Accessory Structures in Longmont, CO

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 3 specific rules on the books covering different aspects of accessory structures, and some of them might surprise you.

Shed & Carport Rules

Longmont exempts small detached accessory structures, tool sheds, storage sheds, playhouses and similar buildings, from a building permit only when they are one story and the floor area is 200 square feet or less. Anything bigger triggers a full building permit under the 2024 International Residential Code as amended by the city, plus the zoning setback rules that apply to accessory structures on the lot.

Key details: Permit-exempt shed size: 200 sq ft (18.58 sq m) or less, one story. Covered uses: Tool sheds, storage sheds, playhouses, similar structures. Governing code: 2024 IRC as amended by Ord. No. O-2026-24. Effective date: June 9, 2026. Setback, over 120 sq ft: Minimum 5 feet from property line.

Building or altering a shed above 200 square feet without a permit is an unlawful act under Section R113.1, as replaced by Section 16.06.130, and it counts as a separate offense for every day the violation continues. Longmont prosecutes unpermitted construction under Chapter 1.12, which sets a default fine of up to $500.00 and up to 90 days in jail under Section 1.12.010 unless another Code section states a different penalty, and the city attorney can also seek a court injunction treating the unpermitted structure as a public nuisance.

ADU Rental Restrictions

Longmont bars accessory dwelling units from being used as short-term rentals outright, under Longmont Municipal Code section 15.02.080.D.2.c. A separate ADU standard in section 15.02.080.E.3.g also blocks selling an ADU apart from the main house, keeping accessory units tied to long-term, owner-linked occupancy rather than the short-term rental market.

Key details: Governing section: Longmont Municipal Code § 15.02.080.D.2.c; E.3.g. ADU short-term rental: Outright banned as a short-term rental. ADU sale rule: Cannot be sold separately from principal dwelling. Long-term rental: Ordinary long-term ADU rental remains allowed. Penalty: Up to $500 fine, 90 days jail (§ 1.12.010).

Operating an accessory dwelling unit as a short-term rental violates section 15.02.080.D.2.c directly; because ADUs are excluded from the dwellings eligible for a short-term rental license, no license can lawfully issue for one, making operation unlicensed activity under section D.3 and a development code violation under section 15.09.030. That carries the general penalty in section 1.12.010, up to $500.00 and 90 days in jail, plus civil penalties under chapter 2.97.

Compared to other cities, Longmont takes a harder line on adu rental restrictions. The enforcement and penalty structure reflects that.

ADU Rules

Longmont allows one accessory dwelling unit per single-family lot, sized between 500 and 800 square feet, with its own setback and building-permit requirements under the Land Development Code.

Key details: Units per lot: 1 ADU maximum. Size range: 500 to 800 sq ft. Rear/side setback: 5 feet. Permit: Building permit required.

Building or occupying an ADU without a permit, exceeding the size cap without qualifying for the 50-percent exception, or placing an ADU on a lot without a principal single-family dwelling violates § 15.02.080.E and can trigger enforcement under chapter 15.09.

The Bottom Line

Longmont's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Longmont is broadly strict or permissive.

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.