Accessory Structures in Thornton, CO (2026)
9 verified accessory structures rules for Thornton, Colorado, sourced directly from the municipal code and official government pages.
Verified from official government sources
ADU Rules
Thornton regulates ADUs through local zoning. Colorado HB 24-1152 requires certain municipalities to allow ADUs. Rules vary by city.
Thornton ADU / Accessory Dwelling Rules
Some RestrictionsADU Permits
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.
ADU Permits in Thornton
Some RestrictionsADU Impact Fees
Thornton collects development impact fees for new dwelling units through the City Development Department and Thornton Water/Infrastructure Division. ADU permits typically trigger a building permit fee scaled to valuation under Chapter 10, plan-review fees, Thornton Water/sewer tap or System Development Fees, drainage and traffic impact fees per the annual Thornton Fee Schedule, and use-tax on construction materials at the Thornton sales tax rate. Thornton has not adopted a categorical ADU fee waiver, though HB24-1152 encourages waivers for small or affordable ADUs through the Colorado DLG ADU Supportive Jurisdiction program.
ADU Impact Fees in Thornton
Some RestrictionsADU Owner Occupancy
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.
ADU Owner Occupancy in Thornton
Some RestrictionsADU Rental Restrictions
Long-term rental of an ADU (30+ day leases) is allowed in Thornton with no city license beyond the ADU permit and no city-imposed cap on rental volume. Short-term rental (under 30 days) of an ADU requires a Thornton STR license under Code Sections 18-54 and 18-174 (Ord. 2020-099) at $180/year, $1M liability insurance, primary-residence operation, and collection of Thornton lodging tax plus the State of Colorado lodger's tax. HB24-1152 preempts ADU-specific rental volume caps as of June 30, 2025.
ADU Rental Restrictions in Thornton
Some RestrictionsShed Rules
Thornton allows small sheds without permits (typically under 120 to 200 sq ft). Larger structures need building permits. Setback requirements apply.
Thornton Shed & Outbuilding Rules
Few RestrictionsGarage Conversions
Thornton may allow garage conversions with permits. Colorado has no statewide garage conversion mandate. Replacement parking may be required.
Thornton Garage Conversion Rules
Some RestrictionsCarport Rules
Thornton requires permits for carport construction. Setback requirements, height limits, and lot coverage maximums apply.
Thornton Carport Construction Rules
Some RestrictionsTiny Homes
Thornton regulates tiny homes differently based on whether they are on a permanent foundation or on wheels. Zoning and minimum square footage requirements apply.
Thornton Tiny Home & Small Dwelling Rules
Some RestrictionsLooking for Adams County county-wide rules?
County ordinances apply to unincorporated areas and may supplement Thornton city rules.
Accessory Structures in Adams County →