Thornton regulates accessory dwelling units under the Thornton Development Code (TDC) and Code of Ordinances Chapter 18 (Zoning), with building permits issued under Chapter 10 (Building Code) which adopts the 2021 IRC. ADUs historically required conditional review in most residential districts; effective June 30, 2025, Colorado HB24-1152 preempts those restrictions and requires Thornton, as a DRCOG MPO subject jurisdiction, to allow at least one ADU on every single-family lot through an administrative-only process. The Thornton City Development Department is updating the TDC to comply.
Thornton's current ADU framework lives in the Thornton Development Code and the Code of Ordinances Chapter 18 (Zoning). Under the long-standing rules, accessory dwelling units have been allowed in limited residential zone districts subject to standards such as one ADU per single-family lot, accessory-only use (no separate sale), conformance with the lot's setbacks, and a size cap typically tied to a percentage of the primary dwelling. Building permits are issued by the City Development Department's Building Inspection Division at the Thornton Civic Center under Code of Ordinances Chapter 10, which adopts the 2021 International Residential Code with Thornton amendments. The package needs a site plan, floor plans, elevations, energy compliance to the 2021 IECC, and confirmation that the lot has adequate Thornton Water service. Colorado HB24-1152 (signed 2024, operative June 30, 2025; codified at C.R.S. 29-32-101 through 109) requires subject jurisdictions inside the DRCOG MPO to allow one ADU on every single-family lot through an administrative review path, prohibit owner-occupancy mandates, prohibit ADU-specific parking minimums, and may not impose more burdensome standards than for the primary dwelling. Thornton qualifies as a subject jurisdiction and is amending the TDC to comply. Plan review typically runs 4-6 weeks for a first round.
Building an ADU without permits is a violation of Code of Ordinances Chapter 10 (Building Code) and Chapter 18 (Zoning), prosecuted in Thornton Municipal Court under Section 38-1 of the Code (general penalty) with fines up to $2,650 per day per offense. Code Compliance typically issues a Stop Work order, requires after-the-fact permitting at doubled fees, or removal of unpermitted construction. Unpermitted electrical, mechanical, or plumbing work must be exposed for reinspection.
Thornton, CO
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