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Residential Infill Options - How Does Floor Area Ratio (FAR) Affect Your Project?

Guide
In exchange for more flexibility and units, the single-dwelling zones code requires that you provide more information about the property’s floor area ratio. The size of new structures or additions are limited based on the size of the lot and size of all structures on the lot.
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How floor area ratio requirements affect a project

Floor area ratio requirements are in place to maintain the relationship between building size and site areas. they ensure development on one lot does not overwhelm development on adjacent lots. The amount of allowed floor area generally increases as units are added to a property.

Maximum allowed floor area ratio 

The number of units can be configured in multiple ways.

For example: One unit is the main house or attached house. Two units could be a house with an attached or detached accessory dwelling unit or a duplex. Three units could be a triplex, a duplex with one accessory dwelling unit or a house with one internal and one detached accessory dwelling unit.

UnitsRFR20*R10*R7R5R2.5
1No limit0.4 to 10.4 to 10.4 to 10.5 to 10.7 to 1
2No limit0.5 to 10.5 to 10.5 to 10.6 to 10.8 to 1
3No limit0.6 to 10.6 to 10.6 to 10.7 to 10.9 to 1
4 or moreNo limit0.7 to 10.7 to 10.7 to 10.8 to 11 to 1

*In the R10 and R20 zones the maximum floor area ratio only applies to sites that are less than 10,000 square feet in area.

Bonus floor area ratio

To provide incentives to build more affordable housing–and to keep existing houses–bonus floor area is available to certain projects:

  • Affordable Housing: Sites that provide at least one dwelling unit to those earning no more than 60% of the median area family income (certified by Portland Housing and restricted by covenant).
  • Existing Houses: Projects on sites with an existing residential structure that received final inspection at least five years ago and alter no more than 25% of the existing street-facing façade.
UnitsRFR20*R10*R7R5R2.5
1Not applicable0.4 to 10.4 to 10.4 to 10.5 to 10.7 to 1
2Not applicable0.6 to 10.6 to 10.6 to 10.7 to 10.9 to 1
3Not applicable0.7 to 10.7 to 10.7 to 10.8 to 11 to 1
4 or moreNot applicable0.8 to 10.8 to 10.8 to 10.9 to 11.1 to 1

*In the R10 and R20 zones the maximum floor area ratio only applies to sites that are less than 10,000 square feet in area.

Small additions- an exception to the rule

Showing the amount of floor area on a lot can require additional drawings or documentation that might not otherwise be necessary. So, the Zoning Code allows for one alteration or addition of up to 250 square feet once every five years for an existing primary structure that received final inspection at least five years ago.

Floor area ratio for specific housing types

Due to variation in lot sizes and the types of housing allowed, the following circumstances have allowed floor area ratios that are different than the base zone (code citations included for reference).

  • Small flag lots (33.110.255.C.2) where the flag portion is less than 3,000 square feet, maximum FAR is 0.5 to 1.
  • Affordable fourplexes and multi-dwelling structures (33.110.265.F), maximum FAR is 1.2 to 1.
  • Cottage clusters (33.110.265.G.5) allow a maximum average floor area of 1,400 square feet per dwelling unit, including the floor area for attached accessory structures. The applicant may choose to exclude the floor area of any existing dwelling units that received final inspection at least five years ago from the average. The maximum floor area allowed for a detached or connected accessory structure is 400 square feet.

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