Feasibility study glossary
What the key site and zoning numbers mean.
These terms help you read a preliminary feasibility study. Each city defines and applies them differently. Use the linked city source in your study as the rule for a specific property.
Floor area ratio (FAR)
FAR compares a building’s counted floor area to the lot area. A 50% FAR on a 6,400 sq ft lot suggests up to about 3,200 sq ft of counted floor area before exclusions, bonuses, or other code provisions.
It does not tell you the building footprint. A two-story 3,200 sq ft home might have a footprint near 1,600 sq ft; a one-story home with the same floor area would need much more ground area.
A garage can change the answer. Some cities count all garage area, some exclude parking, and some allow only a limited exemption. The feasibility fit test assumes 250 sq ft per garage space for the footprint, then separately applies the municipality’s reviewed garage rule to the FAR calculation.
Lot coverage
Lot coverage is the share of the lot covered by buildings and, in some jurisdictions, other defined improvements. A 50% limit on a 6,400 sq ft lot is a preliminary cap of about 3,200 sq ft.
Setbacks, easements, tree protection, and the city’s specific definition of “coverage” can make the usable footprint much smaller. It is not a guarantee that the full percentage can be placed anywhere on the lot.
Impervious surface
Impervious surface generally means ground that does not readily absorb water, such as roofs, driveways, patios, and walkways. It matters for drainage and stormwater requirements.
Impervious surface and lot coverage can overlap, but they are not interchangeable. A feasibility study uses a city’s published standard as an early screen; a civil engineer confirms the actual areas from a site plan and drainage design.
FAR, lot coverage, and impervious surface answer different questions
Assume a 6,400 sq ft lot, a two-story house with 1,600 sq ft on each floor, and 900 sq ft of driveway and patio. Here is how the same property could be measured.
How much counted floor area?
3,200 ÷ 6,400= 50%
Counts floor area across stories. The driveway and uncovered patio normally do not add FAR.How much lot area is covered?
1,600 ÷ 6,400= 25%
Usually starts with the building footprint. A city may include other structures or improvements in its definition.How much ground cannot readily absorb water?
2,500 ÷ 6,400≈ 39%
Often includes roofs plus counted paving and hardscape. Permeable products and exemptions depend on the city.The roof is counted in two different ways. Its footprint can use lot-coverage allowance, while the same roof area also contributes to impervious surface. FAR counts the floor area inside the stories instead.
See which measurement usually moves
These common design changes show why the three percentages cannot be used interchangeably.
Add a second story without changing the roof outline
FAR↑ Increases
Lot coverageNo change
ImperviousNo change
Enlarge the ground-floor footprint and roof
FAR↑ Increases
Lot coverage↑ Increases
Impervious↑ Increases
Add an uncovered driveway or patio
FARNo change
Lot coverageCheck definition
Impervious↑ Increases
“Usually” is intentional. Cities define counted floor area, structures, hardscape, pervious pavement, decks, eaves, pools, and exemptions differently. Apply the local code before relying on the arrows.
Height
Height is measured using a city specific method. The starting point may be average building elevation, finished grade, or another defined point. A “25 ft” maximum is therefore not a simple tape measure limit from the street or the lowest corner of the lot.
Setbacks and buildable envelope
Setbacks are required open areas from property lines. After setbacks, easements, critical-area buffers, utility areas, and tree protection are accounted for, the remaining area is the practical buildable envelope. That envelope needs a current survey and site-specific design review to confirm.
The practical building area is usually smaller than the setback box
Base setbacks create a starting envelope. Easements, protected trees, critical areas, access, and utilities can remove more usable space.
- 1
Draw the base setbacks. Start with current property lines and the jurisdiction’s yard rules.
- 2
Overlay other constraints. Map recorded easements, trees, buffers, access, utilities, and private restrictions.
- 3
Test a real design. The remaining shape still has to support rooms, structure, drainage, construction access, and code-compliant circulation.
A percentage is not a placeable footprint. A current survey and site-specific professional review are needed to confirm the boundaries and workable geometry.
Wetlands, steep slopes, and the calculation area
A critical-area map hit can affect the numbers in two different ways. Some cities keep the protected land in the base FAR or coverage denominator but separately prohibit or limit where the house can go. Other cities subtract a wetland, buffer, shoreline area, steep-slope nondisturbance area, access area, or other unbuildable land before applying the percentage.
The feasibility study therefore shows public-lot math as a theoretical zoning screen. When mapped wetlands or geologic hazards are present, it does not call that number a confirmed building envelope. Seattle, Medina, and Mercer Island can require an adjusted or net area before the square-foot limit is reliable; Bellevue and an ordinary Kirkland detached-home calculation generally keep the base lot-area math but still require the protected envelope to be placed separately.
A city map is not a wetland delineation, surveyed buffer, or geotechnical opinion. Use a qualified wetland professional for wetlands, a geotechnical engineer or engineering geologist for slopes and landslide hazards, and a surveyor to put the resulting boundaries on the site plan.
The same percentage can produce two different screening numbers
Assume a 10,000 sq ft lot, a 2,000 sq ft protected or excluded area, and a 40% limit. The applicable city rule determines which area starts the calculation.
10,000× 40%
= 4,000 sq ftThe protected area may remain in the denominator while still restricting where development can go.8,000× 40%
= 3,200 sq ftThe 2,000 sq ft protected or excluded area is removed before the percentage is applied.Neither number confirms what will fit. The protected area and buffer must also be located, and the city may apply additional adjustment rules.
Why the city matters
The same word can include different surfaces, use a different measurement method, or not be used at all depending on the jurisdiction. Read the percentages with that city’s linked code. Do not transfer them to a neighboring city.
Kirkland
Kirkland’s low-density residential table establishes the zone’s height, yards, and maximum lot coverage. KZC 115.42 separately explains counted FAR, while KZC 115.90 calculates zoning lot coverage from structures and defined hardscape, with stated exemptions and partial exemptions. Enclosed garage and carport area is generally counted in FAR. Because below-grade exclusions depend on surveyed grade and the design, the feasibility study conservatively counts the full assumed garage.
Bellevue
Bellevue treats lot coverage by structures, impervious surface, and hard surface as independent limits. All structure coverage is generally included in impervious surface, and all impervious surface is included in hard surface, subject to the code’s definitions and exceptions. The feasibility study defaults to one FAR-counted dwelling and counts the enclosed garage unless the design establishes a specific exclusion. An attached ADU is FAR-exempt and does not increase the FAR dwelling count; a DADU counts toward both FAR and dwelling count. The exemption for up to 300 sq ft of parking or unheated storage per dwelling is limited to qualifying middle housing and is not automatically applied to an ordinary single-house garage.
Bellevue LUC 20.20.010: dimensional chart
Bellevue LUC 20.20.390: FAR and limited parking exemption
Clyde Hill
Clyde Hill’s R-1 rules use structural coverage and impervious coverage as separate controls. The reviewed base R-1 provisions do not state a general FAR cap, so no garage-in-FAR adjustment applies. A garage still uses structural and impervious coverage.
Medina
Medina does not use FAR for residential feasibility, so no garage-in-FAR adjustment applies. It uses lot-size-based structural coverage and impervious-surface tables, lot-area and width-based setbacks, and a grade-based height plane; a garage consumes structural and impervious coverage. The engine recognizes R-16, R-20, R-30, and SR-30. Medina Heights, planned-development, shoreline, and critical-area rules still require a parcel-specific review.
Mercer Island
Mercer Island’s R-8.4, R-9.6, R-12, and R-15 districts share a 20 ft front yard, 25 ft rear yard, 30 ft base height, and a gross-floor-area limit. Garages and covered parking are included in gross floor area, so the feasibility study counts the full assumed garage. A surveyed design can establish whether a qualifying below-grade portion is excluded. Side yards depend on lot width, while lot coverage depends on lot slope.
Redmond
Redmond consolidated many former residential zones into Neighborhood Residential, Neighborhood Multifamily, and Neighborhood Mixed-Use. Neighborhood Residential does not use one base FAR for this feasibility screen. Where FAR applies, Redmond’s definition excludes area devoted to parking or vehicle circulation, so garage area does not consume FAR capacity; it still consumes building coverage. Incentives, transit-oriented development areas, shoreline jurisdiction, critical aquifer recharge areas, and project type can change the final limits.
Bothell
Bothell’s current residential system uses R-C, R-L1, R-L2, and R-M1 through R-M4 base zones. The base residential table does not use FAR and does not publish a separate building-coverage percentage. Buildings, garages, driveways, and other counted surfaces use the broader hard-surface allowance, but the feasibility study does not present that percentage as a house-footprint ceiling. Residential accessory commercial, manufactured-home, transition, alley, shoreline, and critical-area conditions require separate review and do not receive a generic numerical envelope.
How to use the numbers
- Open the View source link beside each feasibility study metric to see the exact public map or code section.
- Use the numbers to compare concepts and decide what to investigate first. Do not use them to finalize a house size.
- Before committing to a purchase, design, or permit path, confirm the site with the city plus an architect, licensed surveyor, geotechnical engineer, arborist, and civil engineer as applicable.
Build Eastside feasibility studies are preliminary screens based on publicly available data. They are not confirmed feasibility, a survey, tree inventory, geotechnical opinion, engineering design, code interpretation, or permit approval.