The new Colorado state laws for Accessory Dwelling Units (ADUs), primarily established under HB24-1152, introduce several statewide standards and implementation mandates that take effect in 2026. These regulations aim to streamline the approval process and increase the feasibility of building units on existing lots.

Key aspects of these new laws and their impact include:

• Implementation Mandates: The law provides statewide minimum building standards and may override certain local restrictions, such as parking requirements, to encourage development.
• Foundation and Safety Standards: The law mandates that foundations adhere to certified benchmarks, such as ICC-ES AC358. This ensures design capacity, corrosion protection, and structural safety across the state.
• Local Variations: While the state law sets a baseline, individual municipalities in the Denver metro area have specific codes that homeowners must follow:

1. Denver: Allows units up to 800 sq ft with a 5-ft setback and requires an engineered foundation.
2. Littleton: Caps units at 750 sq ft with a 10-ft side setback and recommends helical piers due to expansive clay soils.
3. Centennial: Permits larger units up to 850 sq ft but requires a 7-ft setback and a mandatory geotechnical report.

• Environmental Considerations: Due to Colorado’s expansive clay soils and a 36-inch frost depth, the regulations emphasize the need for professional site evaluations and engineered solutions like helical piers to ensure long-term stability.

• ADU Regulations Colorado 2026: Essential Guide for Homeowners