Stacked slate renovation makes the most of house and garden

Who is the project for and what was the brief?

The project was for a client who grew up in the house – a grade II-listed terraced Georgian townhouse in south London. It was in a state of disrepair – and with the roof falling in, it was in urgent need of renovation.

The project included a rear extension, internal remodelling, and new landscaping. Along with restoration and modernisation, the design ambition was for a rear extension to open up the existing cellular plan. We wanted to create a large, unexpected space that better connects the house with the garden.

Describe the context of the extension

The plot was 5m wide, and the garden that comes off the rear of the house at an angle was long and thin, backing onto a small park. It was also dark and damp, north facing and populated by mature trees.

The site doesn’t receive direct sunlight, so these conditions were embraced in the design. The addition comprises two intersecting volumes – the garden room and gallery. The first is a double height space which connects vertically with the stairwell. The other, nestled beneath it and to the side, is a lower-ceilinged mass, the roof of which arrives just below the windows on the floor above.

A clerestory draws light down into the building from the east and west, and lets you look up and out at the tree canopy above. The gallery mediates between this space and the very low ceilings of the existing lower ground floor and takes you into the garden. It is contrastingly darker and feels more intimate.

What was the planning situation?

We tried to engage playfully with the constraints of a listed building in a Conservation Area. Rather than just conforming to the typical expectations with a subservient addition, we wanted to create a design that challenged and enhanced the historic structure.

The conservation officer encouraged an extension that would respond to the existing building’s sheer rear facade; the unusual arrangement of two intersecting volumes does this by providing a large open space, without excavating below the lower ground floor, allowing the extension to sit on the original garden wall footings.

Explain the external treatment of the project

Materials were chosen that would respond and adapt to the environment they were used in, which as we’ve said was a dark garden with established camellias, Trees of Heaven and Judas Trees. As time passes the copper, slate and lime render used on the exterior have begun to patinate, crumble and stain, blending in with this environment.

The extension facade is made of Ballachulish roofing slates that were reclaimed from another project. The slates turned out to be too damaged to be hung so they were stacked up against the retaining walls. Each slate really varied in size as it was a graduated length slate roof, so we stacked the slates flat – smaller at the top and bigger at the eaves. Stacking them in size order created gently sloping walls that look like buttresses. While each slate is rough and imperfect, the deep facade they form gives the impression of a unified texture, which relates back to the brickwork of the original house.

How have the interiors been designed?

Slabs of travertine, clay plaster, lime washed timber and stripped-back brickwork create a bright reflective space within the extension. The original kitchen has been kept in its existing location in the main house, which frees up the extension for more flexible uses. The custom joinery we designed can be unclipped and moved around, and sliding columns divide up the space.

Provision for plants was a key consideration internally. Rainwater travels through an exposed pipe and an indoor gulley that feeds the plants within the extension. A tank within the wall cavity stores excess rainwater from here to maintain supply throughout the year.

Changes to the existing building were limited to avoid energy-intensive structural changes. We focused on refurbishing the historic fabric and were selective in restoring certain elements to retain parts of its more recent history.

Describe one challenge and how you overcame it

Recycling and repurposing defunct building materials posed design and logistical difficulties, but working through these challenges created solutions that we might not otherwise have thought of. For example, those cladding slates were still on a roof in Scotland when the project started. We did not know what condition they would be in, and in the end, they had to be stacked as they couldn’t be hung, which led to a unexpected outcome.

What is your favourite moment in the project?

We love the view from the garden, looking back at the house through tree trunks and ferns, seeing the extension embedded in its context. The new footprint was designed to protect the established trees which now frame the extension, giving the impression that it has been hidden there for some time.

Grace Fletcher was talking to Michèle Woodger

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Key data
Total contract cost £350,000
Existing building: 115m²
Extension: 30m²
GIFA: £2333 per m²