Basement Conversions
Basement Conversions
Unlock the hidden potential beneath your home. Basement conversions add significant living space without extending upwards or outwards.
- Existing cellar conversions from £75,000
- Excavated basements and major underpinning quoted separately
- Specialist waterproofing and tanking systems
- Can add 20 to 30% to property value
- Typical timeline 8 to 14 weeks
Industry average
£90,000 – £130,000
Our starting price
£75,000
Existing cellar conversion baseline. Excavated basements and major underpinning quoted separately. Tanking and waterproofing done in-house.
Why we're more affordable
- Internal supply chains — we source materials at trade prices
- Optimised systems — efficient planning reduces wasted time and materials
- Strong project management — we complete on time, keeping costs under control
Fixed price quote within 5 working days
Verified client reviews and 350+ completed projects
About this service
About basement conversions
A basement conversion transforms an existing cellar or underground space into habitable accommodation. This involves waterproofing (tanking), insulation, flooring, electrics, heating, and often lowering the floor or underpinning foundations. Basement conversions typically start from £75,000 for existing cellars, with excavated or heavily structural basement schemes costing far more. They add valuable space for cinemas, gyms, bedrooms, or self-contained flats.
Who it's for
Homeowners with existing cellars or those seeking to maximise space on constrained sites. Basement conversions are particularly popular in London where garden space is limited and planning restrictions may prevent upward or outward extension.
GOV.UK: Building Regulations for BasementsScope
What's Included
- Structural survey and engineering design
- Waterproofing design and tanking installation
- Underpinning or floor lowering if needed
- Insulation, electrics, heating, and finishes
- Building Regulations approval and completion
Process
How it Works
Survey & Design
We assess the existing structure, groundwater conditions, and design the optimal waterproofing solution.
Structural & Waterproofing
Underpinning if needed, tanking system installation, and structural floor construction.
Fit Out & Completion
Insulation, stud walls, services, finishes, and final decoration over 8 to 14 weeks.
Our work
Related completed work
Basement conversion
Renovations
Reviews
What homeowners say
What homeowners say about working with us on basements.
“Our basement conversion was complex but they managed it really well. Dry, bright and finished to a high standard.”
Nadia, Twickenham
Basement Conversion · Mar 2025
“The basement was dug out and tanked carefully, with party wall notices handled for us. We now have a dry, light extra room.”
James, Stanmore
Basement Conversion · Feb 2026
“Great communication through a complicated basement project. Drainage, waterproofing and finishes were all well done.”
Helen, Harrow
Basement Conversion · Jan 2024
FAQ
Common Questions
Converting an existing cellar typically starts from around £75,000. Excavated basements, major underpinning, and premium fit outs can increase the cost significantly because waterproofing and structural work make basements far more complex than standard above-ground extensions.
Converting an existing cellar usually falls under Permitted Development. However, excavating new basements or adding lightwells often requires planning permission. We always verify and submit applications where needed.
We use Type C (cavity drain membrane) or Type A (tanking) waterproofing systems depending on groundwater conditions. All systems are installed by certified specialists and come with guarantees. We design to BS 8102:2022 standards.
Most basement conversions take 8 to 14 weeks. Complex projects with underpinning or new excavation can take 14 to 20 weeks. We provide a detailed programme before starting.
Where we build basement conversions
Our team is based in Harrow and works across London and the Home Counties, including:
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Ready to start your project?
Fixed price quote within 5 working days. No obligation.