London Townhouse published in RIBAJ

Corten and glass extension gives sculptural presence to Edwardian terrace.

South London House by Cagni Williams

Can you describe the project?

This project reimagines a large Edwardian terraced house, transforming it into a modern, energy-efficient family home while celebrating both its historic character and connection to nature. 

The traditional entrance and front living rooms lead through to a renovated kitchen and dining area, the vibrant heart of the home, where an expressed steel frame, timber joisted ceilings, and natural light create a sense of openness and flow. 

To the rear, a striking Corten steel and glass extension has replaced an outdated garden conservatory. The gallery’s bold yet refined design uses exposed Corten columns and a plate roof structure to create a sculptural presence, while seamlessly reconnecting the interior with the garden.

Who is South London House for and how did the appointment come about?

The house belongs to a couple who are friends of practice co-founder Edward Williams and I. During a socially distanced garden lunch in May 2020, our friends started talking about their plan to replace the old conservatory and said they had been working with an architect. So Edward and I, tipsy at this point, and free from any professional commitment, started free-flowing ideas. The next morning, we got a call saying that they had fired their architect and wanted us to work with them to create an extension that no one else had!

What emerged from that spontaneous garden conversation was a striking Corten steel and glass gallery that dissolves the boundary between interior and exterior, featuring a central fireplace column and wildflower green roof – proof that sometimes the best architectural briefs begin with good wine and great friends.

The extension’s Corten steel is striking. Why did you choose it and how did material choices influence the design? 

We selected Corten steel for its rich, warm patina, which complements the traditional brickwork while creating a bold contemporary contrast. Its self-weathering properties eliminate maintenance concerns, and the material's tonal depth visually connects the modern extension to the garden landscape while clearly expressing the building's new architectural layer.

The welded roof is a steel tray construction, so there was no need for waterproofing (although the client requested a liquid coating as double safety). 

The gallery is crowned with a wildflower green roof, offering changing views from the first-floor garden room and its new terrace, as well as from the garden below. The client was keen to have wildflowers on the green roof, so the designed system had to be substantial with 150mm of soil over insulation.

How did you design the interiors? 

The kitchen and dining room feature bespoke joinery made of oak veneered panels, designed to lead you towards the gallery and the garden beyond. This creates a carefully choreographed sequence of spaces that draws visitors through the home. Each panel conceals something different, such as a fridge, pantry or technical room, to keep clutter away and the space beautiful.

How have you made the home more energy efficient?

Sustainability underpins the renovation. The home is heated and cooled by a new air-source heat pump in the garden, with underfloor heating across the ground floor. Integrated ventilation ensures comfort year-round, mitigating overheating and condensation within the gallery extension. A year after occupation, the client confirmed that their energy bills were a fraction of what they werey before the works.

When and why did you set up Cagni Williams Energy?  

We founded Cagni Williams Energy in January 2025 in response to a simple but stubborn reality: architecture on its own can’t deliver truly net-zero buildings.

As project teams confronted the growing technical demands of decarbonisation – from ground-source heat pumps and ambient loop networks to complex renewable-energy integrations – it became clear that these systems were too often treated as add-ons, introduced late or managed by engineers operating at a distance from the design vision.

We created the company to close that gap. By embedding specialist energy expertise directly within the architectural process, Cagni Williams Energy ensures that low-carbon infrastructure is conceived from the outset rather than retrofitted at the end. The result is a more coherent design journey and a clearer path to meaningful emissions reduction.

What was the project’s main challenge?   

The unique nature of the gallery structure and material presented some unexpected challenges. In Europe, Corten is only sold in sheets, so we had to change the columns from circular to welded cross shapes. The double-glazed curved glass corners with tight radii are only made in one factory in Germany, so this added lead time to the programme. The pivot patio doors were bespoke, made from Corten frames to fit into the structure, together with several other designed Corten elements, and the whole gallery had to be given a final oil coat to blend them all together.

What is your favourite aspect of the project? 

I love the gallery; it is a magical space, especially at dusk. The client has said that when it rains heavily, they feel outside yet protected in a wonderful warm environment. That made me feel we had done something right and kind of new there. 

I also love the wildflower green roof, which can be enjoyed from the refurbished first-floor garden room leading onto a terrace. It is an extension of green at different levels, which is very coherent with our original vision for the scheme.

What are your reflections on the project?

Edwardian buildings provided residents with a perfect building at the time, but modern needs have changed. The kitchen has is not longer just a room for food prep but has become the centre of the home – a place to gather, to work and to relax.

Added to that, people now want more light, and they can have this due to better insulation. The house’s old conservatory was always either too hot or too cold, so replacing it with our Corten gallery has been perfect for addressing this.

Key data:

Total contract cost Confidential
Area of extension 20m2
GIA 226m2
GEA 185m2

Credits:

Architect Cagni Williams Associates/Cagni Williams Energy
Structural/civil engineer Eckersley O’Callaghan
M&E mstep
Environmental Hammerton Energy Assessors
Planning Logic Planning
Construction Dominik P&D
Joinery WG Studio
Photography Fred Howarth
Landscape Topia Landscapes

Suppliers:

Door handles, pivot glass doors Fabco

Author: Laura Carrara-Cagni