ADU Permits: Thornton vs Westminster
How do adu permits rules compare between Thornton, CO and Westminster, CO?
Thornton, CO
Adams County
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.
View full Thornton rules →Westminster, CO
Adams County
No data available yet for Westminster.
Key Facts Comparison
| Fact | Thornton | Westminster |
|---|---|---|
| Governing Code | Thornton Development Code + Code Ch. 18 (Zoning) | - |
| Building Code | 2021 IRC via Code Ch. 10 | - |
| Units per Lot | One ADU per single-family | - |
| State Preemption | HB24-1152 (eff. 6/30/2025) | - |
| Subject Jurisdiction | Yes (DRCOG MPO) | - |
| Review Path | Administrative under HB24-1152 | - |
Highlighted rows indicate differences between cities.
Thornton FAQ
Can I build an ADU in Thornton right now?
Yes, on most single-family lots, though the path depends on your zone. The Thornton Development Code allows ADUs in defined residential districts subject to size, setback, and accessory-use rules; some configurations required conditional review historically. Colorado HB24-1152 took effect June 30, 2025 and requires Thornton to provide an administrative approval pathway for at least one ADU per single-family lot. Thornton is updating the TDC; contact City Development for current submittal requirements.
How long does plan review take?
A first round at the Thornton Building Inspection Division typically runs 4-6 weeks for a residential ADU. Comments are returned for revision; second rounds are usually 2-3 weeks. Total time from submittal to permit issuance averages 8-12 weeks. Trade permits (electrical, mechanical, plumbing) are pulled separately after the building permit issues. Energy compliance follows the 2021 IECC adopted in Thornton.
Westminster FAQ
No FAQs available.
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