FAR, lot coverage, and the footprint that actually fits
A floor-area calculation can leave you with square footage that a site cannot accommodate in the form you want. To understand the gap, keep total counted floor area, counted ground coverage, and usable building placement separate.
The example below uses a fictional 6,000-square-foot lot. All limits and dimensions are teaching assumptions. They are not Seattle standards or an allowance for any real parcel.
Three questions, three measurements
Floor area ratio, or FAR, compares the floor area counted under a rule with the lot area that rule uses. Multiplying the applicable lot area by an assumed FAR gives a floor-area cap in this example.
Lot coverage measures the ground area counted under a coverage rule. Here, assume it is simply the combined building footprint. On a real project, check the code's definition before deciding how a porch, projection, accessory structure, or other feature counts.
The placement envelope is the part of the site available for the proposed building after the constraints included in the study. It has a shape as well as an area. A long narrow space and a square space of equal area do not fit the same floor plan.
| Teaching input | Assumption |
|---|---|
| Lot area used for both calculations | 6,000 sq ft |
| FAR | 0.8 |
| Maximum counted lot coverage | 40% |
| Floors in the first layout | 2 |
| Existing buildings retained | None |
Assume every floor has the same area, all floor space counts toward FAR, and there are no exclusions or bonuses. These simplifications let us see which limit controls the arithmetic.
Calculate the floor-area and coverage caps
The assumed FAR cap is:
6,000 sq ft × 0.8 = 4,800 sq ft of counted floor area
The assumed coverage cap is:
6,000 sq ft × 40% = 2,400 sq ft of counted ground coverage
If a 2,400-square-foot footprint fits and both floors are that size, two floors provide 4,800 square feet. The two assumed caps happen to meet. We have not yet checked the site shape or a workable building layout.
Now reduce the footprint that fits
Suppose the placement study leaves room for a 30-by-60-foot rectangle. For this example, treat that rectangle as the entire available building footprint:
30 ft × 60 ft = 1,800 sq ft
Two identical floors give:
1,800 sq ft × 2 = 3,600 sq ft
That is 1,200 square feet below the assumed FAR cap. Increasing the FAR input would not add space to this two-floor layout. Its footprint is already limited by placement.
| Check | Result |
|---|---|
| Floor-area cap | 4,800 sq ft |
| Coverage cap | 2,400 sq ft |
| Footprint available in the placement study | 1,800 sq ft |
| Floor area in the two-floor sketch | 3,600 sq ft |
| Unused floor-area allowance in this example | 1,200 sq ft |
A third identical floor would produce 5,400 square feet, exceeding the assumed FAR cap by 600 square feet. A smaller upper floor changes that arithmetic, but raises design questions about height, stairs, accessibility and the building form. The calculation supplies a question for design; it does not answer those questions.
Check the shape before dividing the area
A 1,800-square-foot footprint also does not tell you how several homes would fit. Drawing three equal 600-square-foot rectangles omits circulation and separation. Nor does dividing total floor area by a desired home size establish a legal home count.
Keep these results distinct in a feasibility note:
- The floor-area cap under the selected rule and its measurement definition.
- The footprint used by the particular sketch.
- The proposed number and arrangement of homes, with the eligibility rule still to check.
- The space needed for access and other layout requirements.
For a project retaining a house, add a separate accounting step for the existing building. Determine which limits it shares with the new work, and measure the relevant areas consistently. Do not treat the whole lot's calculated capacity as automatically available for an addition.
The note to carry forward
For this example, a useful conclusion is:
With the assumed 0.8 FAR and 40% coverage, the floor-area cap is 4,800 square feet. The tested 30-by-60-foot footprint supports 3,600 square feet across two identical floors. Reaching more area needs a different layout or building form, followed by the relevant code and design checks.
When applying this method to a real property, write the source for each limit and each measurement beside the calculation. The parcel evidence sheet provides a place to keep them.
Method checked . Arithmetic uses the stated hypothetical inputs. No jurisdiction-specific FAR, coverage, height or dwelling entitlement is asserted.