Basement
conversions
in Marylebone, W1U
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Every Georgian house in the Harley Street area has an open basement lightwell behind cast iron railings, with stone steps to the front door, Westminster's 2008 audit records. Basement work in Marylebone starts with that existing lower floor.
Marylebone sits in the City of Westminster, much of it within the Marylebone Conservation Area and across the Portman and Howard de Walden estates. Westminster's city-wide basement Article 4 direction, in force since 31 July 2016, removes permitted-development rights for basement work everywhere in the borough, so a Marylebone basement — whatever the house — needs planning permission before a spade goes in.
On top of that planning consent, the estate structure adds its own layer: Portman and Howard de Walden estate houses need works agreed with both Westminster and the freeholder under a clear consent strategy, and that applies as much to a basement as to any other alteration. Montagu Square and Bryanston Square carry elegant brick terraces on the Portman Estate, Dorset Square is a leafy Regency set-piece nearby, and Marylebone High Street anchors the mansion flats and conversions above the shops — each needing its estate consent routed alongside the planning application.
Once planning and estate consent are both in hand, the cost split follows the usual pattern: converting existing cellar space in Marylebone and a new excavation with underpinning carry different costs, fit-out not included.
Below a Marylebone town house
Westminster's 2008 audit of the Harley Street Conservation Area describes the Georgian houses as three, four and sometimes five storeys with basements, two or three bays wide behind a parapet cornice, and records that all have open basement lightwells behind cast iron railings, with stone steps up to the front door. Most Marylebone houses therefore already have a lower floor to work with.
Those front railings, the audit explains, enclose the lightwells and mark the distinction between the main and service areas of a house; they are cast or wrought iron, generally painted black and set into low stone plinths. A kitchen, utility room or plant room fits that historic order, and the railings, plinths and steps stay in place throughout the work.
Two areas of special archaeological priority touch Marylebone: the Marylebone Village area, largely around the north end of Marylebone High Street, and the Tyburn Settlement, which lay close to Manchester Square. Any new excavation in either is checked for archaeology at the feasibility stage.
West of the High Street, the Portman Estate mini guide says the 18th century fabric has largely survived and that a very large proportion of its 18th century buildings are listed Grade II, so the lower floor of a listed house is altered only once Westminster has confirmed the consents needed.
Every new basement in Westminster has needed planning permission since July 2016. For a listed house, the City Plan adds that a basement must protect the heritage asset and not upset the original order of rooms where that order matters to the building's significance.
Where a house is held on a Portman Estate lease, the estate's licence to alter guidance says structural works make the leaseholder liable for its consultant structural engineers' fees and that, depending on the wording of the lease, a premium may be payable. Those costs are raised with the client at the start.
A related project, not in Marylebone, is Belsize Park Gardens, NW3: a whole-house refurbishment in Belsize Park that took in basement, extension and loft works. It stands for the type of work only; nothing on this page presents it as work in Marylebone.
We open up the existing basement first, testing the walls and floor for damp, tracing the drains under the lightwell and checking the vaults below the pavement, then agree with the client whether refurbishing that floor alone meets the brief.
Sources: Westminster City Council: Harley Street conservation area audit (SPD, adopted 16 June 2008); Westminster City Council: Harley Street conservation area mini guide (May 2004); Westminster City Council: Portman Estate conservation area mini guide (May 2004); Westminster City Council: Article 4 directions in Westminster; Westminster City Plan 2019-2040 (2026), adopted 21 January 2026; The Portman Estate: licence to alter, frequently asked questions.
Basement Conversions in Marylebone: Questions
Do I need planning permission for a basement conversion in Marylebone?
Yes, always: Westminster's city-wide basement Article 4 direction, in force since 31 July 2016, removes permitted-development rights for basement development throughout the city, so every Marylebone basement scheme needs a full planning application.
Does the Portman or Howard de Walden estate also need to approve a Marylebone basement?
It does: estate terraces on the Portman and Howard de Walden estates need works agreed with both Westminster and the freeholder under a clear consent strategy, and a basement falls under that same estate-consent process as any other alteration.
Which Marylebone streets sit on the Portman Estate, and does that change a basement scheme?
Montagu Square and Bryanston Square carry elegant brick terraces on the Portman Estate, Dorset Square adds a leafy Regency square, and Marylebone High Street anchors the mansion flats above the shops — stock where estate consent runs alongside the planning application for any basement.
Is the Marylebone Conservation Area a separate hurdle from the estate consent for a basement?
It sits alongside it: Marylebone lies within the Marylebone Conservation Area and across the Portman and Howard de Walden estates in the City of Westminster, so conservation status, estate consent and the Article 4 planning requirement all apply together to a basement scheme.
Cellar conversion or new dig — what's the cost difference for a Marylebone basement?
Converting an existing cellar in Marylebone and digging a new excavation with underpinning are priced differently, each quoted after a survey and before fit-out.
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