Basement Conversions London
Create premium below-ground living space with expert basement excavation, underpinning and waterproofing across London.
Specialist Basement Conversion Builders in London
Alstruct delivers high-specification basement conversions across London, transforming unused voids into beautiful living spaces. Whether you need a home cinema, gym, extra bedroom, home office, or additional reception room, a basement conversion maximises your property's footprint without altering its external appearance.
Basement projects demand specialist expertise in structural engineering, waterproofing, and ground conditions. Our experienced team manages every stage -- from initial site investigation and trial pits through to excavation, underpinning, tanking, structural works, mechanical and electrical installation, and final fit-out.
Types of Basement Conversions We Build
Light Well Basements
Light wells are excavated at the front or rear of the property to allow natural daylight into the basement. This creates a bright, habitable space that feels connected to the outside. Light wells can include staircases providing direct garden access.
Full Basement Excavation
A complete dig-out beneath your property to create a full-height basement level. This is the most comprehensive option, providing maximum additional floor area. Suitable for creating multiple rooms including bedrooms, bathrooms, and entertainment spaces.
Underpinning
Existing foundations are deepened to create additional headroom in an existing cellar or partial basement. A cost-effective way to convert a low-ceiling cellar into a usable living space without full excavation.
Waterproofing & Tanking
Essential for every basement project. We install multi-layered waterproofing systems including cavity drain membranes, sump pumps, and drainage channels to ensure your basement remains completely dry for decades to come.
What's Included in Every Basement Conversion
- Free site survey and feasibility assessment
- Ground investigation and trial pits
- Architectural and structural engineering design
- Planning permission applications
- Party wall agreements coordination
- Excavation and spoil removal
- Underpinning and structural reinforcement
- Waterproofing and tanking systems
- Mechanical ventilation and dehumidification
- Electrics, plumbing and heating
- Plastering, flooring and decoration
- Full project management throughout
Basement Conversion Cost Guide - London 2026
| Conversion Type | Price Range | Duration |
|---|---|---|
| Light well basement | £80,000 - £150,000 | 12-20 weeks |
| Full excavation | £150,000 - £300,000+ | 20-36 weeks |
| Underpinning | £40,000 - £80,000 | 8-12 weeks |
Prices are indicative for London in 2026 and vary based on ground conditions, depth, waterproofing requirements, and specification. Contact us for an accurate quote for your project.
Frequently Asked Questions
How much does a basement conversion cost in London in 2026?
Basement conversion costs in London vary significantly by scope. Light well basements typically cost £80,000-£150,000, full excavations £150,000-£300,000+, and underpinning projects £40,000-£80,000. The final cost depends on ground conditions, depth, waterproofing requirements and specification. Contact Alstruct for a free assessment.
Do I need planning permission for a basement conversion?
Most basement conversions in London require planning permission, particularly if they involve excavating beneath an existing property or adding light wells. Many London boroughs also have specific basement development policies. Alstruct handles all planning applications and ensures compliance with local authority requirements.
How long does a basement conversion take?
A light well basement typically takes 12-20 weeks, a full excavation 20-36 weeks, and underpinning projects 8-12 weeks. Basement projects require careful sequencing of excavation, structural support, waterproofing, and fit-out. Alstruct provides a detailed programme before work begins.
Is my property suitable for a basement conversion?
Most London properties can accommodate a basement conversion, but suitability depends on ground conditions, water table level, proximity to neighbouring foundations, and local planning policies. Alstruct conducts a thorough site investigation including trial pits and soil analysis before recommending the best approach.
Will a basement conversion cause disruption to my neighbours?
Basement excavations involve significant groundworks and can cause noise, vibration and traffic from spoil removal. Alstruct works closely with neighbours, provides party wall notices where required, conducts condition surveys on adjacent properties, and implements a Construction Management Plan to minimise disruption.