Basement Conversion Case Study

This basement conversion case study follows the £2.8 million transformation of a period house at Strand on the Green, Chiswick.

Over a 52-week programme, Proficiency Design & Build's team is delivering a basement enlargement, rear extension, new second floor and whole-house refurbishment, involving complex structural engineering, underpinning and groundwater management.

At a Glance: Project Overview

The Client's Vision

The owners wanted to create a more spacious, modern home through a premium whole-house transformation. The project involves alterations across every level, including an enlarged basement, a new second floor and a rear ground-floor extension.

The new lower-ground accommodation is being integrated into the main house rather than developed as a separate basement flat. The aim is to create additional living space that feels like a natural extension of the home and works for everyday use, while adding to the home's perceived value.

With construction taking place across multiple levels, careful structural coordination and sequencing are essential to ensure the basement works integrate with the wider renovation.

Challenges We Faced

Here are a few site assessment, planning and project challenges we faced:

• Planning Permission: The house sits in the Strand on the Green conservation area, so we needed planning permission for a basement enlargement as well as approval for the extensions.
• Site Access: The site is tight, so we arranged parking suspensions and traffic management for the crane lift and deliveries. We also used specialist machinery and manual handling where space ran out.
• Party Wall Agreement: Surveyors Callum Escott and Michael Clark led the party-wall management with the adjoining owners. Together with Croft Structural Engineers, we're finalising the temporary works drawings and monitoring schedules, which gives the scheme its technical credibility.
• Neighbouring Properties: The house shares structures with the homes next door. We're monitoring properties on Hearne Road throughout the build and had special foundation details approved at the neighbour interface.
• Ground Conditions: Surveys found loose to medium-dense sand and gravel, with groundwater confirmed. Disturbed, this ground can turn into running sand, so we supported every stage of the dig.
• Sequencing Challenges: Temporary propping, needling, segmental drives and underpinning follow a fixed order, because each step holds the house up for the next. Managing that sequencing complexity is key to the project's deliverability.

How We're Building It

Croft Structural Engineers designed the structure around extensive temporary works and underpinning, which keep the house and its neighbours standing while the ground comes out.

Basement construction under a house you're keeping takes far more engineering than a kitchen conversion or loft. That's why structural design is at the centre of our complete design-and-build basement service.

The build runs in five stages:

1. Enabling works
2. Demolition
3. Excavation
4. Underpinning and structural steel installation
5. Retaining wall construction

On site, that means:

• Underpinning: We're taking the perimeter wall foundations down below the new basement floor.
• Temporary works: Lateral bracing, needling, cross-props, and temporary ring frames hold up the front walls and party walls as the ground comes out. Basement structures in loose sand can't stand unsupported until the permanent structure takes over.
• Spoil removal: Skips and conveyors carry excavated soil away from the dig.
• Crane lift: A contract crane lifts the large Sky-Frame glazing units over the restricted access.

Underpinning and controlling the water table have been the toughest stages. When water came in at changing levels, our basement contractors brought it under control with local dewatering from sumps and kept basement-level delivery on track.

Waterproofing, Environmental Control & Building Services

Borehole logs confirmed wet ground, and the tidal Thames means groundwater levels rise and fall. That's why we specified Grade 3 waterproofing, which is the strictest level in BS 8102, so the basement stays completely dry.

The house's core systems are being relocated and reconnected, and every system goes through testing and certification for Building Control and Party Wall sign-off.

The Design Features that Make It Work

These design choices make the new space feel like home:

• Sky-Frame glazing: Large, frameless glazing units fill the living space with natural lighting and keep it bright and connected to the outside.
• An integrated lower ground floor: The basement links straight into the house like any other floor, so it feels naturally usable every day.
• Comfort below ground: Ventilation is built into the services design, so the lower ground floor feels as fresh as the rest of the house.
• More room on every level: The new second floor and rear extension add liveability above ground to match the new rooms below.

The project was still on site at the July 2026 update, so the completed rooms aren't ready to show yet. Most basement conversion case studies lead with finished interiors, but this one shows the engineering behind them.