When we developed our internal proposal for 24 Sussex in 2017, one of the central ambitions was to test whether the property could be reimagined through a net zero carbon lens. At the time, that was still an ambitious framing. Much of the sustainability conversation in architecture remained focused on energy efficiency, operating performance and mechanical systems. Those issues matter, but carbon changes the discussion. It asks us to consider not only how a building performs after construction, but also what is consumed, retained, emitted and avoided through the act of building itself.

That distinction is especially important for existing buildings. An existing structure already represents an investment of material, labour and carbon. Its walls, floors, structure, finishes and assemblies carry embodied value. When we retain and improve an existing building, we are not simply preserving memory or architectural character. We are also leveraging the carbon already spent in its construction. From a lifecycle perspective, that gives many existing buildings a head start. It does not mean every building can or should be retained in every circumstance, but it does mean that demolition and replacement should not be treated as environmentally neutral.

For the historic residence at 24 Sussex, the sustainability strategy began with retention. Keeping as much material as possible was the first and most important environmental move. From there, the proposal explored building envelope and air-tightness improvements, window rehabilitation and glazing replacement, improved internal systems and compatible passive strategies. It also considered how the building’s existing characteristics could be strengthened rather than erased. Overhangs, for example, could be retained and enhanced to better protect masonry. Thermal mass could be used as part of a broader environmental strategy. Rainwater collection, on-site renewables and careful servicing upgrades could be introduced in ways that respected the character of the house.

The new pavilion allowed a different set of sustainability ideas to be tested. Because it would be purpose-built, it could integrate systems and assemblies from the outset. The proposal explored geothermal heating and cooling, solar thermal hot water heating, passive ventilation focused on a central spine and stack effect, greywater recycling, a vegetated roof, sustainable materials and the use of vegetation and overhangs to help manage heating and cooling loads. We also looked at the potential for hydrokinetic energy in the Ottawa River, recognizing that the site’s relationship to the river is not only visual and symbolic, but potentially environmental as well.

Another important sustainability move, although not a significant consideration when we prepared the original proposal, is the potential dismantling and salvage of the existing indoor pool pavilion. If that later structure was to make way for a more useful new addition, its removal should be approached as a material recovery exercise rather than a simple demolition. That distinction has become even more important as the building industry has paid closer attention to embodied carbon, construction waste, circularity and stewardship. A site like 24 Sussex should model that shift. Its public importance makes it an opportunity to demonstrate that carbon management is not only achieved through visible technology or improved operating performance, but also through restraint, reuse, careful sequencing, material salvage and respect for what is already present.

Architecturally, the proposal aimed to create a clear relationship between the new pavilion and the historic residence without resorting to imitation. We looked closely at the evolution of the house, including features that had been removed as it was adapted to serve as the official residence. Elements such as the rounded bay window and the projecting central second-floor window became useful points of reference. They were not treated as details to reproduce, but as clues to the building’s form, rhythm and character. The goal was to let the new work acknowledge the existing residence through proportion, material relationship and architectural influence, while remaining recognizably contemporary.

Material selection played an important role in that relationship. Grey limestone and supporting materials of similar tone were selected to visually connect the lower pavilion to the existing house, with localized use of wood and copper to provide warmth and refinement. The intent was to create a building that felt of its place and time while still belonging to the larger composition of the property. Scale was equally important. Because the lower pavilion would sit near the existing residence, it needed to align with the two-to-three-storey scale of the house while still providing the necessary sense of generosity for its public and formal spaces.

The broader architectural challenge was to avoid overwhelming the site. 24 Sussex is not a blank property waiting for an object building. It is a layered place where house, landscape, river, road, ceremony and privacy all matter. The architecture therefore needed to be deferential without being timid, contemporary without being disconnected and sustainable without relying on sustainability as an aesthetic. In that sense, the proposal treated sustainability and architecture as inseparable. The decision to retain, the decision to add, the placement of new functions, the choice of materials and the environmental systems all formed part of the same argument: the future of 24 Sussex could be both lower carbon and more meaningful if it began by working intelligently with the place that already exists.

This article was cross-posted with the TRACE Journal on Linked In.