What ninety-one per cent means

Porto Atelier retained 91% of its existing structure by mass. That number appears in our project sheet without explanation, so this piece is the explanation.

The 1927 frame — columns, beams, slabs, two chimneys and the north sawtooth glazing — weighs approximately 6,400 tonnes. Reproducing it in new reinforced concrete to current standards would have carried an embodied carbon cost of roughly 4,180 tCO₂e, before accounting for demolition, haulage and landfill of the original.

For scale: the projected operational emissions of the completed building, across all forty-one apartments, run to about 134 tCO₂e per year. The demolition-and-rebuild option would therefore have spent thirty-one years of the building's entire operational footprint before a single resident moved in.

This is the calculation that most viability assessments never run, because embodied carbon does not appear in the appraisal and operational carbon does. A new building with a better energy rating scores well on the metric that is measured and disastrously on the one that is not.

The counter-argument is real and we should state it fairly: retained structures constrain the plan, complicate the services, and typically cost eight to fifteen per cent more to deliver. At Porto Atelier the premium was eleven per cent. The frame also forced apartment depths we would not have chosen freely.

We accepted those constraints because the alternative was to spend three decades of carbon on the convenience of a clean site. Stated that way, it was not a difficult decision. Stated the way the industry usually states it — as a viability question — it would have gone the other way.

The full model, including assumptions and the RICS methodology applied, is available on request.