What is the proportion of the floor area?
The floor space index is a commonly used real estate metric. Also known by the acronym FAR, the floor area ratio is the ratio of the available square footage of a building to the total area of the lot on which the building stands. Sometimes referred to as the Floor Space Index (FSI), the Floor Area Index helps commercial property developers and owners determine whether a property is economically viable. Cities and towns often use this ratio in their zoning laws as well. In many cases, the FSI or Floor Area Ratio is used to control congestion or help enforce other municipal ordinances.
Calculating the ratio of the floor area of a building is a simple exercise. Take the total space in a structure and divide it by the square footage of the building lot. For example, a 1,000 square feet (approximately 93 square meters) building located on a 2,000 square feet (approximately 186 square meters) lot would have an area ratio of approximately 0.5. Likewise, a five-story building with each floor measuring 200 square feet (approximately 19 square meters) would have 1,000 square feet (approximately 93 square meters) of floor space. If this building is on a plot of land of 2000 square feet (approximately 186 square meters), the area ratio would also be 0.5.
The floor area ratio is often used in zoning laws. Setting an area ratio for a lot corrects the total floor area of a building, regardless of the building's design. Total square footage gives a city or town an indication of the municipal services that will be needed. For example, a city or town that knows the total square footage of an office building can estimate how much water will be needed to support the building's occupants.
Some cities or towns use the usable area ratio to enforce quality of life initiatives as well. For example, a new building in a congested area of the city may be zoned with a low area ratio. To achieve this, the designer would have to leave a large amount of open space on the lot. Likewise, a low area to building ratio in the country would ensure that there is adequate parking. This would help ensure that neighbors weren't bothered by cars parked on their streets.
Developers often use the usable area ratio to determine the revenue-generating capacity of a vacant lot. A larger area ratio means more space will be available to sell or rent to tenants. In this way, the area relationship can help establish whether a new construction project is economically viable.
In addition, developers take the proportion of floor area into account in their design plans. For example, suppose a 1000 square foot lot (approximately 93 square meters) is zoned with an area ratio of 0.5. The developer has several options. You can build a one-story structure of 500 square feet (approximately 46 square meters). An alternative would be a two-story building with each floor measuring 250 square feet (about 23 square meters). You can also propose a four-story building with each floor measuring 125 square feet (about 12 square meters). All these buildings would have an area ratio of 0.5. This gives the developer great flexibility in designing a building. You can add floors to your project by reducing the square footage of the floors.