Thornton's STR ordinance (Code Sections 18-54 and 18-174) already requires short-term-rental use of an ADU to be at the licensee's primary residence, which functions as an owner-occupancy condition when an ADU is used as an STR. The Thornton Development Code historically conditioned some ADU types on accessory residential use rather than absolute owner occupancy. Colorado HB24-1152, operative June 30, 2025, preempts owner-occupancy requirements on ADUs in subject jurisdictions including Thornton; the city is amending the TDC to comply.
Two threads run through Thornton's owner-occupancy posture. First, the Thornton Development Code provisions on accessory dwelling units treat the ADU as accessory to the primary residence on the lot, which has historically been interpreted to require that at least one of the two units (primary or accessory) be occupied by the owner, particularly where the ADU was approved through a conditional or accessory-use review. Second, Thornton Code Sections 18-54 and 18-174 (the STR ordinance adopted by Ord. 2020-099) require that any short-term rental (including an STR operated in an ADU) be the licensee's primary residence, defined as the place where the licensee resides more than 50% of the calendar year. Colorado HB24-1152 (codified at C.R.S. 29-32-101 through 109, effective June 30, 2025) prohibits subject jurisdictions from imposing owner-occupancy requirements as a condition of building, permitting, or operating an ADU. Thornton sits within the DRCOG MPO and qualifies as a subject jurisdiction. The Thornton TDC amendment process must remove ADU-specific owner-occupancy conditioning to align with state law. The STR ordinance's primary-residence rule may continue to apply to STR operation generally, but cannot lawfully condition simple ADU ownership or long-term ADU rental on owner occupancy. Deed restrictions are not currently recorded with ADU permits.
Operating an ADU as a short-term rental without the licensee occupying the primary residence violates Thornton Code 18-54/18-174 and can result in STR license revocation and Municipal Court fines up to $2,650 per day under Section 38-1. Long-term ADU rental without owner occupancy is permitted under HB24-1152 and cannot be enforced. Code Compliance monitors listing platforms (Airbnb, Vrbo) for STR violations.
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See how Thornton's adu owner occupancy rules stack up against other locations.
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