When a development application is submitted to the planning department, it contains pertinent information regarding the proposed structure. The height, total gross floor area, and lot coverage—the percentage of the property to be occupied by buildings—are attributes that are typically specified in the application's data sheet. Figuring out the proposed density of the development is a little trickier, but can be determined by calculating the floor space index (FSI).

Also known as the floor area ratio (FAR), FSI is the relationship between the total amount of usable floor area and the total area of the property on which it stands. It is determined by dividing the gross floor area of the building by the gross area of the lot. For example, a 10,000-square-foot building on a 10,000-square-foot lot will have an FSI of 1.0, whereas a 50,000-square-foot building on a 10,000-square-foot will have an FSI of 5.0. 

Each of these building blocks has the same floor area ratio, image via City of Toronto

A one-storey building occupying the entire property, from lot line to lot line, would have an FSI of 1.0 — the same as a two-storey building occupying 50 percent of the property and a four-storey building occupying 25 percent of the property. This scenario is outlined in the image above. 

Local governments use the FSI of a building as a density measure: a higher ratio indicates a denser development. Where land costs are high, developers need to build at a higher FSI to cover those costs. Downtown areas, which are typically highly sought after locations for both office and residential buildings, typically foster much higher FSI construction.

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