Basement Conversions
London & Hertfordshire
Unlock the hidden potential beneath your property. From converting an existing cellar to a full basement excavation, TCM Building delivers structurally sound, beautifully finished basement spaces across London and Hertfordshire.
Create Extraordinary Space
Below Ground
A basement conversion is the process of transforming an existing cellar or underground space — or creating a new one through excavation — into a fully habitable, finished room. In London and Hertfordshire, where above-ground space is limited and property prices are high, basement conversions represent one of the most dramatic and high-value ways to expand a home.
Basement conversions are technically the most complex type of domestic building project. They involve structural engineering, specialist waterproofing, and careful management of groundwater and drainage. Choosing an experienced, specialist contractor is critical — a poorly executed basement conversion can result in serious structural damage and persistent damp problems.
TCM Building has an experienced structural team and works with specialist waterproofing designers on every basement project. We manage the entire process — from the initial feasibility survey and structural engineering through to Building Regulations approval, construction, and final decoration.
Which Type Is Right for Your Property?
Existing Cellar Conversion
If your property already has a cellar or basement, converting it into a habitable room is the most straightforward and cost-effective basement project. The structure already exists — the work involves lowering the floor if necessary to achieve adequate headroom, waterproofing the walls and floor, installing services, and finishing to a habitable standard.
Popular Uses for Converted Basements
Additional Living Room
A large, open-plan basement living room or kitchen-diner — creating a dramatic new heart for the home.
Home Office / Studio
A quiet, naturally insulated workspace away from the rest of the house — ideal for focused work or creative projects.
Cinema / Media Room
A dedicated home cinema with acoustic insulation, projector, and surround sound — the ultimate entertainment space.
Wine Cellar / Leisure
A temperature-controlled wine cellar, gym, or games room — making use of the naturally cool basement environment.
Waterproofing: The Most Important
Element of Any Basement Project
Waterproofing is not optional — it is the foundation of every successful basement conversion. Here's what you need to know.
Type A — Barrier Protection
Tanking slurry or bituminous membranes applied directly to the structure. Suitable for low water table conditions. Relies on the structural integrity of the substrate.
Type B — Structurally Integral
Waterproof concrete or reinforced concrete structure. The concrete itself is the waterproofing. Requires specialist mix design and careful detailing at joints.
Type C — Cavity Drain
A dimple membrane is fixed to the walls and floor, creating a cavity that channels water to a sump pump. The most reliable system for existing basements with variable water pressure.
TCM works with specialist waterproofing designers on every basement project. All waterproofing is designed in accordance with BS 8102:2022 (Code of Practice for Protection of Below Ground Structures Against Water from the Ground) and backed by a 25-year manufacturer's guarantee.
From Feasibility to Handover
Initial Survey & Feasibility
We carry out a detailed survey of the existing structure, assess soil conditions, and advise on feasibility, planning requirements, and indicative costs.
Structural Engineering
A qualified structural engineer designs the underpinning sequence, reinforced concrete specification, and any temporary works required.
Planning & Building Regs
We advise on planning requirements and submit all necessary applications. Building Regulations approval is required for all basement conversions.
Waterproofing Design
A specialist waterproofing designer specifies the appropriate waterproofing system — typically a cavity drain membrane system or structural waterproofing.
Construction
Our team carries out all structural works, waterproofing, services installation, plastering, and finishing under one contract.
Finishing & Handover
We complete all second-fix works, decoration, and snagging. Building Regulations sign-off and waterproofing guarantees are provided at handover.
Basement Conversion Cost Guide
Basement conversion costs vary more than any other type of building project, due to the variability of soil conditions, water table, and existing structure. These are indicative guides for London and Hertfordshire.
| Project Type | Typical Cost | Added Value | Build Time | Planning |
|---|---|---|---|---|
| Existing Cellar Conversion | £20,000 – £60,000 | 15–20% | 8–14 weeks | Usually not required |
| Basement Lowering | £30,000 – £100,000 | 15–25% | 12–20 weeks | Usually not required |
| Full Basement Excavation | £100,000 – £300,000+ | 20–30% | 20–40 weeks | Required |
| Side Extension + Basement | £150,000 – £400,000+ | 25–35% | 24–48 weeks | Required |
All prices exclude VAT and are indicative only. Basement costs are highly site-specific. A fixed-price quotation is provided after the feasibility survey and structural engineer's assessment.
Experienced Basement
Conversion Specialists
Basement conversions are the most technically demanding domestic building projects. Choosing the right contractor is not just about price — it's about structural expertise, waterproofing knowledge, and the experience to manage complex projects without costly mistakes.
We work with qualified structural engineers on every basement project. All underpinning and excavation work is designed and supervised by a chartered engineer.
We work with specialist waterproofing designers on every project. All waterproofing is designed to BS 8102:2022 and backed by a 25-year guarantee.
All structural, waterproofing, services, and finishing works are managed under one contract. One team, one invoice, one point of accountability.
Over 50 basement projects completed across London and Hertfordshire, with a 5-star Checkatrade rating from verified clients.
"TCM converted our existing cellar into a stunning open-plan kitchen and living room. The structural work was immaculate, the waterproofing is flawless, and the finish is exceptional. Two years on and not a drop of damp. Absolutely outstanding work."
Book a Free Feasibility Survey
Our structural specialist will visit your property, assess feasibility, and advise on the best approach — completely free of charge.
Basement Conversion FAQs
Ready to Unlock Your
Basement's Potential?
Book a free feasibility survey today. Our structural specialist will assess your property and advise on the best approach.