Basement Conversions
Build down. Add an entire floor to your home.
Basement conversions and excavations across London. Full underpinning, structural engineering, waterproofing and fit-out — creating high-quality habitable space beneath your property.
Basement conversion specialists in London.
Where extending outward is not possible or permitted, building downward can unlock an entire floor of new living space. Basement conversions are among the most technically demanding construction projects in residential building — but when executed properly, they deliver exceptional value.
The process involves underpinning the existing foundations, excavating to the required depth, installing structural waterproofing (tanking), constructing a reinforced concrete structure, and fitting out the space to habitable standards including ventilation, drainage, lighting and fire safety.
RCB manages basement conversions from initial feasibility and structural engineering through planning, construction and handover. These are complex projects that require an experienced principal contractor with the structural capability and site management discipline to deliver safely and to standard.
What's included
A basement conversion can add 25-30% to your property value and create an entire new floor of living space
How It Works
The RCB process for this service.
Feasibility & Surveys
Structural assessment of existing foundations, geotechnical investigation, planning review and initial cost appraisal to confirm the project is viable.
Design & Engineering
Detailed structural engineering design, waterproofing strategy, planning application (if required), Building Regulations submission and party wall notices.
Estimate & Contract
A comprehensive itemised estimate covering underpinning, excavation, structure, waterproofing, services and fit-out. Basement projects require detailed scoping — no shortcuts.
Build & Complete
Phased underpinning and excavation, structural concrete works, waterproofing, services installation, internal fit-out and Building Control completion certificate.
Planning & Regulations
Basement conversions and excavations are subject to strict regulatory requirements. Planning permission is required in many London boroughs — some have specific basement development policies limiting depth, extent and construction methodology. Building Regulations approval is mandatory, covering structure, waterproofing, fire safety, ventilation and drainage. Structural engineering design must be carried out by a chartered engineer. Party wall notices are required where underpinning or excavation occurs near a boundary. Some boroughs require a Construction Management Plan. RCB coordinates all regulatory, structural and compliance requirements.
FAQs
Common questions.
Do I need planning permission for a basement conversion?
It depends on your borough and the scope of work. Converting an existing cellar may not require planning permission, but excavating a new basement or lowering the floor level almost always does. Many London boroughs — including Kensington and Chelsea, Camden, and Westminster — have specific basement policies. RCB will assess the planning position for your property and borough at the feasibility stage.
How much does a basement conversion cost in London?
Basement conversions in London typically start from £120,000 for a straightforward conversion of an existing cellar and can reach £300,000+ for a full new excavation with high-end specification. Costs are driven by the depth of excavation, underpinning methodology, waterproofing requirements and specification of the fit-out. RCB provides a detailed itemised estimate once the structural design is complete.
How long does a basement conversion take?
A typical basement conversion takes 20–40 weeks on site, depending on the scope. Simple cellar conversions are faster; full excavations with underpinning take longer. Pre-construction — surveys, structural design, planning and party wall — adds 12–24 weeks. Basement projects have long lead times and should not be rushed.
Will a basement conversion cause problems for my neighbours?
Basement works involve vibration, noise and structural work near boundaries. Party wall notices must be served, and a condition survey of adjacent properties is standard practice. RCB manages the construction methodology to minimise impact, maintains communication with neighbours throughout, and ensures all party wall obligations are met.
How do you keep a basement dry?
Structural waterproofing is the most critical element of any basement conversion. RCB uses a combination of Type A (barrier), Type B (structurally integral) and Type C (drained cavity) waterproofing systems depending on the site conditions. A properly designed sump pump and drainage system provides an additional safety layer. All waterproofing is designed by a specialist and backed by insurance-backed guarantees.
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