Floor area ratio, commonly shortened to FAR, compares the floor area of a building with the area of the lot on which it sits. Local governments use FAR to regulate the amount of development permitted on a property.
The basic formula is:
Floor Area Ratio = Counted Floor Area ÷ Zoning Lot Area
For example, a building with 2,500 square feet of counted floor area on a 5,000-square-foot lot has a floor area ratio of 0.5.
The arithmetic is simple. Determining which measurements belong in the calculation is more complicated. Local zoning regulations establish which parts of a building count as floor area, how lot area is measured, and what maximum FAR applies to the property.
FAR can help you estimate how much floor area zoning may permit. However, it does not account for every restriction that could affect a new home, addition, or other construction project.
What Does Floor Area Ratio Mean?
Floor area ratio measures development intensity rather than the physical footprint of a building.
A FAR of 0.5 means the counted floor area equals half the area of the lot. A FAR of 1.0 means the counted floor area equals the entire lot area. A FAR of 2.0 means the counted floor area is twice the lot area.
That floor area may be distributed across one or several stories. For example, a 10,000-square-foot lot with a permitted FAR of 1.0 could theoretically support 10,000 square feet of counted floor area. That could take the form of one 10,000-square-foot level, two 5,000-square-foot levels, or another configuration.
Those examples only demonstrate the FAR calculation. Setbacks, height limits, lot coverage, open-space requirements, and other regulations may prevent any of those exact configurations from being built.
The American Planning Association describes FAR as the floor area of buildings on a zoning lot divided by the area of that lot. It also notes that FAR can limit development intensity without requiring one specific building shape.
How Do You Calculate Floor Area Ratio?
To calculate the existing or proposed FAR, divide the floor area counted under the applicable zoning code by the zoning lot area.
Suppose a two-story house has 1,200 square feet of counted floor area on each level. The property contains 6,000 square feet of zoning lot area.
First, add the counted area of both floors:
1,200 + 1,200 = 2,400 square feet
Then divide the total counted floor area by the lot area:
2,400 ÷ 6,000 = 0.4 FAR
The house has a floor area ratio of 0.4.
Both measurements must use the same unit. If floor area is measured in square feet, lot area must also be measured in square feet.
How Do You Calculate the Maximum Floor Area Allowed by FAR?
You can reverse the formula when you know the maximum FAR assigned to a property:
FAR-Derived Floor Area = Zoning Lot Area × Maximum FAR
Suppose a zoning district permits a maximum FAR of 0.5 on a 6,000-square-foot lot:
6,000 × 0.5 = 3,000 square feet
The FAR-derived ceiling would be 3,000 square feet of counted floor area.
If the existing house already contains 2,400 square feet that count toward FAR, the basic calculation would leave 600 square feet:
3,000 − 2,400 = 600 square feet
That does not automatically mean the owner can build a 600-square-foot addition. The proposed design would still need to satisfy the property’s other zoning, site, and building requirements.
What Is a Good Floor Area Ratio?
There is no universally good floor area ratio.
A suitable FAR depends on the zoning district, property use, surrounding development, desired building form, and the needs of the people using the property. A lower FAR is not inherently better, and a higher FAR is not automatically more desirable.
For a homeowner, a good FAR is one that allows the home to provide the needed space without forcing a poor layout or creating conflicts with other property restrictions. For a developer, a higher permitted FAR may allow more floor area, but construction feasibility, cost, market demand, and other regulations still affect whether that additional area is useful.
The more important questions are whether the proposed FAR is permitted and whether the resulting design serves the property well.
What Counts as Floor Area?
There is no national definition that determines what must be included in every FAR calculation. The applicable zoning code controls the answer.
Depending on the jurisdiction and property, the calculation may address spaces such as garages, basements, attics, enclosed porches, stairways, mechanical rooms, accessory buildings, balconies, and areas with limited ceiling height.
A garage may count in one city and be partially or fully excluded in another. A basement may be excluded when it is sufficiently below grade but counted when more of it sits above ground. An attic may count only when it meets a specified ceiling-height requirement.
“Floor area,” “gross floor area,” “living area,” and “finished square footage” should not be treated as interchangeable terms. The square footage used in a real estate listing may not match the floor area used by a zoning department.
Before calculating FAR, find the exact definition used by the authority that governs the property. Technical references such as DataDrivenAEC’s FAR guide also caution that local rules control the measurement boundaries, inclusions, exclusions, and adjustments used in the calculation.
Floor Area Ratio vs. Lot Coverage
Floor area ratio and lot coverage both regulate development, but they measure different parts of a building.
FAR compares the total counted floor area with the lot area. Lot coverage compares the building footprint with the lot area.
Consider two houses that each contain 3,000 square feet of counted floor area. The first house has one 3,000-square-foot level. The second has two 1,500-square-foot levels.
If both houses sit on identical lots, they may have the same FAR. However, the one-story house would have a larger footprint and therefore greater lot coverage.
This distinction matters when deciding whether to build outward or add another level. A second story may increase floor area without increasing the existing footprint, but it may face height, structural, or design restrictions. A ground-level addition increases the footprint and may be more affected by setbacks and lot-coverage limits.
Homeowners comparing these options can also read Should I Build My Addition Up or Out?.
Why Do Cities Use Floor Area Ratio?
Local governments use FAR to regulate the overall intensity of development. It gives designers some flexibility in arranging floor area while placing a limit on the total amount of counted building space.
A zoning district intended for lower-density development may have a lower maximum FAR than an area intended for larger residential buildings, offices, or mixed-use development. However, permitted ratios and the policies behind them vary by jurisdiction.
FAR does not control every aspect of a building’s appearance. Two buildings can have the same ratio and look considerably different depending on their height, footprint, setbacks, roof shape, and distribution of floor area.
How Does FAR Affect a Home Addition?
FAR can determine whether zoning leaves enough floor-area capacity for a proposed addition. It can also affect where that additional space should go.
Suppose an existing home is already close to its maximum permitted FAR. A large ground-level addition may not be possible without reducing its size or revising another part of the design. Building upward does not avoid FAR because counted floor area on upper levels still contributes to the calculation.
In some cases, a homeowner’s initial request may need to change after the property restrictions are reviewed. The design might use the remaining floor area more efficiently, reorganize existing rooms, reduce the proposed addition, or pursue another configuration.
This is why an addition should begin with property and home evaluation rather than a finished floor plan. Lamont Bros.’ home addition services account for zoning, structure, existing conditions, and how the new space should connect with the rest of the house.
Does Meeting the FAR Limit Mean You Can Build the Project?
No. Meeting the maximum FAR is only one part of zoning and construction feasibility.
A project may satisfy FAR and still conflict with setbacks, lot coverage, building height, easements, environmental restrictions, access requirements, historic protections, or other property-specific rules. The existing home must also be capable of supporting the proposed work, particularly when the project involves adding another story.
FAR gives you an area limit. It does not tell you where that area can go, what shape it can take, or whether the existing structure can support it.
For examples of how additions can respond to different homes and property conditions, visit the Lamont Bros. home addition portfolio.
How Do You Find the Maximum FAR for Your Property?
Start with the planning or zoning department responsible for the property. Many jurisdictions provide an online zoning map that allows you to search by address.
Once you identify the zoning designation, follow these steps:
- Find the zoning table or development standards that apply to the property.
- Check for overlays, special districts, or property-specific restrictions.
- Locate the zoning code’s definition of floor area.
- Confirm how the code defines and measures zoning lot area.
- Calculate the existing counted floor area.
- Verify the calculation with the local planning department or a qualified professional before relying on it for design.
Property records and real estate listings can provide useful starting information, but they may use square-footage definitions that differ from the zoning code. A survey may also be necessary when lot dimensions, property lines, or easements are uncertain.
Common Floor Area Ratio Mistakes
One common mistake is using finished living area as the numerator without checking the zoning definition. Finished square footage and counted floor area can produce different totals.
Another is confusing FAR with lot coverage. A property can comply with its FAR limit while exceeding its permitted footprint.
Homeowners may also subtract the existing listed square footage from the FAR-derived maximum and assume the difference can become an addition. That calculation does not account for excluded or included spaces, setbacks, building height, structural conditions, or the shape of the available building area.
Finally, do not assume that a neighbor’s addition proves the same project will be permitted on your property. The properties may have different zoning, lot dimensions, overlays, existing floor area, approval histories, or site constraints.
Frequently Asked Questions
What does a floor area ratio of 0.5 mean?
A FAR of 0.5 means the counted floor area equals half the zoning lot area. On an 8,000-square-foot lot, a 0.5 FAR produces a FAR-derived ceiling of 4,000 square feet. Other regulations may reduce what can actually be built.
Can floor area ratio be greater than 1?
Yes. A FAR greater than 1 means the counted floor area exceeds the lot area. This is commonly achieved by distributing the floor area across multiple levels.
Is floor area ratio the same as building coverage?
No. FAR measures counted floor area across the building’s applicable levels. Building or lot coverage measures the portion of the lot occupied by the building footprint.
Does a garage count toward floor area ratio?
It depends on the local zoning code. Some jurisdictions count garages, while others exclude all or part of their area. Check the definition that applies to the property rather than relying on the home’s advertised square footage.
Does a basement count toward floor area ratio?
Basement treatment varies by jurisdiction. Factors may include how much of the basement is above grade, its ceiling height, and how the space is used. The applicable zoning definition provides the controlling answer.
Does FAR determine how many stories you can build?
No. FAR limits counted floor area in relation to lot area. Separate height, setback, building-code, and design requirements may control how many stories are permitted.
Is maximum FAR the same as maximum buildable square footage?
Not necessarily. Multiplying lot area by maximum FAR gives an area ceiling based on that ratio. Other development standards and site conditions may result in less buildable floor area.
Planning a Home Addition?
Floor area ratio can tell you whether a property appears to have room for additional floor area, but it cannot determine the right addition by itself. The existing structure, zoning requirements, site conditions, and desired layout must support the same plan.
Lamont Bros. Design & Construction evaluates these factors during the early planning and design stages of a home addition. Our design-build process keeps property evaluation, design, engineering coordination, and construction connected as the project moves forward.
If you are planning an addition in the Portland metro area or Southwest Washington, contact Lamont Bros. to discuss your home and the space you need.
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