize Park that took in basement, extension and loft works.
Where a scheme does go deeper, the party wall surveyors on both sides, the structural engineer and Westminster's building control are involved before the design is fixed, and the neighbours' basements are surveyed with their owners' agreement. Spoil and materials move through the front area by hand or conveyor, timed around the residents of the neighbouring houses, and the lightwell paving is lifted, numbered and relaid at the end. Monitoring points on the party walls are read before, during and after any excavation. Their readings are shared with the neighbours. Final readings close the job. A copy goes in the house file.
Sources: Westminster City Council: Smith Square conservation area audit (SPG, adopted 14 January 2005); Westminster City Plan 2019-2040 (2026), adopted 21 January 2026; Westminster City Council: Article 4 directions in Westminster.
Basement Conversions in Westminster: Questions
Is a basement conversion around Smith Square in Westminster treated as a listed-building matter?
Yes — Smith Square and Lord North Street sit within the Westminster Abbey & Parliament conservation areas, among the most tightly-protected ground in the country, so a basement scheme there is judged on heritage terms from the first sketch.
Does listed-building consent cover the inside of a Westminster basement, not just the facade?
It does — external change here is exceptionally tightly controlled and listed-building consent applies widely, so an excavation beneath a listed Westminster townhouse needs consent for the internal work as much as for anything visible from the street.
How does being near Parliament affect a basement dig in Westminster?
Security and access constraints around Parliament add their own demands on top of the conservation controls, so a basement excavation on the streets near Smith Square has to plan site access and deliveries around that setting.
What about a basement beneath a Crown Estate house in Westminster?
Many of Westminster's Georgian townhouses and listed terraces are Crown Estate houses that are heavily listed and often leasehold, so a basement conversion in one is heritage work governed by planning, listed-building consent and, in places, the Crown.
How much does a basement conversion cost in Westminster?
Converted cellar space in Westminster and a wholly new excavation with underpinning are priced differently, with fit-out not included.
