Building ADUs in Longmont
Backyard cottages, garage apartments and basement suites, built to Longmont’s ADU rules
Longmont ADU quick facts
- One ADU allowed per lot
- Up to 800 sq ft, or larger if it stays within 50% of the main house’s finished above-ground floor area
- No size cap for an ADU in an existing basement
- Usually no extra parking required
- Rentals of 30 days or more only
- HOAs can no longer prohibit ADUs
- City pre-approved stock plans expected in 2027
Thinking about an ADU in Longmont?
An accessory dwelling unit, or ADU, is a second, complete home on your lot: a backyard cottage, an apartment in or above the garage, or a basement suite with its own entrance. People build them for rental income, for an aging parent, for grown kids, or to downsize into while renting out the main house.
Longmont updated its ADU rules in June 2025 to match Colorado’s 2024 ADU law, and the City is now creating pre-approved ADU plans to make them faster and cheaper to build. We help Longmont homeowners figure out what fits on their lot and build it right.
St. Vrain Construction is a second-generation, licensed general contractor (Longmont license C01009653) serving Longmont, Boulder County and Weld County.
We get to you fast. We typically return calls the same or next business day and can usually walk your lot within 3 business days, so you know what fits before you spend money on plans.
Types of ADUs we build
🏡 Detached Backyard ADU
- A separate cottage in your yard
- Can’t be taller than the main house
- 5 ft from side and rear property lines
- Can’t sit closer to the street than the main house
🚗 Garage ADU
- Inside, above or attached to a garage
- Garage and ADU square footage count together
- An existing detached garage doesn’t have to meet new setbacks unless it’s expanded
🏠 Basement or Attached ADU
- A suite in an existing basement or an addition to the house
- No size cap for a basement ADU
- Attached ADUs need 5 ft side and rear setbacks
- See our basement finishing page
Longmont’s pre-approved ADU plans: what we know
The City of Longmont is creating a set of ADU stock plans. Each plan will be reviewed in advance against the City’s building and fire codes, so homeowners who use one can skip much of the architect cost and get a faster review. The City is also building an incentive program to go with the plans.
- When: The City expects the plans to be available in 2027.
- Who pays for it: The program is funded by a state ADU grant from the Colorado Department of Local Affairs, created under the 2024 ADU law (HB24-1152).
- What it won’t cover: A pre-approved plan covers the building, not your lot. Every project still needs site-specific work like a survey, soils report, engineered foundation, utility connections and a site plan that meets setbacks and size limits.
Should you wait? Not necessarily. If you want rental income or space for family soon, you can build now with custom plans. If you want to use a stock plan, we can do the lot check, survey and soils work now so you’re ready to go when the plans are released. We’re following the program closely and will update this page as the City shares details.
Longmont ADU rules at a glance
- Who can apply: Longmont residents who own a legally permitted detached home in a zone that allows ADUs, which includes most residential zones.
- How many: One ADU per lot.
- Size: Up to 800 square feet, or larger if it doesn’t exceed 50% of the main house’s finished above-ground floor area. All accessory structures together, including a detached ADU, can’t exceed 75% of the main house’s floor area, so an old shed may need to come down. An ADU in an existing basement has no size cap.
- Height: A detached ADU can’t be taller than the main house.
- Parking: Usually no additional parking is required.
- Renting and selling: An ADU can be rented for 30 days or more, but not as a short-term rental, and it can’t be sold separately from the house.
- HOAs: HOAs can no longer prohibit ADUs. Submitting plans for HOA review is still recommended.
- Access: A 3-foot all-weather walkway must connect the ADU entrance to the public sidewalk.
- City fees: The City estimates its fees typically run $5,000 to $10,000, including plan review, permit fees, taxes and community investment fees.
- Historic homes: If the main house is a designated landmark, Historic Preservation approval is also required.
Based on the City of Longmont ADU Guide, updated December 2025. Rules change, so we confirm current requirements for every project. See the City of Longmont ADU page for the latest.
How we build your ADU
- We check your lot. Zoning, setbacks, utilities, existing sheds and garages, and size limits, so you know what fits before you spend money on plans.
- We talk about your goals and budget. Rental income, family, a home office or aging in place. Converting an existing basement or garage often costs far less than a new detached build, and we’ll tell you honestly which makes sense.
- We get the plans ready. Custom plans now, or a City stock plan once they’re available. [Your answer: who draws your ADU plans, you or a designer you work with?]
- We put together the permit package. Survey, soils report, engineered foundation, energy compliance and the rest of the City’s checklist, and we handle the plan check back-and-forth.
- We build it. Foundation, framing, utilities and finishes, with licensed electricians and plumbers where required and every City inspection scheduled.
- We walk it with you. A final walkthrough so the unit is ready for move-in.
What affects the cost of an ADU
- ADU type. Converting a basement or garage usually costs much less than building a new detached unit.
- Size and finishes. Square footage and the level of finish drive most of the cost.
- Utilities. Running water, sewer and power to a backyard unit can be a big line item, depending on distances and your existing service.
- Site conditions. Soils, slope, access and removing old structures.
- Design and engineering. Stock plans should reduce this once the City releases them.
- City fees. Typically $5,000 to $10,000, according to the City.
[Optional: typical price ranges for a basement ADU, garage ADU and detached ADU]
Frequently asked questions
We typically return calls the same or next business day and can walk your lot within 3 business days. We start the lot check and permit package right away. Construction begins once the City issues your building permit.
The City expects them in 2027. We’re following the program and will update this page as details are released.
It depends on your timeline and budget. You can build now with custom plans, or start the lot check, survey and soils work now so you’re ready when stock plans are available.
Yes, for stays of 30 days or more. Short-term rentals aren’t allowed in ADUs in Longmont.
To apply for an ADU permit, the City requires proof that you live in Longmont and own the detached home on the property.
No. HOAs can no longer prohibit ADUs, though the City recommends submitting your plans for HOA review.
Often, yes, and an ADU in an existing basement isn’t subject to the size limit. See our basement finishing page.
We take a 50% deposit to start, which covers materials. Larger jobs include a progress payment, and the balance is due when the work is done.
Find out what fits on your lot
Tell us about your property and what you want the ADU for, and we’ll walk you through your options, costs and timeline.
Or call (720) 310-0783