Basement Conversions in South Woodford & East London — cellar tanking, underpinning and full digs

Basement work is the most technically demanding category we run. Done properly, it adds a fully habitable storey — typically 30–50 m² of new floor area — without changing the visible footprint of your house. Done wrong, it floods, undermines the existing structure, or fails Building Regs and stays unusable.

We deliver three distinct types of basement project. Cellar conversions: where there's already a usable below-ground volume that needs damp-proofing, lighting and finishing. Underpinning: lowering an existing cellar floor to achieve full habitable head height (minimum 2.4m), which involves sequenced excavation under the existing walls and casting new structural foundations. Full basement digs: excavating an entirely new below-ground room under a house with no existing cellar — the most complex and expensive option, almost always requiring full planning and structural engineer sign-off at every stage.

Most basement clients are owner-occupiers in larger Edwardian and inter-war stock in Hackney, Wanstead, Loughton and Buckhurst Hill, where plot footprints are already maximised above ground. Typical end uses: home cinema, gym, second living room, guest suite with ensuite, or self-contained annexe. We always work to BS 8102 Grade 3 (suitable for habitable use) using Type A (barrier waterproofing), Type B (integral structural waterproofing) or Type C (cavity drain membrane with sump and pump) — usually a combination depending on water table and structural detail.

BS 8102 Grade 3 waterproofing

Combined Type A and Type C systems sized for actual water table conditions — the only spec considered safe for habitable basement use.

Structural engineer-led every stage

Underpinning calcs, sequencing and bay sign-offs by a chartered structural engineer — not a builder making it up as they go.

Sump and pump with battery backup

Twin-pump systems with battery-backed alarms, so a power cut during heavy rain doesn't become a flood.

Insurance-backed warranty available

On qualifying waterproofing systems — gives you something to claim against if a leak ever develops.

Underpinning is the controlled, sequenced excavation of small sections (typically 1–1.2m wide bays) under existing load-bearing walls, casting new structural concrete foundations beneath each bay before moving to the next. The sequence — alternating bays, leaving piers undisturbed — is critical: dig adjacent bays simultaneously and the wall above can drop, cracking the structure. A structural engineer designs the sequence and signs off each bay before the next opens.

Type A waterproofing is a barrier system: cementitious slurry coatings or bitumen membranes applied to the inside or outside of the structural wall. Cheap and simple, but the only thing keeping water out is the barrier itself — any breach means a leak.

Type B is integral structural waterproofing: the concrete itself is mixed and detailed to be water-resistant (using waterstops at construction joints, hydrophilic strips, and admixtures). Used most often in new-build basements and full digs, less in retrofit.

Type C is cavity drain membrane: a dimpled HDPE membrane is fixed to the inside of the wall, creating a small cavity behind a finished wall surface. Any water penetrating the structural wall runs down the cavity to a perimeter drainage channel and into a sump, where pumps discharge it. The wall finish stays dry even if the structural wall is wet.

BS 8102:2022 Grade 3 — required for habitable basement space — requires the waterproofing system to keep the internal environment dry and humidity-controlled. In practice, almost every East London retrofit basement combines Type A (cementitious tanking on the structural wall) with Type C (cavity drain membrane behind the finishes), giving two independent layers of defence. Sumps must have twin pumps with battery-backed alarms.

Do I need planning permission for a basement?

How long does a basement project take?

What does BS 8102 Grade 3 actually mean?

Will my house be safe during underpinning?

What happens if there's a power cut and the pumps stop?

Will my home insurance cover the basement once finished?

Can you handle the structural engineering, or do I need to bring my own?