Basement
conversions
in Marylebone, W1U
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We design and build Marylebone basement conversions with basement excavation, cellar conversion, and underpinning for properties on Marylebone High Street, Montagu Square, Bryanston Square - structural engineering, party wall coordination, underpinning, waterproofing, drainage pumps, lightwells and full interior fit-out for period homes, townhouses and prime London properties where ground conditions, neighbour risk and sequencing need handling properly.
Basement Conversions in Marylebone: What You Need to Know
Every basement conversion project in Marylebone is designed to respect the Marylebone Conservation Area and the character of Georgian townhouses, Portman Estate terraces, mansion flats. Our in-house RIBA Chartered Architects handle Westminster planning applications, our chartered structural engineers specify any load-bearing work, and our directly-employed trades deliver the build under one fixed-price contract. We coordinate Building Control, Party Wall awards where needed, and all certifications through to handover.
Elegant village feel with Georgian architecture, Marylebone High Street boutiques, and Regent's Park proximity. Howard de Walden Estate properties. Westminster conservation area. Westminster's basement planning policies require careful attention to structural impact, drainage, and neighbouring properties. We prepare comprehensive basement impact assessments and manage the entire planning and construction process.
Engineering basement conversions in Marylebone demands expertise in underpinning, waterproofing, and structural reinforcement. Our team manages party wall agreements, Westminster building control, and construction logistics — critical in Marylebone's densely built environment.
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 runs £750–£3,000 per square metre of converted area, rising past £6,000/m² finished, while a new excavation with underpinning is priced at £2,100–£4,000 per square metre of excavated area, fit-out not included.
Basement Conversions in Marylebone: Applied to the Local Stock
The Harley Street Conservation Area - first designated in 1968 around Portland Place, Harley Street and Wimpole Street, and extended west to Marylebone High Street in 1981 - covers the Howard de Walden Estate's Marylebone grid, with medical uses of international renown in and around Harley Street and Marylebone High Street as its main shopping thoroughfare; it is bounded by the Regent Street, Mayfair, Portman Estate, Regent's Park and East Marylebone conservation areas. The Portman Estate Conservation Area was first designated in 1967 to cover the late 18th and early 19th century residential area developed by the Portman Estate, and was extended in 1979 to include parts of Seymour Place and Marylebone Road and in 1990 to include Portman Square; Portman Square was laid out in 1764 and the estate's characteristic 18th century grid extended to Manchester Square (1776) and Baker Street (completed c. 1800). 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 Marylebone basement schemes need planning permission.
The Marylebone Village Area of Special Archaeological Priority lies largely within the Harley Street Conservation Area around the north end of Marylebone High Street, and the strategic view from Primrose Hill to the Palace of Westminster cuts through the conservation area from Park Crescent to Oxford Street.
Sources: Westminster City Council, Harley Street conservation area mini guide (May 2004) (2026); Westminster City Council, Portman Estate conservation area mini guide (May 2004) (2026); Westminster City Council, Article 4 directions in Westminster (2026)
What a Basement Conversion Involves
Digging out a basement adds a complete new storey without touching the garden or the street elevation — and it is the most engineering-led work we do. Ground conditions, foundation depths and the water table are investigated before a price is ever fixed, because below-ground surprises are expensive precisely when they are unplanned. Each project moves through four disciplines, delivered by one team under one contract.
- Excavation — the dig itself, planned around access, spoil removal logistics and temporary support of the house above
- Underpinning — foundations deepened in short, engineer-sequenced sections, with movement monitoring on your property and your neighbours’ throughout
- Waterproofing — a tanked, integral or cavity-drain system (Type A, B or C to BS 8102) selected for your site’s ground conditions, not by habit
- Fit-out — insulation, ventilation, light wells, staircase, plaster and finishes brought to the same standard as the floors above
How We Run Your Project
Below-ground work punishes improvisation, so every stage is closed out before the next one starts.
- Ground investigation — bore holes establish soil type, bearing capacity and water table before feasibility is confirmed.
- Design & consents — layout, light wells and stair position from our architects; underpinning sequence from our structural engineers; planning and any Basement Impact Assessment handled for you.
- Fixed-price contract — priced after the soil investigation, so ground conditions are our risk rather than your variation.
- Excavation & underpinning — sequenced digs and pours with live structural monitoring, never more than the engineer’s design allows at once.
- Waterproofing & shell — membrane or structural system installed and inspected before anything conceals it, with sump and pump provision where the site needs it.
- Fit-out & handover — services, finishes, building control sign-off and a certification pack at completion.
On site, expect 12–16 weeks for a cellar conversion and 16–24 weeks for a full dig-down — the curing time between underpinning sections cannot safely be compressed. Read the full basement conversion guide for waterproofing systems, ceiling heights and light-well design in depth.
The regulatory load is real: a chartered structural engineer must design and certify the underpinning methodology, and most excavations within three metres of a neighbouring building engage the Party Wall etc. Act 1996 — notices, schedules of condition and often a formal award before work can start. Many boroughs add a Basement Impact Assessment covering hydrology and construction management. Our planning guide library sets out what applies where.
Why Hampstead Renovations
Basements fail when design, engineering and waterproofing are handled by separate firms working from different assumptions. Here they sit under one roof: in-house RIBA Chartered Architects shape the space, our own structural engineers design and supervise the underpinning, and RICS surveying support through our sister company covers party wall and condition matters. Every project carries £10M public liability cover, a genuinely fixed price agreed after ground investigation, and a 10-year structural and waterproofing warranty. Send an enquiry before 2pm on a weekday and you will hear back from us that same day.
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 existing cellar space in Marylebone is guided at £750–£3,000 per square metre of converted area, over £6,000/m² once finished, while a new excavation with underpinning runs £2,100–£4,000 per square metre of excavated area, before fit-out.
Basement Conversions in Marylebone: Planning & Local Context
Westminster City Council · W1
Planning for basement conversions in Marylebone
Basement and underpinning schemes in Marylebone are assessed against Westminster City Council's subterranean-development policy and almost always require a Basement Impact Assessment, structural method statements, and Party Wall awards with neighbouring owners before work can begin.
Westminster City Council context
Marylebone W1 sits within Marylebone's conservation areas, and Westminster applies a demanding conservation and listed-building regime, and many mansion blocks and townhouses here are listed or sit within tightly controlled estates such as the Grosvenor, Howard de Walden and Crown estates. Much of Marylebone is managed by the Howard de Walden and Portman estates. Our in-house RIBA Chartered Architects prepare and submit every application to Westminster City Council and handle pre-application discussions where early planning advice is useful.
Building Regulations
Basement Conversions in Marylebone must satisfy Building Regulations — covering waterproofing and structural design to Part A and Part C, ventilation, and means of escape to Part B. Our in-house engineers manage Building Control from initial notice to completion certificate.
Recent work & local links
See recent Marylebone work: Home refurbishment, Marylebone. Explore our Kitchen Renovation Loft Conversions in Marylebone, or the Marylebone hub.
Our in-house planning team prepares and submits every Westminster City Council application and manages building control end to end. View our planning track record →
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