Knowledge Base

Are accessory dwelling units, also called "mother-in-law" apartments, allowed in Thornton?

Accessory dwelling units are permitted for single-family detached dwellings subject to the following conditions:

  1. Any request to incorporate accessory dwelling units in a new development must be received by the City at the time of application for a CSP or PD Zoning.
  2. Accessory dwelling units may be located within the principal dwelling unit, attached to the principal dwelling unit, or located separately on the same lot as the principal dwelling unit.
  3.  Accessory dwelling units shall comply with all established setbacks for the principal dwelling unit.
  4. Only one accessory dwelling unit is allowed per lot.
  5. Accessory dwelling units shall utilize the same utility connections as the primary residence.
  6. Any accessory dwelling unit shall meet the same development standards required for the principal dwelling unit.
  7. A certificate of occupancy (CO) will only be granted to an accessory dwelling unit after a CO has been granted to the principal dwelling unit.
  8. Required parking shall be located on the same lot as the principal dwelling unit and identified on the site plan required as part of a minor development permit.
  9. The minimum size for an accessory dwelling unit is 500 square feet; the maximum size for an accessory dwelling unit shall be 1,000 square feet or 50% of the gross floor area of the primary residence, whichever is less.
  10. The property owner must occupy either the principal dwelling unit or accessory dwelling unit.
  11. Home occupations are prohibited within an accessory dwelling unit.
  12. Manufactured homes, campers, travel trailers, and recreational vehicles are prohibited as accessory dwelling units.
  13. Prior to the issuance of a building permit, the property owner shall file with the county clerk and recorder a declaration of restriction to the deed for the property where the accessory dwelling unit will be located.

For more information on Accessory Dwelling Units, contact the City Development Department at 303-538-7295.

Updated 10/17/2023 1:32 PM
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