Units/Acre
I am having trouble understanding and calculating how to increase units per acre when total land acreage stays the same. Obviously the amount of open space and infrastructure space would need to decrease giving a larger amount of net developable area, but when not given the amount of infrastructure space and open space how do I work the algebra. Is there some absent expected variable or information or calculation I am missing or need to know? I am having trouble finding this info and how to calculate in any of the listed NCARB references and or third party or intranet sources.
So far I have this:
Total Project Acreage = Total land area of a project
Net developable area = Total project acreage minus open space and infrastructure acreage
Gross Density = # of Dwelling Units/Total Project Acreage
Net Density = # of Dwelling Units/Net Developable Acreage
Maybe I am making this more complex than it has to be (which I tend to do).
Any help would be appreciated.
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The FAR, which is determined by the municipality, and the IBC determine density.
The FAR = floor area ratio. It's a # like 2.0 or 4.0 for example. The building can be 4 x the sf of the land area if the FAR = 4.0. The municipality will also have setbacks and height requirements. In cities like NYC, the FAR can be unlimited.
Then the IBC also factors in. Chapter 5 of the IBC has tables that define the maximum building height, # of floors, and area. The use group and bldg construction type determine this. It'd help you to read this chapter.
Both the municipal zoning restrictions and the IBC apply, so whichever is most stringent rules.
Hope this helps.
Rebekka O'Melia, Registered Architect, NCARB, B. Arch, M. Ed, Step UP, Step UP ARE 5.0 Courses
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Rebekka,
I appreciate the comment. I understand Chapter 5 of the IBC and how it relates to an individual parcel, but I am having trouble relating it to an entire subdivision where the architect or developer is tasked with maximizing units per acre. How is it possible to calculate max units per acre when the required amount of infrastructure space (green space,streets, pedestrian walkways, flood control channels, etc…) is not known? It seems straight forward when given the zoning ordinance that requires say 15% of developable land required for infrastructure space. But absent that information how does the architect or developer calculate. Are there any available resources other than the IBC that show how to make detailed calculations concerning maximizing units per acre when subdividing land? -
John, let's approach your question with an example question from Amber Book's practice exam. . . An architect is developing a 150-acre site with a goal for the entire site of 15 units per acre. He’s developed 50 acres of the site so far at 11 units per acre. What should the density be, in units per acre, for the remaining not-yet-developed portion? Choose the value closest to the target; you may round up or down.
11 units per acre
13 units per acre
15 units per acre
17 units per acre
Scroll way down to see the answer. . .
Answer: 17 units per acre
Explanation
11*(50/150) + x*(100/150) = 15. . . then solve for X
To watch a similar animated problem solved from the Amber Book course, click here.
. . . or you could simply note that if the goal is 15 units per acre, and we’ve developed only 11 units per acre so far, we must have some catching up to do in order to get to our goal of 15. The answer therefore must be more than 15—and there is only one choice available that is more than 15. If you see a calculation test item that offers you a multiple choice lifeline, like this one does, remind yourself what the numbers actually mean and see if you can use process of elimination, the smell-test, or common sense to take the easy win.
Want another one to test yourself? An architect is developing a 90-acre site with a goal for the entire site of 21 units per acre. She’s developed 44 acres of the site so far at 3 units per acre. What should the density be, in units per acre, for the remaining not-yet-developed portion? Scroll down for the answer (don’t peek).
3*(44/90) + x*(46/90) = 21
38.22 units per acre
To watch more examples I made, and to see the link between this calculation and other area-weighted calculation on these exams (thermal, acoustic), click here.
Get licensed!
Michael Ermann, Amber Book creator.
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John,
I think you are confusing 2 different things.
Planning regulations in a municipality define the planning of neighborhood. The planning regulations will spell out the width the street can be, open spaces needed, the length of new streets by type, how wide the sidewalks need to be, etc. It works with the zoning regulations. So the zoning map (or overlay) defines what the buildings' use groups can be in each area of a city. Part of the city is just residential. Part is commercial. A small area might be zoned for Industrial. The FAR (density) for each is defined by the zoning.
Then Zoning regulations define the maximum height, area, and setbacks in each area. The IBC also defines maximum height, area, and number of floors for each use group by construction building type; so the most stringent of both IBC and zoning applies.
It's very import for a municipality to have well developed planning and zoning regulations to protect it from bad development.
For the PA exam, you would be given a list of planning regulations and design criteria in order to answer any questions. There may be old-grown trees that are protected, and maybe 20% open space is required. For PA you should be prepared to look at a site plan, read a scenario, and put an X on the best location for the building. You seem to be over-thinking things - trying to design a community and site at the same time. Check out the demonstration exam when you log into your NCARB account for good example questions.
Hope this helps!
Rebekka O'Melia, Registered Architect, NCARB, B. Arch, M. Ed, Step UP, Step UP ARE 5.0 Courses
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Rebekka,
I think the confusion lies in the scenario that these question types present and that I did not do a good job in explaining what was being asked.
When a developer owns a large tract of undeveloped land say 1000 acres that is already zoned for residential but not yet subdivided and wants to construct a residential subdivision or maybe construct a PUD that would require redefinition of the existing zoning, the architect will be tasked with planning infrastructure space in a way that maximizes the amount of net developable area. Yes the architect will have to adhere to detailed infrastructure and zoning requirements (min. Street widths, min. sidewalk dims., min. amount of green space, FAR, setbacks, heights, etc..) and along with the developer would have to take the plan to the Planning and Development departments to determine how feasible the project would be. But depending on the infrastructure planning design of the site, the architect may or may not maximize the amount of net developable area which would affect the number of units per acre. Obviously the developer wants the max. number of units allowable. I think the ARE, in a disconnected confusing way is trying to determine competency within this particular design skill.
Thanks for the comments. Having this conversation and trying to properly describe possible scenarios has helped advance my knowledge.
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