Longmont ADU Zoning Rules & Permit Guide

ADU Regulations in Longmont, Colorado

Updated September 29, 2026

In Boulder County, Longmont has an established ADU program under local ordinance. Colorado's lack of a statewide mandate means Longmont's municipal code sets all requirements for accessory dwelling units.

Below is a summary of the key zoning rules applicable to Accessory Dwelling Units in Longmont 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 Longmont

• Minimum lot size may apply
• Minimum 200 sq ft
• Owner occupancy may be required
• ⚠️ CO has NO statewide ADU mandate — verify with city

Frequently Asked Questions About ADUs in Longmont

Can I build an ADU in Longmont?

This Colorado city regulates ADUs through local ordinance. Confirm the minimum lot size and owner occupancy rules with the planning department.

What is the maximum ADU size allowed in Longmont?

ADUs in Longmont 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 Longmont planning department before designing your project.

What are the setback requirements for an ADU in Longmont?

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