Today’s show is another AMA episode. Except, on today’s show I’ve had virtually the same question from several listeners in different geographies. I’ve synthesized a composite question which basically aims to cover the landscape of the question.

The question is how to determine the density of apartments that you can build on a site having rather large dimensions. In one case, the property is in a dense urban location and measures about 200 feet by 500 feet. How many apartments can I get if the building is limited to 5 stories in height?

This is a great question. Of course each municipality has rules that determine the setbacks of the building from the property line, front, back and sides. But the setbacks are not the constraint in this case.

When you’re evaluating the utility of a building site for residential apartments, you need to think about the basic apartment as a building block. A one bedroom apartment is going to need about 22-25 feet of perimeter space on the building exterior in order to have windows in the living room and the bedroom. A two bedroom apartment is going to need about 35 feet of exterior wall space for two bedrooms and a living room. Apartments are not that deep. On the interior, you’re going to locate the kitchen, the bathrooms, closets, hallways, laundry facilities. None of those items require windows. Your most expensive real estate in an apartment building is the exterior walls because that’s where you have windows. At most, your apartment is going to be 30 to 40 feet deep. If you allow 6 feet for a hallway between apartments, then at most your building will be 85-90 feet deep from one side of the building to the other. If the property is too large, you have space in the core that is not really usable. Making a larger laundry room in an apartment is only going to add cost and is not going to bring you additional rent.

So there are certain land dimensions that naturally lend themselves to building an apartment building, and then other dimensions that are just plain awkward.

So if you have a property that is deeper than you need it to be for an apartment building, you’re not naturally going to be able to make use of that extra land. A site that is 200 feet deep is too deep for a single building to span the entire depth of the site and not deep enough to have two towers.

You can use some of the extra depth in a dense urban setting to create a sense of open streetscape by setting the building further back from the street. You can also make a larger footprint on the ground floor for the amenities like the lobby, the party room, parking entrance, a gym, and so on. But carrying that extra depth up the to the top of the tower is wasteful.

I’ve seen many rookie developers multiply the width of the property by the height restriction in and then divide by the size of an average apartments order to determine how many units are possible. But this simple calculation neglects the real constraints in designing an apartment building.

A property that is 400 feet deep may be a candidate for two towers on top of a podium ground floor with a space between them. Perhaps a horseshoe shaped or U shaped building on top of a podium. The goal is to maximize the perimeter area of the building so you can get as many bedrooms and living rooms on the exterior of the building as possible. That one constraint is usually the limiting factor on the number of apartments that are possible.

The second limiting factor for any apartment building is going to be the parking ratio. The city will determine the minimum parking allowed in the zoning code. But you also need to take market demand into account as well. If your building height and perimeter calculations can allow, say, 200 apartments, but you only have space for 100 cars, you might be limited by parking.