The cheapest moment is the first
Sustainability is cheapest at concept stage, when a consultant can shape orientation, massing, envelope and strategy before anything is drawn in concrete. Every major green-building target is easier and cheaper to set as a design principle than to recover in a finished design.
At design stage
If design has started, a consultant's role shifts to coordination: feeding requirements into design packages, reviewing specifications and calculations, and building the documentation trail. The rating is still achievable, but the margin narrows and the effort rises.
At tender and construction
Late engagement is not pointless. A consultant can define the tender requirements so contractors price compliance in, then support construction-stage verification, material submittals and audits to protect the rating. It is harder and more costly, but it protects the certification you have already invested in.
The rule of thumb
Appoint early enough that sustainability directs the design rather than adapting to it. If you are unsure, bring the consultant in for a scoping review before design decisions lock. The review is cheap; the recovery rarely is.
Sources & references
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