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What is the FSI of Paris?

In the world of development, there is something known as a floor space index (FSI). Some places call it a floor area ratio (FAR), but they mean the same thing. It is one measure of density. To calculate it, you simply divide the total building area by the site area. For example, if you had a 25,000 square foot piece of land and you were to build a single storey building that occupied every bit of the site (also 25,000 sf), you would have an FSI or FAR of 1.0. If you built a two storey building on only half of the site, you would similarly have an FSI or FAR of 1.0. The area didn’t change, you just moved things around. That’s how FSI’s work.

A ratio like this tells you how intensely you may be using a piece of land, but it doesn’t tell you everything or necessarily give you the full picture. Which is why I find it silly when too much emphasis is placed on this singular number. I don’t think anyone in the history of the world has ever traveled to a city — let’s take Paris — and remarked how beautiful it is because of its floor space indices. Nobody thinks like this. It’s way too esoteric. What guides our experiences is built form, the ground plane, relationships to streets, materiality, light, context, and many other important things.

To give a specific example, let’s take One Delisle. This project was in effect approved twice. After it was approved by City Council in July 2020 an adjacent land parcel was acquired. It wasn’t absolutely necessary to do this, but we felt it made for better city building and so we did it. (We wanted to look back knowing we did the right thing.) That meant that we needed to go back to Council to revise our approvals, which ended up happening at the beginning of this year (public staff report, here). There was no change to the tower and and no change to any of the key setbacks or stepbacks. But the overall FSI did go down!

Will anyone notice or care about this lower ratio? I doubt it. Which is why I think it’s silly to try and plan our cities around them. It feels like design by spreadsheet. Thankfully, I think many people recognize this.

Photo by Thibault Penin on Unsplash

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