Mount Prospect, IL Building Safety: Anti-Mansionization (2026)

Section 14.905 E caps floor area ratio in the R-1 single-family district: FAR shall not exceed 0.5. The code does not use the word mansionization. Its tool against oversized houses is FAR, which § 14.2401 defines as the number of square feet of floor area in all buildings on a lot divided by the square feet of area of that lot. The ratio counts floor area in all buildings on the lot, not just the house.

The caps by district are: R-X (§ 14.805 E) 0.35; R-1 (§ 14.905 E) 0.5; R-A (§ 14.1005 E) 0.5; R-2 (§ 14.1105 F) 0.5; and R-3 (§ 14.1205 F) 0.5. Height limits sit beside the ratio. A residential building in R-X cannot exceed forty feet, and in R-1 and R-A thirty five feet. The R-1 and R-A histories list Ord. 6528, 9-2-2020 among their amendments.

Accessory buildings feed into the ratio. Section 14.306 B.2.d requires all solid roofed accessory structures, including gazebos, to be included in any Floor Area Ratio calculation. Section 14.306 B.1 limits a detached private garage to 672 square feet and a shed, gazebo, pergola, greenhouse or similar accessory structure to the smaller of two percent of the lot area or 200 square feet.

Garage design is also limited. Section 14.906 provides that no garage serving a single-family detached dwelling and fronting on a public right of way can have more than one of the following: three single garage doors, a combination of a single and double garage door, or a triple garage door. Impervious surface caps in § 14.905 C apply alongside the FAR limit.