ADU rules in Thornton

Thornton allows detached ADUs (backyard cottages, converted outbuildings, units above garages) as a permitted accessory use to a single detached home (the code's Dwelling, 1 Unit Detached use) in the RE Residential Estate, RL Residential Low Density, RM Residential Mid Density, RH Residential High Density, AG Agricultural, and SFD-L Single-Family Detached Legacy districts. Only one ADU is allowed per lot, and once an ADU is added the principal dwelling may keep no more than one kitchen. Manufactured homes, campers, travel trailers, and recreational vehicles are prohibited as ADUs. These rules sit in Section 18-339 of the Development Code that Thornton repealed and reenacted effective September 1, 2025.

ADUs may be located within the principal dwelling (for example a basement unit) or attached to it as an addition, in the same districts where detached ADUs are permitted. An attached or internal ADU must have its own exterior entrance separate from the primary dwelling and receives its own address (the lot is addressed as Unit 1 for the house and Unit 2 for the ADU). The unit must provide independent living facilities for living, sleeping, eating, cooking, and sanitation with no interior access to the primary unit.

Section 18-339(k) caps an ADU at 1,000 square feet or 50 percent of the gross floor area of the primary residence, whichever is greater, and excludes any garage, porch, or similar area from the calculation. ADUs are exempt from the general rule that an accessory use may occupy no more than 5 percent of the lot or main floor area. Detached ADUs in most residential districts are limited to the accessory structure height of 16 feet with 5 foot side and rear setbacks, but an ADU built above a garage may use the principal structure height (typically 35 feet); the RE estate district instead allows 30 feet for accessory structures with 15 foot setbacks. The reenacted code sets no minimum ADU size; the 500 square foot minimum still shown on one city webpage is from the pre-September 2025 code.

Section 18-339(l) requires the property owner, as shown in title records and evidenced by voter registration, vehicle registration, or similar means, to occupy either the principal dwelling or the ADU. A deed restriction recorded with the Adams County Clerk and Recorder before the building permit keeps the ADU's certificate of occupancy valid only while the owner of record occupies one of the two units. State law cuts against this: Thornton is a subject jurisdiction under HB24-1152 (population over 1,000 and within the Denver area MPO), and since June 30, 2025 C.R.S. 29-35-103(2)(b) bars such jurisdictions from requiring the ADU or any dwelling on the lot to be owner occupied, allowing the city only to verify that the owner resides on the parcel when the ADU application is submitted or for a short-term rental license. Thornton nonetheless kept the ongoing requirement in the Development Code it reenacted on September 1, 2025, so plan on owner occupancy in practice while the preemption question stands.

Table 18-650.1 requires one off-street vehicle space per accessory dwelling unit, on top of the two spaces per dwelling (one of them in a garage for new development) required for a single detached home, and Section 18-339(h) requires the ADU parking to be on the same lot and shown on the Development Plan site plan. In the residential districts, required parking must sit on a driveway or parking pad rather than on lawn areas. The city's ADU Handbook translates this to at least three spaces on the property with one in a garage. State law narrows enforcement: C.R.S. 29-35-103 lets a subject jurisdiction require designating an existing driveway, garage, or tandem space for the ADU, but it may compel construction of a new space only where the lot has no existing off-street parking, the district required parking for the primary dwelling as of January 1, 2024, and on-street parking is prohibited on the block.

An ADU is a permitted (P) accessory use in Table 18-334.1, and every city approval involved is an administrative staff decision with no public hearing. A building permit is always required, and before it issues the owner must record an ADU deed restriction with the Adams County Clerk and Recorder. Planning review is director-level: a Development Plan is needed for a detached ADU over 500 square feet that is visible from the right-of-way and for additions over 500 square feet (smaller or non-visible projects are exempt), while dwelling units otherwise go through Residential Design Review. Approval is also subject to verification of adequate water and sewer capacity, though a complying ADU is exempt from the separate water and sewer connection and separate meter normally required for structurally independent buildings (utility connection fees still apply).

Section 18-339 places no restriction on renting the ADU for 30 days or longer; because the owner must occupy one of the two units, the other unit is the one available to rent. Short-term rentals (stays under 30 days) require an annual city license under Section 18-46, limited to one license per owner and per property and issued only to the fee-title owner (the city's licensing page lists a $180 annual fee through the MUNIRevs portal), plus a sales and use tax business license. An ADU may operate as a short-term rental only when the principal dwelling is the owner's primary residence, and the ADU may not be short-term rented if any portion of the principal dwelling is also used as a short-term rental. More generally it is unlawful to operate a short-term rental anywhere that is not the licensee's primary residence, defined as the place where the licensee resides more than 50 percent of the year.