Project Type
Retrofit Basement Conversion
Duration
22 weeks
Project Value
£175,000
The Challenge
Westminster's basement policy requires a detailed basement impact assessment for any new basement excavation, covering hydrology, structural impact, and neighbourhood effects. The property's geology included a layer of Terrace Gravel above London Clay at 3m depth — the groundwater table was at 2.8m, just at the proposed basement floor level. Dewatering the site during excavation without affecting neighbouring properties required specialist engineering.
Our Solution
A specialist geotechnical engineer designed a temporary dewatering system using wellpoints around the excavation perimeter, with monitoring of neighbouring properties throughout. The permanent waterproofing was a combined Type B tanked system with Type C cavity drain backup — belt and braces for a high groundwater site. A lightwell was introduced at the front elevation to bring natural light into the guest suite. Westminster approved the application in 11 weeks.
Results
- ✓Westminster planning approval in 11 weeks
- ✓Geotechnical dewatering scheme — zero impact on neighbouring properties confirmed
- ✓Combined Type B/C waterproofing system — independently certified
- ✓Guest suite with en-suite — 28m², 2.4m ceiling height
- ✓Home office — 22m² with bespoke fitted joinery and concealed AV
- ✓Lightwell at front bringing natural daylight to guest suite
- ✓Plant room housing boiler, heat pump, and electrical distribution
Key Features
- ✓Westminster basement planning consent — impact assessment approved
- ✓Specialist geotechnical dewatering — high groundwater site
- ✓Combined Type B tanked and Type C cavity drain waterproofing
- ✓Guest suite with direct lightwell access
- ✓Bespoke home office joinery with integrated cable management
- ✓New plant room — consolidating all building services
Specification
| Item | Specification |
|---|---|
| Waterproofing (Type B) | Sika Bituseal bonded sheet membrane, fully adhered to poured concrete structure |
| Waterproofing (Type C) | Delta MS Drain cavity drain membrane with perimeter channel and dual sump |
| Lightwell | Bespoke structural steel lightwell, powder-coated, with toughened glass cover |
| Guest suite | Duravit en-suite, Porcelanosa tiles, wall-hung WC, rainfall shower, heated towel rail |
| Office joinery | Bespoke oak-veneered fitted desk and shelving, integrated cable trays, media panel |
| Ceiling | GRG smooth ceiling, recessed LED downlighters, 2400mm finished height throughout |
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