What makes a green, sustainable building? Does it depend only on the carbon footprint of construction, and then how much energy the building uses once it’s inhabited? Is it better for construction materials to be 100-per cent natural, or are recycled plastics OK too? Is average environmental impact the best gauge for “greenness,” or should buildings be judged by worst-case scenarios?

These aren’t easy questions to answer, though organisations like the US Green Building Council, creator of the LEED green building certification, have put a lot of time and effort into defining what is or isn’t sustainable. Still, it can seem all but impossible to come up with a standard that satisfies everyone, especially since any decision can affect — for better or for worse — large segments of one industry or another.

For example, hempcrete, a building material of hemp and lime made by companies like Britain’s Lime Technology and France’s La Chanvriere de L’Aube, is carbon-negative … meaning it absorbs more carbon dioxide over its lifetime than it give off. But it’s a hard sell in places like the US, where the Drug Enforcement Administration prohibits any hemp cultivation, THC or no.

And then there’s polyvinyl chloride, or PVC. It’s sturdy, lightweight and inexpensive — three big advantages when faced with the challenge of building sustainable houses that are also affordable. In fact, a demonstration project — the Ozzano Project –  in Italy aims to show how PVC-based construction can be used to build residences that are “nearly zero energy.” Despite ongoing improvements in the manufacturing process over the years, however, PVC earns a zero-tolerance rating by NGOs like the Healthy Building Network. Among their objections: PVC in construction can release toxic compounds like dioxin during a building fire.

Ultimately, there’s likely no one “best” method of green, sustainable construction. A lot will always depend on local conditions, legal or natural, as well as on economics. Considering the recently burst housing bubbles in the US and parts of Europe, though, the truth remains — as architect Carl Elefante has said — “the greenest building is … one that is already built.”

Moving forward, that will be the greatest challenge for municipal planners and smart-city planners: making sustainable the housing and infrastructure we already have, not building whole new communities from scratch.

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