McLean County, IL Accessory Structures: ADU Rules (2026)

McLean County treats an accessory dwelling as a 'guest household unit' under § 350-43A(3)(e), one of the residential accessory uses permitted on a lot in the unincorporated county alongside the principal single-family home. The ordinance allows exactly one such unit per lot, and unlike an ordinary bedroom suite it may include a kitchen facility separate from the kitchen in the principal household unit, the feature that functionally makes it an ADU rather than an addition. Four conditions apply. First, the guest unit has to be located within the same building as the principal dwelling unit, so a detached backyard cottage does not qualify under this subsection; it must be internal, such as converted space that is part of the house or an attached wing.

Second, access should run through the principal dwelling's own entrance; if the unit instead gets a separate exterior door, that door has to face a side yard or the rear yard, never the front of the lot. Third, the unit's livable floor area is capped at 900 square feet, regardless of the size of the principal home. Fourth, occupancy is restricted to guests of the family in the main house rather than the general public. Because the guest household unit is an accessory use, it also has to comply with the accessory-use development standards in § 350-43A(5): it cannot exceed the underlying zoning district's height limit, must respect the standard interior setbacks, and counts toward the lot's maximum building coverage. The Director of Building and Zoning reviews compliance.