Key Facts Ordinance Text Full Breakdown Violations FAQ Sources More in Longmont

Longmont, CO Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified August 2026

ADU rules in Longmont, CO, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

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

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Colorado under recent ADU streamlining laws. The Longmont, CO rules below cover local size, height, and setback requirements.

Summary

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.

b.Number and size of accessory dwelling units.i.Limit per lot: Only one ADU is allowed per lot, in addition to the principal single-family detached dwelling.ii.Minimum and maximum size: Regardless of the principal dwelling's size, an ADU may be between 500 and 800 square feet.iii.Maximum size exception: An ADU may be larger than 800 square feet if the proposed ADU does not exceed 50 percent of the finished, above-ground floor area of the principal dwelling.iv.Basement exception: If the ADU is located in a basement of the principal dwelling structure, the size restriction does not apply.c.Height.i.Accessory dwelling units located in detached structures shall not exceed the height of the principal structure as determined by section 15.03.080.D.d.Setbacks.i.New detached ADU. New detached accessory structures containing ADUs shall comply with the following setbacks from property lines:(A)Front: ADUs shall not be located closer to the front property line than the principal dwelling unit.(B)Rear: five feet.(C)Side: five feet. Exception: For detached ADUs on corner lots, the ADU may be located closer to the front property line than the principal dwelling unit as long as the ADU meets the setback requirements that are applicable to the principal dwelling structure for that zone district.

View official code

Full Breakdown

Longmont's Land Development Code permits one accessory dwelling unit per lot as an accessory to a single-family detached dwelling, built into the principal structure or into a detached structure like a garage. A standard ADU must fall between 500 and 800 square feet, though it can exceed 800 square feet if it stays under 50 percent of the principal dwelling's finished above-ground floor area, and the size cap does not apply to a basement ADU. Detached ADUs must sit no closer to the front property line than the principal dwelling and keep 5-foot rear and side setbacks. Mobile homes, RVs, and travel trailers cannot serve as ADUs, and a building permit is required before one is established.

Violations & Fines

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.

Frequently Asked Questions

Can I build more than one ADU on a Longmont lot?

No: only one ADU is allowed per lot in addition to the principal single-family dwelling, per § 15.02.080.E.3.b.

How big can a Longmont ADU be?

Generally 500-800 square feet, though it may exceed 800 sq ft if under 50% of the main home's floor area.

Sources & Official References

[1]Longmont Municipal Code (Municode)

Other rules in Longmont

Shed & Carport RulesSome Restrictions ADU Rental RestrictionsHeavy Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Noise Ordinances: Aircraft NoiseLight Restrictions Parking Rules: Driveway RulesSome Restrictions Parking Rules: Abandoned VehiclesSignificant Restrictions Parking Rules: Loading ZonesSome Restrictions Fence Regulations: Fence Permit RequirementsSome Restrictions

All Longmont rules

How Longmont compares: Easiest Cities to Build an ADU·Compare Longmont to another location·View the Colorado accessory structures overview

See something wrong?

Help us keep this page accurate. If you notice an error or outdated information, let us know.

ADU Rules in Nearby Cities

How other cities in Boulder County handle adu rules.

Boulder, CO