ADU & Garage Apartment Longmont CO — What’s Allowed & What It Costs
Accessory Dwelling Units (ADUs) are one of the hottest trends in Northern Colorado real estate — and Longmont has some of the most permissive ADU rules in Boulder County. Whether you want a garage apartment, a detached backyard cottage, or a basement in-law suite, Josh Brooks Construction and Renovation Inc. builds ADUs throughout Longmont and the surrounding communities.
What Is an ADU in Longmont?
An ADU is a secondary housing unit on a single-family residential lot. In Longmont, ADUs can take several forms:
- Attached ADU: Connected to the main home, often a converted garage or home addition
- Detached ADU: A separate structure on the same lot — garage apartment, backyard cottage, or carriage house
- Interior ADU: A finished basement or converted space within the existing home footprint
- Junior ADU (JADU): Smaller unit (under 500 sq ft) within the primary home
Longmont ADU Rules — What’s Allowed
The City of Longmont allows ADUs on most single-family residential lots. Key requirements include:
- Minimum lot size requirements apply depending on zone
- Owner-occupancy required in most zones (owner must live in primary home or ADU)
- Maximum ADU size: typically 850–1,000 sq ft or 50% of primary home, whichever is less
- Separate utility meters may be required for rental ADUs
- Building permit required — we handle all permitting
ADU Construction Costs in Longmont 2026
- Basement ADU conversion: $45,000–$90,000 depending on existing condition and finishes
- Garage apartment above existing garage: $80,000–$150,000
- Detached backyard ADU (new construction): $150,000–$280,000
ADUs typically return their investment within 5–8 years through rental income, and add significant resale value to the property.
Why Josh Brooks Construction for Your Longmont ADU
As a full-service general contractor, we manage the entire ADU process — design consultation, permit application, foundation, framing, plumbing, electrical, HVAC, insulation, drywall, flooring, kitchen, and bath. One contract, one contractor, one point of accountability. We’ve built ADUs, basement apartments, and garage conversions throughout Longmont, Boulder, Erie, and Northern Colorado.
Start Your ADU Project Today
Veteran-owned. Boulder County Licensed CON-25-0089. Senior and Veteran discounts available.
Call 720-828-7997 | 2167 Frontier St, Longmont, CO 80501 | joshbrooksconstructionllc.com