Polysmiths completes flood-resilient basement home in London

Limewash has been used throughout the scheme to allow the walls to remain breathable, while all external walls have been treated with a damp-resistant slurry and lined with insulation. Channel drains have been laid around the extension, connected to an oversized basement water pump which pumps water to the main drainage system during a storm surge.

The project aimed to resolve challenges including extending, insulating, flood-proofing and bringing light into the basement flat, while retaining as much of the garden area as possible.

The existing Tufnell Park property was a one-bedroom flat that had been left vacant, due to flash flooding damage to its floor and walls.

We had to move out of our home when it was damaged by a flood and were extremely concerned when water was sipping through the walls and floor boards after a particularly heavy rain. With the renovation, we see the potential to extend our home to accommodate a small family, as well as making it brighter and more flood-resilient. Charles and his team worked out a design that allows in daylight from above and views of the garden from every angle of the flat. It is now a dry, quiet and worry-free home for our family.
Christoph, owner


Project data

With a narrow and deep plan typical of a London terraced house and long, south-facing garden, the practice has designed a scheme with stepped double and triple-glazed windows to an extended elevation that follows the contour of the garden, allowing planting to come up against the windows.

Percentage of floor area with daylight factor >2% Not supplied
Percentage of floor area with daylight factor >5% 60%
On-site energy generation Nil
Annual mains water consumption 80 m3/occupant
Airtightness at 50Pa 4.52 m3/h.m2
Heating and hot water load Not supplied
Overall area-weighted U-value 0.16 W/m2K
Design life 50 years
Embodied/whole-life carbon Not supplied
Annual CO2 emissions Not supplied


The extension’s roof structure ‘lifts up’ to accommodate three angled rooflights. A Siberian larch louvre sits on the rear elevation, hiding the half-sunken flat from the garden side.

According to the Mayor of London’s office, at least 200,000 properties in London are at risk of surface water flooding, mostly basement flats of this kind.

Architect’s view

Garden topsoil surface area has been maximised to allow rainwater to soak away, reducing pressure on the drainage system during heavy rainfall.

The scheme takes a cabin-like form and has been built using lightweight and natural materials for a journalist gardener.


Client’s view

While we are thinking about bolstering energy efficiency of our housing stock, it is equally important to strengthen our homes against the effects of climate change. Heavier rainfall and flash flooding is increasingly becoming an issue, affecting more than 200,000 properties in London alone. Our project considers the garden as a mechanical and visual extension of the home: it is a natural rainwater soakaway that acts as part of the flood defense for the home; it also turns a basement flat into a cabin enveloped by a lush, calming landscape – Walden. We believe this could become a new ground-level extension typology.
Charles Wu, director, Polysmiths

Start on site October 2022
Completion July 2023
Gross internal floor area 75m²
Gross (internal + external) floor area 160m²
Form of contract JCT MW 2016
Construction cost Undisclosed
Architect Polysmiths
Client Private
Structural engineer Price & Myers
Landscape consultant Polysmiths
Approved building inspector All Building Control
Main contractor Omniscient London
CAD software used AutoCAD, Rhino

Environmental performance data