“Many of our best neighborhoods would be illegal to build today.” This single sentence offers powerful insight into the challenge facing American cities, exposing, in one breath, the perverse restrictions involved in conventional zoning and subdivision regulation. In many old neighborhoods, built form, lot dimensions, and land uses are "non-conforming", meaning that they are non-compliant with land use regulation. One sees this line frequently across the urbanism blogosphere, usually coupled with an explanation of how outstanding neighborhoods like the French Quarter or Beacon Hill fly in the face of conventional urban planning.

But the problem is worse. Forget about the superstar neighborhoods—even most run-of-the-mill inner suburban neighborhoods would be next to impossible to build today. You probably have a few such neighborhoods in your city—neighborhoods just outside of downtown where structures cover most of the lot, where lots are perhaps slightly narrower and smaller, and where single-family homes casually mix with other uses. Understanding this variation is key to freeing up developers to replicate what works in new neighborhoods and to unlocking the potential of some of our best existing neighborhoods.

Let’s take an example from my hometown of Lexington, Kentucky. Kenwick is a wonderful inner suburban neighborhood to the southeast of downtown. The neighborhood was originally built out in the 1920s and 1930s, though waves of teardowns and rebuilding—particularly post-World War II—have gradually changed the appearance of parts of the neighborhood. While Kenwick was once serviced by the private streetcar, car-ownership was fairly widespread at the time this neighborhood was built. The neighborhood’s streets follow a tried-and-true grid pattern, with blocks measuring 100 meters in depth and 350 meters in length.