The ConstructionLtd

London's Construction Experts

The ConstructionLtd
Basement Conversions

Basement Conversions London — Cellar Conversions & New Dig Basements

In London's most valuable postcodes, the only direction left to extend is down. Basement conversions create entirely new floors of living space beneath your existing property — space for kitchens, cinemas, gyms, home offices, guest bedrooms, utility rooms, or wine cellars. In a city where land is the scarcest commodity, going underground is the most space-efficient investment you can make. Our team has the structural engineering expertise, the specialist plant, and the project management discipline to deliver basement conversions safely, on time, and on budget.

Starting From£100,000
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Types of Basement Conversions

Cellar Conversion

Converting an existing cellar into a habitable room. The most affordable option where cellars already have adequate headroom (2.1m minimum, 2.4m ideal). Involves damp-proofing, insulation, lighting, ventilation, and finishing.

Typical cost:£100,000–£180,000

Lowering an Existing Cellar

Many London properties have shallow cellars with insufficient headroom. Underpinning the foundations and lowering the floor slab creates a full-height basement. More complex than a simple conversion but avoids the cost of a full new excavation.

Typical cost:£150,000–£280,000

Full New-Dig Basement

Creating a basement where none exists — the most complex option, requiring full excavation beneath the existing building with sequential underpinning. Requires structural engineering and specialist waterproofing.

Typical cost:£200,000–£500,000+

Basement Conversions Costs & Pricing

TypeSizePrice Range
Cellar conversion (existing adequate height)8–14 weeks£100,000–£180,000
Cellar lowering (underpinning + conversion)14–22 weeks£150,000–£280,000
New-dig basement (single storey)20–36 weeks£200,000–£500,000+
Light well addition3–4 weeks£15,000–£30,000

* Prices are indicative for London construction in 2026. Final price depends on specification, finish, and site-specific factors. All our projects are delivered on a fixed-price contract.

Frequently Asked Questions

Is my property suitable for a basement conversion?
Most London houses can accommodate a basement, but suitability depends on ground conditions, water table level, structural feasibility, and local planning policy. We conduct a detailed feasibility assessment including trial pits and soil analysis before providing a quote.
Do I need planning permission for a basement?
In many cases, yes. Most London boroughs now have specific basement planning policies that require planning applications, Structural Impact Assessments, and Construction Management Plans. Some boroughs (Camden, RBKC, Westminster) have particularly detailed requirements. We manage the full planning and regulatory process.
Is a basement conversion worth the investment?
In prime London postcodes, basements are extremely cost-effective on a per-square-metre basis when compared to above-ground extension alternatives. A well-executed basement adds substantial value — often 1.5–2x the construction cost in inner London.
How long does a basement conversion take?
Cellar conversions: 8–14 weeks. Cellar lowering: 14–22 weeks. Full new-dig basements: 20–36 weeks. Timescales include structural work, waterproofing, fit-out, and snagging.

Get a Quote for Your Basement Conversion Project

Tell us about your property and what you're planning. We'll respond within 48 hours with a preliminary estimate and arrange a free site visit.

Free site visit and assessment
Fixed-price quote — no surprises
In-house planning and design team
Dedicated project manager throughout

Basement Conversions Quote

Free consultation within 48 hours.

Or call us now: 07438 319930

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Get a free, no-obligation quote within 48 hours. We cover every London borough and the Home Counties.

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Free consultation • Fixed-price contract • No obligation