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.
| Units | RF | R20* | R10* | R7 | R5 | R2.5 |
|---|---|---|---|---|---|---|
| 1 | No limit | 0.4 to 1 | 0.4 to 1 | 0.4 to 1 | 0.5 to 1 | 0.7 to 1 |
| 2 | No limit | 0.5 to 1 | 0.5 to 1 | 0.5 to 1 | 0.6 to 1 | 0.8 to 1 |
| 3 | No limit | 0.6 to 1 | 0.6 to 1 | 0.6 to 1 | 0.7 to 1 | 0.9 to 1 |
| 4 or more | No limit | 0.7 to 1 | 0.7 to 1 | 0.7 to 1 | 0.8 to 1 | 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.
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.
| Units | RF | R20* | R10* | R7 | R5 | R2.5 |
|---|---|---|---|---|---|---|
| 1 | Not applicable | 0.4 to 1 | 0.4 to 1 | 0.4 to 1 | 0.5 to 1 | 0.7 to 1 |
| 2 | Not applicable | 0.6 to 1 | 0.6 to 1 | 0.6 to 1 | 0.7 to 1 | 0.9 to 1 |
| 3 | Not applicable | 0.7 to 1 | 0.7 to 1 | 0.7 to 1 | 0.8 to 1 | 1 to 1 |
| 4 or more | Not applicable | 0.8 to 1 | 0.8 to 1 | 0.8 to 1 | 0.9 to 1 | 1.1 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.