Basement & Lower Ground
Construction, N2

The Brief

Building a Lower Ground Level Worth Living In

The Bishops Avenue is one of the few streets in London where the plot rather than the postcode sets the ambition. This private commission created a full basement and lower-ground level beneath a reinforced concrete frame — habitable space with natural light, not a plant room with a ceiling.

Space below ground only works if three things are resolved together: keeping water out permanently, getting daylight in, and burying the services before anything is finished. Each is straightforward alone. Delivered on the same programme, in the same volume, they compete for the same millimetres.

Crown Building was engaged for the substructure and lower-ground construction, working alongside the client's design team. The photographs on this page were taken during construction and are published with the client's consent; further detail remains confidential.

Waterproofing

Type C: Managing Water Rather Than Resisting It

The lower ground is protected with a cavity drain system — dimpled membrane to walls, floor and soffit, taped at every joint, discharging to a perimeter channel and sump. Type C manages groundwater rather than trying to hold it back, which is why it remains serviceable: a drainage channel can be maintained, whereas a failed tanking coating buried behind finishes cannot.

The critical decision is the fixing strategy. Every plug, batten and service penetration through a tanked wall is a potential breach of both the system and the warranty behind it, so the layout is agreed with the system designer before first fix rather than improvised on site. A defect found after screed, plaster and joinery are in place is not a repair — it is a strip-out.

Crown Building is DELTA Membrane and Mapei registered. Our approach to subterranean work is set out on the basement conversion service page.

Structure

A Concrete Stair Cast in Place

A curved concrete staircase rises through the lower ground, cast in situ rather than assembled from precast flights. Formwork for a helical stair is a joinery exercise before it is a concrete one: the soffit curves in two directions at once, and every riser is set out from the finished floor level rather than the slab.

Cast in place, the stair is monolithic with the structure — no bearing joints, no applied treads, and a soffit that reads as a single continuous surface once struck. It also has to be propped for far longer than clients expect, which is why it is programmed early: the space around it cannot be finished until the props come out.

The frame itself is exposed concrete — columns, downstand beams and slab soffits left as struck, with the quality of the shuttering permanently on display. Full-height glazing to a board-marked retaining lightwell brings daylight into a level that sits below the garden.

Services

Everything Buried Before the Screed

Above the waterproofing sits insulation, then the services that make a basement habitable: galvanised ducting to floor plenums, trench heaters set along the glazing line, and distribution for lighting, power and controls. All of it is set to level and signed off before the screed goes down.

Trench heating along a fully glazed elevation is the correct answer to a technical problem — a glazed wall below ground loses heat and attracts condensation, and a heater at the floor line addresses both without an object in the room. It also has to be positioned to the millimetre, because the finished floor and the glazing threshold are both fixed points.

This is the stage where a basement is won or lost. Once the screed is poured, every route is permanent. See our new build service for how we sequence substructure and services from the outset.

On Site · The Bishops Avenue, N2