Boulder, CO Building Safety: Anti-Mansionization (2026)

C. 1981 is Boulder's direct answer to mansionization pressure on smaller residential lots. It applies to all construction of principal and accessory buildings, including new construction and expansions, in the RR-1, RR-2, RE, RL-1, and RMX-1 zoning districts. The bulk plane itself begins twelve feet above the side lot line, then rises at a forty-five-degree angle until it reaches either the zone's permitted building height or the mirror-image plane rising from the opposite side lot line. The city allows two ways to fix the starting elevation: the 'grade level point method,' measured from the natural grade at the midpoint between the front and rear yards, or the 'parallel points method,' using measurement points spaced ten feet apart along the side property line.

No building or portion of a building may extend beyond the resulting plane, though the section carves out limited encroachments: roof overhangs and eaves up to thirty inches, flush-mounted or shallow-angle rooftop solar systems, gable roof ends that project up to eight feet with a maximum forty-foot width, dormers limited to eight feet wide and six feet beyond the plane, chimneys no wider than seventy inches, and small appurtenances such as antennae. The rule does not apply everywhere: lots averaging less than forty-five linear feet wide, lots under four thousand square feet, and interior side yards next to a nonresidential use or multi-unit building within twenty feet of the property line are exempt. Landmark-alteration certificates under Chapter 9-11 can also authorize an encroachment for individually landmarked buildings or those in a historic district.