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FAR and density
2019-12-16 12:27Another exercise, regarding Floor Area Ratio.
Imagine a model km2, 36% of which is road (not unusual for the US), so 64% is buildable lot -- 640,000 m2. Say half is zoned for residential. (I don't know why so low, but I recall Seattle being about that.) Americans apparently have 80 m2 of housing per capita, which seems high to me, but let's use it.
Say the FAR is 1.0 -- every lot is filled with a one-story building, or a 2-story takes half the lot, or a 3 story takes 1/3 the lot. 320,000 m2, 80 m2/person, so 4000 people/km2. Not particularly dense -- Boston is 5500, Chicago used to be and is still around 4500 -- but not terrible.
Of course, residential often clumps, so if we imagine a mostly residential zone, then even with grocery stores and schools, we would approach doubling the density locally -- 8000 people/km2. Which is significant for supporting small businesses in a walkable neighborhood. One supermarket per km2 or so, a 10 minute walk away for everyone.
If the FAR is 2.0 -- two story terraced housing filling the lot, 3-story on 70% of the lot, 4-story on 50% -- then double both numbers. 8000/km2 for the city, now denser than San Francisco, 16,000/km2 for the residential clump. A real city!
OTOH, if the FAR is 0.5 -- a one story building on half the lot, or 2-story on 1/4 the lot -- then the city is at 2000/km2. Like most of Silicon Valley, as it happens. Also modern Detroit.
Emotionally I would say you need a FAR of 1 to even qualify as a city, and really more like 2. One story buildings that don't even fill their lot isn't a city, it's a village.
What about parking? The whole point of a walkable city is that not everyone needs a car! But part of that 36% land use for roads is going to be curbside parking, so lots of spaces there -- not enough for everyone to have a car, but maybe enough for everyone who needs a car, especially if you price them right. You can also have garages a la Japan to meet further demand.
If you didn't build specifically for cars, then your roads are probably 15-25% of the land, not 36%. Which allows even more low-rise density.
Imagine a model km2, 36% of which is road (not unusual for the US), so 64% is buildable lot -- 640,000 m2. Say half is zoned for residential. (I don't know why so low, but I recall Seattle being about that.) Americans apparently have 80 m2 of housing per capita, which seems high to me, but let's use it.
Say the FAR is 1.0 -- every lot is filled with a one-story building, or a 2-story takes half the lot, or a 3 story takes 1/3 the lot. 320,000 m2, 80 m2/person, so 4000 people/km2. Not particularly dense -- Boston is 5500, Chicago used to be and is still around 4500 -- but not terrible.
Of course, residential often clumps, so if we imagine a mostly residential zone, then even with grocery stores and schools, we would approach doubling the density locally -- 8000 people/km2. Which is significant for supporting small businesses in a walkable neighborhood. One supermarket per km2 or so, a 10 minute walk away for everyone.
If the FAR is 2.0 -- two story terraced housing filling the lot, 3-story on 70% of the lot, 4-story on 50% -- then double both numbers. 8000/km2 for the city, now denser than San Francisco, 16,000/km2 for the residential clump. A real city!
OTOH, if the FAR is 0.5 -- a one story building on half the lot, or 2-story on 1/4 the lot -- then the city is at 2000/km2. Like most of Silicon Valley, as it happens. Also modern Detroit.
Emotionally I would say you need a FAR of 1 to even qualify as a city, and really more like 2. One story buildings that don't even fill their lot isn't a city, it's a village.
What about parking? The whole point of a walkable city is that not everyone needs a car! But part of that 36% land use for roads is going to be curbside parking, so lots of spaces there -- not enough for everyone to have a car, but maybe enough for everyone who needs a car, especially if you price them right. You can also have garages a la Japan to meet further demand.
If you didn't build specifically for cars, then your roads are probably 15-25% of the land, not 36%. Which allows even more low-rise density.