In the built environment, sustainability is often treated as a problem of performance. We measure, compare, and optimise, relying on indicators to assess energy efficiency, water management, carbon footprint, or biodiversity.
These tools are necessary. But they also tend to focus attention on a moment that comes too late in the life of a project: when the system is already largely defined, and its trajectory is difficult to change.
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What ultimately determines the sustainability of a place, however, is not only its measured performance but its ability to remain coherent over time. In other words, its capacity to function, adapt, and retain meaning without requiring a constant succession of corrective interventions once it is in use.
This distinction matters. A system can be highly performant and yet fragile. Conversely, a coherent system can absorb variations, unforeseen uses, or climatic changes without losing its overall stability. Sustainability, therefore, is not merely a matter of optimisation levels, but of a deeper mode of structuring.
In many projects, technical systems, landscape, water, and uses are conceived separately and assembled later. Living systems are often added as an extra layer: green roofs, planted areas, and compensatory ecological devices. This additive logic creates an illusion of control, but it frequently produces environments that depend on continuous maintenance, where every malfunction calls for a new intervention.
By contrast, designing water, vegetation, and spatial organisation as a single integrated system fundamentally alters the trajectory of a project. Living systems are no longer decorative or compensatory elements; they become structuring constraints. They impose rhythms, temporalities, and limits, but they also introduce forms of self-regulation that purely technical systems struggle to provide.
This shift in perspective moves the core question of sustainability. It is no longer simply about whether a building or district reaches a given performance threshold, but about identifying the moment when a project becomes irreversible. There is always a point—often difficult to see—at which choices permanently close off certain options: the orientation of the built form, the organisation of flows, the relationship to the ground, or dependence on specific technical systems.
This point can be described as a knot. The knot is not the moment of formal approval, nor the delivery of the project. It occurs much earlier, when the overall structure is committed, and real margins for change disappear. Beyond this threshold, any improvement tends to be corrective rather than truly design-led.
When sustainability is addressed solely through performance indicators, this knot remains invisible. Efforts then focus on optimising systems that are already locked in. This helps explain why many projects labelled as sustainable paradoxically require constant monitoring, costly adjustments, and repeated interventions to maintain their operation.
Recognising the existence of this knot fundamentally changes the question of responsibility. The issue is no longer only how to improve a project, but when and where the decisions are made that determine its capacity to endure. This approach does not reject measurement tools; it places them in their proper role—as instruments of verification, not as the foundation of design.
For both industry leaders and public authorities, this requires a clear shift in perspective. Sustainability can no longer be treated as a quality added after the fact, nor as a goal achieved through the accumulation of solutions. It becomes an emergent property of a system whose coherence has been engaged from the very earliest stages.
Before seeking to improve the performance of a project, public decision-makers and industry actors must first ask at what point their choices cease to be reversible.
This knot—often invisible within design and decision-making processes—ultimately determines whether a system can remain coherent over time or whether it will later require permanent correction.
Recognising this moment is not an additional constraint; it is the condition for designing environments capable of lasting without being continuously repaired.
