FAR (Lesson 3) tells you HOW MUCH total floor area you can build. But it doesn't tell you the SHAPE. Two buildings with identical 3.0 FAR on identical lots can look completely different — one squat and square, one tall and skinny.
The buildable envelope is the maximum lawful shape, defined by four overlapping geometric rules:
- Yard setbacks — empty space required at the lot edges
- Maximum height — vertical ceiling in feet
- Sky-exposure plane — invisible plane leaning back from the street, above which you can't build
- Lot coverage — what fraction of the lot the building can sit on
You take the intersection of all four — that's your envelope.
Yard setbacks
Most R and C districts require empty space at the front (along the street), at the rear (away from the street), and along the sides.
Front yard: typical R1-R5 districts require 15-25 feet. Higher- density R6+ usually has NO front yard requirement (zero setback — buildings sit at the property line, "street wall").
Rear yard: nearly every district requires 30 feet rear yard for residential buildings. This is where backyard light comes from. In deep lots, this rule is what keeps anyone from filling the whole parcel.
Side yards: low-density (R1-R5) require side yards. Mid + high density (R6+, C, M) usually permit zero side yards if the building is "attached" (party-wall construction — think Brooklyn brownstone row).
Maximum height
Pre-CoY, most R districts had complicated height-factor formulas. Post- CoY (Dec 2024), the Quality Housing option (the default for new R buildings in most cases) uses a simpler max base height + max building height:
- Max base height is how tall the "street wall" can go before the sky-exposure setback kicks in. Typical R6A: 60 ft. R10A: 95 ft.
- Max building height is the absolute ceiling. R6A: 75 ft. R10A: 185 ft. R11A: 245 ft.
Sky-exposure plane
This is the most-misunderstood rule. Imagine an invisible plane that starts at the lot line at the height equal to the max base height and leans back into the lot at a specified angle.
Above this plane, you cannot build. So a tower above the base height must set back from the street to clear the sky plane.
The angle varies by district:
- R6A: ~ 30° from vertical (steep — barely any setback)
- R10A: ~ 55° (substantial — big tower setback)
- C6-4 Midtown: ~ 65° (extreme — towers must step back hard)
That's why Midtown high-rises have those classic tapered crowns and setback wedding-cake shapes — they're literally tracing the sky plane.
Lot coverage
The cap on the FRACTION of the lot the building footprint occupies. Typical numbers:
- R1-R5: 30-50% (lots of yard)
- R6A: 65% (compact urban)
- R10A: 70-100% depending on subzone
- Commercial: usually 100% (no coverage cap)
Lot coverage is the cap that catches you when yards alone wouldn't. A 100x100 lot with no front yard and 30ft rear yard leaves 7,000 sqft of "yards-allowed" footprint. But if coverage is 65%, you can only USE 6,500 sqft.
Putting it all together
For your lot, the maximum building shape is:
- Start with the lot area
- Subtract yard setbacks → "yards-allowed footprint"
- Cap by lot coverage % → "effective footprint"
- Stack floors up to the max base height
- Above the base height, step back per the sky-plane angle
- Hit the absolute max height ceiling
The 3D envelope you can see in Lesson 3's widget IS this shape, live- morphing as you drag the sliders.
Two real examples
Park Slope brownstone (R6B): 25 ft wide × 100 ft deep lot. Zero front yard, 30 ft rear yard. So footprint = 25 × 70 = 1,750 sqft. At 65% coverage, that's already 70% of the lot — coverage isn't binding. Stack 4 floors of 1,750 sqft each = 7,000 sqft total. At 2.0 FAR (R6B), max GFA is 25 × 100 × 2.0 = 5,000 sqft. So the FAR is what binds (5,000 < 7,000) — you can only USE 3 floors of buildable area.
R10A Manhattan tower: 100 × 100 lot. 10.0 FAR = 100,000 sqft total. At 70% coverage = 7,000 sqft footprint. So ~14 floors at that footprint. Max building height 215 ft = ~17 floors at 12 ft FTF. The top 3 floors must step back per the sky plane (55° angle, 60 ft base height → ~85 ft setback at the top floor). So your envelope shapes into a stepped tower.
Try it yourself
Use the widget below — pick a district, drag the sliders, watch the envelope morph. The most counter-intuitive moves: dropping FAR makes the building SHORTER, not always smaller in footprint; raising the sky-plane angle makes the TOWER setback bigger (more lean-back at the top); dropping coverage makes the building thinner.