Residential Construction Cornwall
What is a basement conversion?
A basement conversion creates habitable living space below ground level — either by converting an existing cellar or excavating beneath an existing home. In Cornwall, ground conditions including granite bedrock, high water tables on coastal sites, and clay soils require specialist structural and waterproofing expertise. Done correctly, a basement conversion adds significant usable space without extending the building’s footprint.
Basement conversion price
Most Cornwall basement conversions cost £1,200 to £2,500 per square metre — converting an existing cellar or lower-ground space, not excavating a new one. A typical 25–30m² conversion lands somewhere around £35,000 to £75,000, depending on waterproofing method, ceiling height, and finish level.
New basement excavation — rare outside dense terraces — runs significantly higher, from £3,000 to £5,000+ per square metre.
We don't quote a headline number that won't reflect your actual project. Every basement conversion is priced after a structural and waterproofing survey, so the figure you receive is real — not a guess that moves once work starts.
→ Basement Conversion Cost UK: Full Guide → Get in touch for a realistic price conversation
Basement Conversion Specialists Cornwall
A basement conversion isn't a standard build-it's a structural and waterproofing project first, a living space second. That's why it needs a specialist, not a general contractor treating it as an afterthought.
What makes a genuine specialist:
- Structural understanding — knowing when underpinning is needed and when it isn't
- The right waterproofing system for the site — Type A, B, or C, matched to ground conditions, not applied by default
- Ventilation designed in, not bolted on — moisture control that prevents damp rather than masking it
- A survey before a single drawing — pricing and design follow the structural reality, not the other way round
We treat every basement conversion this way. TrustMark registered, with structural and waterproofing surveys carried out before any design or price is finalized.
What We Build
- Cellar conversions — converting existing below-ground space into habitable rooms
- Full basement excavations — digging out beneath an existing home to create new lower-ground floor space
- New build basements — incorporating basement level into a bespoke new build from the outset
- Basement extensions — extending the footprint below ground alongside above-ground works
- Garage-to-basement conversions — where site levels allow below-ground access
What a Basement Conversion Involves
- Structural surveys and ground investigations before any design is finalised
- Underpinning or temporary works where required to support the existing structure
- Excavation and removal of spoil — managed carefully on constrained Cornish sites
- Structural waterproofing (tanking) — cavity drain or cementitious systems specified for Cornwall’s ground conditions
- Concrete or masonry structural walls and floor slab
- Drainage, ventilation, and services installation
- Full internal fit-out to the same standard as above-ground living space
Why Warvena
- In-house tradespeople — groundworkers, structural carpenters, and fit-out trades all employed directly
- Cornwall ground condition experience — granite bedrock, high coastal water tables, and clay soils handled correctly
- Structural coordination from the outset — we work with structural engineers and architects before a spade goes in
- TrustMark registered · AECB members
- Clean, low-disruption sites — managed around occupied homes wherever possible
Where We Work
Across Cornwall — Truro, Falmouth, Redruth, St Ives, Padstow, Newquay, Penzance, and throughout the county including coastal and AONB locations.
Considering a basement conversion in Cornwall?
TrustMark registered. In-house tradespeople. Experience on Cornwall’s most demanding ground conditions.
Get in touch → info@warvenaconstruction.co.uk | 01872 300856
Frequently Asked Questions
What is a basement conversion?
A basement conversion creates habitable living space below ground level — either by converting an existing cellar or excavating beneath an existing home to create new space. It adds usable floor area without increasing the building’s above-ground footprint
How much does a basement conversion cost in Cornwall?
Basement conversions are among the most complex and costly forms of home improvement. In Cornwall, costs typically start from £3,000–£4,500 per m² depending on ground conditions, depth of excavation, waterproofing specification, and fit-out standard. Granite bedrock and high coastal water tables add to costs compared to easier ground
Do I need planning permission for a basement conversion in Cornwall?
Basement conversions often fall within Permitted Development for single dwellings, but this depends on the scope of works, the property type, and the local planning designation. Listed buildings and properties in conservation areas or AONBs require listed building consent or planning permission regardless of scale. We advise on the planning position as part of the initial project discussion.
How long does a basement conversion take?
A straightforward cellar conversion typically takes 8–14 weeks on site. A full excavation beneath an existing home — including underpinning, structural works, waterproofing, and fit-out — typically runs 20–32 weeks depending on ground conditions and complexity. Ground investigations and structural design add time before work starts on site.
Is basement conversion possible on Cornwall's granite bedrock?
Yes, but it requires specialist plant and structural expertise. Granite excavation is significantly more complex and costly than soft ground. We have experience working in Cornwall’s granite geology and factor this into programme and cost planning from the outset — not as a surprise on site.
What waterproofing system is used for basement conversions?
The two primary systems are cavity drain membrane (Type C) and cementitious tanking (Type B). The right choice depends on the ground conditions, water table level, and intended use of the space. On Cornwall’s coastal sites with high water tables, cavity drain systems are often the more reliable long-term solution. We specify the appropriate system for each site based on the ground investigation results.
THINKING ABOUT A BASEMENT CONVERSION IN CORNWALL?
Warvena Construction are TrustMark registered builders based in Redruth, Cornwall. In-house tradespeople, experience on Cornwall’s most demanding ground conditions, and the structural expertise to deliver basement projects correctly.