Residential Infill - Understanding Floor Area in Single Dwelling Zones

Guide
How to calculate the floor area ratio in single-dwelling zones.

Floor Area Ratio 

Floor area is the total area of all floors of a building. Also: 

  • Floor area is measured for each floor from the exterior faces of a building or structure.
  • Garages and other enclosed structures also count as floor area.
  • Because floor area is a way to measure or control size, areas that continue from one floor to another, such as stairwells, shafts, or chases, are included as floor area on each story of a building.
  • Lofts and portions of a house with vaulted ceilings are only counted as floor area for the area of the additional floor.
  • Bays and bay windows are counted toward floor area if they extend to a finished floor or have a step acting as an additional floor.
  • Portions of buildings that are below ground or with lower ceilings may or may not count as floor area.

How to calculate Floor Area Ratio

Floor area ratio is a relationship between the size of a lot and the size of allowed development on the lot. For example:

This image is a chart that illustrates how to calculate floor area ratio, which is a relationship between the size of a lot and the size of allowed development on the lot.
This image is another illustration of how to calculate allowable floor area ratio.

Example of Floor Area Ratio 

Diagram showing an example of what is counted in the floor area ratio, which is anything within 6 feet 8 inches from the ceiling.

Areas included in Floor Area Ratio

  • Portions of a house with vaulted ceilings are counted as floor area only for the lower level with the floor.
  • Lofts are included as floor area only for portions with additional floor and a ceiling height of 6 feet, 8 inches.
  • Stairs that lead from one story of a house to another story count toward floor area one time (not at both levels).
  • Bays and bay windows are counted toward floor area if they extend to the adjacent finished floor and have a ceiling height of 6 feet, 8 inches.

Areas not included in Floor Area Ratio 

  • Basements that are partly or completely below grade. At least half of the total combined area of the basement walls must be below grade to be considered a basement. Only one basement level may be partly below grade; additional basement levels must be completely below grade.
    Diagram showing what constitutes a basement as not being counted in the floor area ratio.
  • Areas where the ceiling height is less than 6 feet, 8 inches such as dormers or attics
    Diagram showing what counts as floor area ratio in an attic or dormer.
  • Sloped sites, areas where the elevation of the floor is 4 feet or more below the adjacent right-of-way
    Diagram showing what isn't counted as floor area ration if the ground level is over 4 feet below the adjacent street.
  • Covered porches or balconies are not included unless they are enclosed by walls that are more than 42 inches high for 75 percent or more of their perimeter.
     
    Diagram showing balcony comparisons and what counts as being included in the floor area ratio.
  • Rooftop decks do not count toward floor area if they are not covered or enclosed.