Accessory Structures in Longmont, CO: What Residents Actually Need to Know

If you live in Longmont or are thinking about moving there, accessory structures are one of those things you probably won't think about until they affect you directly. Longmont has 5 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

ADU Rules

Longmont adopted new ADU regulations on June 3, 2025, in LMC § 15.02.080.E to comply with Colorado HB24-1152. ADUs are allowed in R-RU, R-SF, R-MN, R-MF, MU-D, and N-AG zones; an ADU is capped at 800 sq ft unless it does not exceed 50% of the principal dwelling's above-ground floor area; the owner must reside in the main house or the ADU; sale of the ADU separately from the main home is prohibited.

Key details: Code Section: LMC § 15.02.080.E (also § 15.04.040). Ordinance Adopted: June 3, 2025 (HB24-1152 compliance). Max Size: 800 sq ft, or up to 50% of principal dwelling above-grade area. Allowed Zones: R-RU, R-SF, R-MN, R-MF, MU-D, N-AG. Owner-Occupancy: Owner must live in main house or ADU.

Building or occupying an ADU without permits is a Title 15 and Title 16 violation enforced by Code Enforcement and Building Inspection: up to $2,650 fine and/or up to 364 days imprisonment, plus a stop-work order and an order to remove the unpermitted structure.

Garage Conversions

Converting a garage into habitable space in Longmont requires a building permit from Building Inspection (Title 16) and zoning approval under the Land Development Code (Title 15). If the conversion creates a second dwelling unit, it must comply with the ADU standards in LMC § 15.02.080.E. Off-street parking requirements for the principal dwelling must still be met.

Key details: Building Permit: Required (LMC Title 16 — IRC). ADU Standards Trigger: Cooking facilities + separate entrance → LMC § 15.02.080.E. ADU Size Cap: 800 sq ft, or up to 50% of principal dwelling. Off-Street Parking: Principal dwelling parking must be replaced on-site. Owner-Occupancy (if ADU): Required.

Converting a garage without permits is a Title 15 and Title 16 violation: stop-work order, double permit fees, up to $2,650 fine and/or up to 364 days imprisonment, and order to restore the garage to its permitted condition or bring it up to code.

Shed Rules

Longmont does not require a building permit for a one-story shed under 120 sq ft of floor area, but anything 120 sq ft or larger requires a building permit and engineered foundation. Sheds and other detached accessory structures must sit at least 5 feet from rear and side property lines and behind the front facade of the principal building.

Key details: Permit-Exempt Size: Under 120 sq ft, one-story. Permit Required At: 120 sq ft or larger (engineered foundation). Setback (rear/side): 5 ft minimum from property lines. Location Rule: Behind front facade of principal building. Total Accessory Coverage: Up to 75% of principal structure floor area.

Building without a required permit is enforced by Building Inspection and Code Enforcement under LMC Title 16 and Title 15: stop-work order, double permit fees, up to $2,650 fine and/or up to 364 days imprisonment, and order to remove or modify the noncompliant structure.

If you are coming from a city with tighter rules, you will find Longmont gives residents more flexibility on shed rules.

Carport Rules

Carports in Longmont are detached accessory structures regulated by the Land Development Code (Title 15) and the building code (Title 16). A building permit is required for any carport with a footprint over 120 sq ft. Carports must sit at least 5 feet from rear and side property lines and behind the front facade of the principal dwelling.

Key details: Permit Threshold: 120 sq ft (any larger requires permit). Rear/Side Setback: 5 ft (detached accessory). Location: Behind front facade of principal building. Front-Yard Carport: Prohibited. Enclosing into Garage: Separate permit required (full IRC garage standards).

Building a carport without a required permit triggers a stop-work order and double permit fees. Setback or location violations are enforced under LMC Title 15: up to $2,650 fine and/or up to 364 days imprisonment per the general penalty.

The rules around carport rules in Longmont lean permissive, but that does not mean anything goes.

Tiny Homes

Tiny homes on a permanent foundation in Longmont are treated as either a single-family dwelling or an accessory dwelling unit depending on use, and must comply with the building code (LMC Title 16, adopting the IRC including Appendix Q for dwellings under 400 sq ft) and the Land Development Code (Title 15). Tiny homes on wheels (RVs) cannot be used as a primary residence in residential zones.

Key details: Building Code: LMC Title 16 — IRC + Appendix Q for dwellings ≤400 sq ft. Tiny Home as ADU: Permitted (≤800 sq ft, LMC § 15.02.080.E). Tiny Home as Primary Dwelling: Permitted on permanent foundation if zone standards met. Tiny Home on Wheels (RV): Not permitted as residence in residential zones. Required Utility Connections: City water & sewer (ADUs share principal home's taps).

Living in an RV-classified tiny home in a residential zone is a Title 15 violation enforced by Code Enforcement: up to $2,650 fine and/or up to 364 days imprisonment, plus an order to vacate. Building a site-built tiny home without permits triggers Title 16 building enforcement and a stop-work order.

The Bottom Line

Compared to many U.S. cities, Longmont gives residents more room on accessory structures. 2 of the 5 rules here are rated permissive. But permissive does not mean unregulated. There are still requirements, and the city does enforce them when violations are reported.

This guide is based on Longmont's current municipal code. Local rules can and do change, so check the individual ordinance pages for the latest details, penalties, and FAQs.