This colorful mosaic represents Cambridge's 60 different zoning districts. Five of those are residential-only districts, the remainder are commercial, industrial, office, or mixed-use districts. The five residential zones are graded as "A", "B" and "C" zones (light and darker yellow) "A" zones allow only single-family homes. "B" zones allow only single or two-family homes. "C" zones allow single, two-family and multi-family housing.
The vast majority of homes in Cambridge’s neighborhoods could not be built today, and are only allowed because they were constructed before the current zoning ordinances took effect. The zoning standards were imposed on existing neighborhoods to discourage further density, and to effectively ban apartments in much of Cambridge.
As a result, almost all Cambridge houses are non-conforming and owners are required to seek variances or special permits to build additions, add accessory apartments, or even add a porch or a window in many cases!. This is why most developers and wealthy home-owners hire lawyers to navigate the zoning process for them. "N" zoning is designed to legalize most existing homes so that owners can apply for building permits without facing the delays, costs or rejections of our current zoning policies.
When properties are re-developed - in the case of badly deteriorated houses ("tear-downs") or the occasional vacant lot, owners are held to the same low-density standards.