In London, adding space to a home is rarely straightforward. Planning restrictions, tight plots, and neighbouring properties often limit how far a house can be extended above ground. For many homeowners, building below ground becomes a practical way to gain meaningful space without compromising the character of the property.

 A basement can dramatically improve how a home functions. It might create room for family living, additional bedrooms, leisure space, or a dedicated area to work from home. When designed well, a basement should feel like a natural extension of the house rather than an afterthought. 

From a value perspective, additional usable floor area is particularly valuable in London, where space is at a premium. However, basement construction is a technically demanding process, and understanding the likely costs is an important part of deciding whether it is the right solution. 

Before looking at cost ranges, it’s important to understand what these figures typically cover. Basement construction costs usually relate to the building works themselves and do not always include professional fees such as architectural design, structural engineering, party wall matters, planning costs or VAT. These should be considered separately as part of the overall project budget.

In the current London construction market, the cost of building a basement typically ranges from £3,000 to £5,000+ per square metre for the structural shell, with fully finished basements often costing between £4,500 and £7,500+ per square metre. The final figure depends on the property, ground conditions, access constraints, and how the space will be used. 

Construction costs continue to change due to labour availability, material pricing, and regulatory requirements, which is why early professional advice is always recommended. 

With nearly four decades of experience delivering complex basement and below-ground projects across London and the South East, MJ Rooney provides this guidance based on current market conditions and real project delivery rather than generic averages. 

Basement construction should be approached as a long-term investment. Costs reflect the technical complexity of working below ground and the level of coordination required to deliver a safe, durable, and well-performing space.

 As a broad guide for London properties:

Newly excavated basement (structural shell)

£3,000 – £5,000+ per m²

Excavation, underpinning or piling, reinforced concrete structure and waterproofing

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Newly excavated basement (fully fitted)

£4,500 – £7,500+ per m²

Structural shell plus heating, electrics, joinery and full internal fit-out

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Existing cellar or basement conversion

£750 – £3,000 per m²

Upgrading existing space, possible floor lowering, damp protection and internal works

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These figures are intended as realistic guidance rather than fixed prices. Construction costs typically relate to building works only; professional fees such as architectural design, structural engineering, party wall matters and planning costs are usually additional and should be allowed for within the overall budget.

Basement projects vary more than almost any other type of home improvement. Two neighbouring properties can have very different costs, even if the finished spaces appear similar. 

Ground conditions are a major factor. London clay, gravel or high groundwater levels all influence how excavation and waterproofing must be designed. Access is equally important; tight urban sites often require specialist equipment, additional labour and carefully planned sequencing of works. 

Depth also affects cost. A shallow basement used for storage or plant space is very different from a full-height living space designed to feel like a natural extension of the home. 

In London, neighbour proximity and site logistics also play a role. Shared boundaries, party wall considerations, and the need to protect surrounding properties require careful planning and experienced management. 

Finally, the intended use and level of fit out make a significant difference. A utility area or gym will generally cost less to complete than a cinema room, guest suite or high-quality family living space with bespoke joinery and integrated services.