Basement Conversion Company in London

New Excavation Basements

Building down where there is nowhere left to build out.

A new excavation basement is dug beneath an existing property, or beneath its garden, where no usable below-ground space currently exists. It is the most structurally demanding project type Sovran delivers, requiring underpinning of existing foundations, temporary works, and careful sequencing to protect the structure above throughout the dig.

Existing Cellar Conversions

Space that already exists, made habitable.

Many London period properties have an existing cellar, currently used for storage rather than as living space. Converting it typically involves lowering the floor level for full head height, structural underpinning where required, and bringing the space up to the same waterproofing and building regulation standard as a new excavation.

Cost by Project Type and Depth

Basement conversion costs in London are shaped more by ground conditions and structural scope than by finish specification alone. As a guide:

• Existing cellar conversion: £150,000–£280,000+
• New excavation basement: £250,000–£500,000+ — depending on depth and footprint

Costs run highest in Kensington, Chelsea and Notting Hill, where ground-level space is most constrained and basement demand is highest. For an indicative figure against your own property, use the cost calculator.

What Drives the Price, Waterproofing, Structural Engineering, Party Wall

Depth, soil conditions and the number of adjoining party walls are the primary cost drivers, followed by waterproofing specification and finish. A single-storey dig beneath an existing footprint costs meaningfully less than a full-width excavation extending beneath the garden. At Sovran, every quote is fixed after a site investigation confirms ground conditions, not adjusted upward once excavation is underway.

Permitted Development vs. Full Planning

A small number of basement works, such as converting existing cellar space without external alteration, may fall under permitted development, but any excavation that changes the external appearance of the property, adds a lightwell, or extends beneath the garden will almost always require full planning permission. Basements beneath conservation areas and listed buildings face further scrutiny, particularly around impact on the water table and neighbouring structures.

Party Wall Agreements for Basement Digs

Basement excavation is the project type most likely to require Party Wall Act agreements with every adjoining owner, since underpinning work directly affects shared foundations. Sovran manages the full process, from serving notice through to agreeing the award with each adjoining owner's surveyor, and sequences the notice period into the programme from the outset rather than treating it as a delay risk.

BS 8102:2022 Waterproofing Standard

Every Sovran basement is specified to BS 8102:2022, the British Standard for protecting below-ground structures against water ingress. Waterproofing is assessed and specified following a site investigation, not applied as a standard package regardless of ground conditions.

Type A, B and C Waterproofing Systems

Depending on the site, Sovran specifies a combination of Type A (barrier), Type B (structurally integral) and Type C (drained cavity) waterproofing systems, often layering more than one system where ground conditions demand it. The specification is made on a property-by-property basis, not assumed from a standard basement package.

London Clay and High Water Table Conditions

Much of London sits on London clay, which shrinks and swells with moisture content and requires careful underpinning sequencing, while parts of west and south-west London carry a genuinely high water table that raises the waterproofing specification required. Sovran's structural engineering team assesses both conditions at survey stage, before any cost or waterproofing specification is confirmed.