Thornton ADU Zoning Rules & Permit Guide

ADU Regulations in Thornton, Colorado

Updated September 29, 2026

Adams County's largest city, Thornton regulates ADUs through its municipal code. Without statewide ADU legislation, the city imposes a 6,000 sq ft minimum lot size and requires property owners to occupy the residence.

Below is a summary of the key zoning rules applicable to Accessory Dwelling Units in Thornton as of 2026. Regulations are governed by local zoning ordinances, as Colorado has no statewide ADU mandate (HB 24-1152 addresses transit density).

Additional Local Requirements for Thornton

• Minimum lot size: 6,000 sq ft
• Minimum ADU size: 200 sq ft
• Owner must occupy property
• Building permit and utility connections required

Frequently Asked Questions About ADUs in Thornton

Can I build an ADU in Thornton?

Thornton permits ADUs in residential neighborhoods. Verify the specific zone and minimum lot size before applying.

What is the maximum ADU size allowed in Thornton?

ADUs in Thornton can be up to 1,000 square feet, subject to the property's zoning and lot coverage limits. Always confirm the exact allowed size with the Thornton planning department before designing your project.

What are the setback requirements for an ADU in Thornton?

ADUs in Thornton generally require setbacks of 5 ft rear, 5 ft side. Colorado has no statewide ADU mandate, so the exact setback figures come from Thornton's own zoning code — confirm them with the city planning department before you design.