Tuesday, May 13, 2014

Parking and density: how minimum parking requirements cap density

 One of the major regulations present in most North American cities is the minimum parking requirement. I spoke about it a few times, like when I talked about the "door-to-door" versus "neighborhood-to-neighborhood" concept of motorized transport, but that was about commercial parking. I would like to point out how difficult it is to reconcile parking minimums with residential density.

Ways to increase density

First of all, how does one go about increasing density? There aren't a thousand ways of doing so, you can either:
  1. Increase the number of units per lot, by building a bigger, taller building and having many apartments.
  2. Decrease the width of lots.
  3. Decrease the depth of lots.
So, essentially you can play with three dimension, but often you are constrained by the existing street grid which dictates the depth of the lots. Like in this example:
Street grid imposing a certain lot depth
There are only 60 meters between the top and bottom street, so place for 2 lots both 30-meter deep, if you stick to the idea of having buildings facing the existing streets. So you lose option 3, the depth of lots is fixed.

If you are limited to single-family homes, then you lose option 1, because even if you build higher, you're still stuck with one unit per lot.

So if you want more density with single-family housing on existing street grids, you are limited to one way of doing it: reducing the width of lots. You could still get a pretty high density with rowhouses that are 5-meter (17 feet) wide. Overall, each lot would be 5-meter wide and 35-meter deep (30 m for the lot, plus 5 meters for half the street in front of the lot), so that would give around 50 dwelling units per hectare (20 per acre). With each house being 2-story high and 14-meter deep, each could offer around 120 square meters (1 300 square feet) of living space.

Rowhouses on the previous grid
Of course, you can do denser. You could build an alley between the lots and double density by building two other rows between the existing ones. You could also build higher multi-family buildings.You can pile things on and get many times that density. Except that in this case, I omitted one very important fact: recent housing must house not only humans, but also cars.

Housing and parking


In most cities, recent constructions need to offer 2 parking spaces for family-sized units. The traditional way of providing this parking is a driveway and garage, and the parking spaces are built side-by-side, because most families do not share cars equally, you often have "dad's car" and "mom's car", and people don't want to have to get a car out of the way just to access their own car.

How much space does a car need for a parking spot? Well, cars can go up to 2,1-meter wide (7 feet) and 5-meter long (17 feet), though most are about 1,9-meter wide and 4-meter long. Anyway, to provide enough space to park safely, you need a width of about 2,5 meters and a length of 6 meters.

So what happens if you suppose 2 parking spots per unit, in a driveway/garage with a fixed street grid like previously shown? Well, you have to have a 5-meter driveway in front of every house. Essentially, parking acts like single-family houses, the depth is determined (30 meters in the example), you can't build one atop another (too expensive), and the width of each parking space is minimum 2,5 meters. So each parking space takes in fact nearly 90 square meters:
Space occupied by two parking spaces in a driveway
Remember, this is PER UNIT. So each dwelling unit must have two parking spaces, which takes up about 90 square meters in total (around 1 000 square feet), for a total of 180 square meters. And to avoid having entire streets be unbroken driveways, you'll have maybe a 2-meter buffer between lots, so that's another 70 square meters lost. The result? The minimum space each unit occupies will be around 250 square meters, allowing for a maximum density of only 40 dwelling units per hectare, or 16 per acre.

What about on-street parking? It's even worse as they require more width:
Space occupied by two on-street parallel parking spaces
Here, each parking spaces takes no less than 210 square meters, for a total of 420 square meters. At least, you don't require buffers here, but anyways, each UNIT will require 420 square meters for parking, a maximum density of about 24 dwelling units per hectare, or 10 per acre.

Again, this is per unit, not per lot. So you can't even increase density by increasing the number of stories and building many units per lot, because then you will have to multiply the amount of parking spaces per lot too. So if you build a duplex, you'll be forced to provide 4 parking spaces, taking at least 450 square meters.

Example of this dynamic: Houston

Houston in Texas is a famous city for urbanists as it has notoriously lax zoning regulations, land uses are basically unregulated (at least not by the city itself). However, what is not lax is the minimum parking requirements, which are of 2 parking spaces minimum for residential units with 3 or more bedrooms.
Houston's residential parking ordnance
Currently, Houston is proceeding to the greatest densification of low-density residential areas in North America that I know of. They are converting a lot of older low-density houses to townhouses. But despite the freedom the lack of regulations allows developers, the density of their townhouses is pretty underwhelming... The problem is exactly what I pointed out about how parking limits density.
Houston townhouses

Houston townhouses 2

Houston townhouses: seen from the sky
First, okay, they look pretty bad. One huge garage door and a little side door for humans. But even though they're built very close, even though they have little to no courtyard, their density is around 35 to 40 units per hectare (14-16 per acre). That's decent mind you, but not all that great.

The big problem here is that as they need to provide 2 parking spots per house, they are about 7 meters wide at least, and the existing street grid determines lot depth. These townhouses are also pretty big, but even if they tried to make each housing unit smaller, they couldn't accommodate any more because the parking, not the the livable space, limits density.

They sometimes try to compensate when the lot is too deep by building new alleyways to reduce the depth of lots:
New alleyway to reduce lot depth
Same, seen from above
In this example, the creation of the alleyway/courtyard between the town houses allowed 24 units to be built instead of the 18 or 20 that could have been built otherwise. Of course, it means some houses are surrounded by houses on three sides. The density here is over 50 dwelling units per hectare (20 per acre).

Mitigating methods

One way to avoid this issue is to be able to have many rows of parking spaces, which can be done through side-loading garages and parking lots, which are more efficient in that aspect.

Here is an example from Lasalle in Québec, around where I currently reside:
Side-loading garage
Schematic representation of the previous

These blocs have 4 2-bedroom units and 1 1-bedroom unit in the basement. They have 4 parking spots in the garages under the building, 1 in the driveway and 2 on-street parking spots, for a total of 7. Each building is about 15 meters wide. If they had all this parking in one big driveway, the driveway would be 18-meter wide.

Here is another example, a more recent one, but in Lasalle also:
Recent condos in Lasalle
These two semi-detached blocs have 12 units and a density of around 70 dwelling units per hectare (32 per acre). They have about 18 parking spaces, but the side-loading parking spots save enough space to salvage a front yard and a backyard.
Schematic representation of parking spaces of the previous
Here is another example with townhouses and parking tucked under a backyard patio, leaving the front without parking and preserving yards. Contrast these townhouses with the Houston ones, which largely have the same density of 35 to 40 units per hectare.
Modern townhouses in Lasalle, back-loading garages with patios built over the access to the garages

The same townhouses in front: yards, trees and stairs
Schematic representation of the previous

Parking lots either adjacent to, behind or below buildings can also compensate for this. However, in general you will have to build a 6-meter wide alley between parking rows, which means that each parking spot will actually take 22,5 square meters, including part of the alley, maybe more. Including all the bits of lost space due to corners and access to the street, the parking lot may be 25 to 30 square meters per parking spot.

Low-rise condos with shared parking lot between them in Lévis near Québec City
Schematic representation of the previous
Here, the 12 dwelling units have no less than 34 parking spots. The density is around 50 dwelling units per hectare (20 per acre) while preserving a lot of green space.

Condos in Longueuil, a suburb of Montréal
Schematic representation of the previous
Here, the building has 14 units and 32 parking spots. This is a density of about 80 units per hectare, or 32 per acre.

High-rises may be able to have garages with many stories, but this is extremely expensive, with costs per stall between 30 000 and 50 000$ from what I have seen.

Two densities

The result of all of this is that there are two densities that matter: the density of dwelling units which is often used by urbanists to describe the desired density, and the density of parking places. When you implement parking minimums, you essentially cap the population density by parking density. For instance, if your parking "scheme" allows for only one parking spot per 100 square meters and you impose 2 parking spots per unit, then by definition, you allow a maximum density of just 50 dwelling units per hectare, regardless of anything else you do.

To solve that issue, there are only two choices:
  • Find a way to be able to build more parking spaces per area
  • Reduce the parking minimums so you don't require a high parking density
And that's the gist of it, the reason why recent constructions generally have such low density compared to old constructions, especially in car-dependent areas. Parking requirements impose an hard cap on density. It is extremely hard to find an affordable way to incorporate a lot of parking in a dense residential area in a manner that doesn't result in an ugly wasteland of parking lots and lines of parked cars everywhere you look. Before the car, they didn't have to worry about it and so made neighborhoods much denser.


  1. Again, this stuff gets even more problematic in older cities, where many of the existing houses don't have the required parking, and there may not be enough street parking to match the level of demand (at least at a price of zero). So new construction is forced to make up the difference, except of course everyone wants to park for free on the street anyway. The existing residents hate that the newcomers are taking "their" parking and complain until minimum parking requirements are raise. New construction ends up with ever larger and emptier garages at tremendous added cost.

    1. I've actually talked about that dynamic before in a previous article, as an intro to how the Japanese have dealt with that problem through a proof of parking system:

      Parking is an issue in older urban neighborhoods, yes, because they weren't built with cars in mind, since cars didn't exist at the time, or were a rich man's oddity. So people with cars have to make do with less facilities than they're used to. There are other ways to get around that problem, I'm thinking of writing an article on ways of managing parking in cities. One of the big problem is that we try too much to manage the supply of parking, when managing demand for it could be just as effective and maybe yield better results.

  2. Really interesting info. Many thanks.