How to Choose an ADU Contractor in Colorado (Questions to Ask)
How to hire an ADU contractor in Colorado, the questions to ask, the red flags to avoid, how licensing works here, and modular versus site-built compared.
Longmont’s real rules, taxes and rents — read from the codified ordinance, every figure sourced and dated. Here: a HUD benchmark rent of $1,795/mo for a 1-bed. Run your own numbers below, free.
Read from the Longmont Land Development Code itself — the operative legal text, not a website summary. Longmont is an HB24-1152 subject jurisdiction: since June 30, 2025, state law requires it to allow an ADU wherever it allows a single-family home, with administrative approval — the local standards below still apply.
| Maximum ADU size | 800 sq ft standard cap (or up to 50% of the primary dwelling's above-ground floor area if larger); no size limit applies if the ADU is located in a basement of the principal dwellingverified September 2026Longmont Land Development Code §15.02.080the ordinance textMinimum and maximum size: Regardless of the principal dwelling's size, an ADU may be between 500 and 800 square feet. 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. Basement exception: If the ADU is located in a basement of the principal dwelling structure, the size restriction does not apply. |
| Minimum ADU size | 500 sq ft (does not apply to an ADU in the main home's basement)verified October 2026Longmont Land Development Code §15.02.080the ordinance textMinimum and maximum size: Regardless of the principal dwelling's size, an ADU may be between 500 and 800 square feet. |
| Height limit | Detached ADU: no taller than the main home (height measured per Sec. 15.03.080.D)verified October 2026Longmont Planning confirmed this reading in October 2026.Longmont Land Development Code §15.02.080the ordinance textc. 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. |
| Owner occupancy | No ongoing owner-occupancy rule; when you apply, you must show proof that you live in Longmont (not required when the ADU is built at the same time as a new main home)verified October 2026Longmont Planning confirmed this reading in October 2026.Longmont Land Development Code §15.02.080the ordinance text2. Proof of residency. a. At the time of application, the applicant shall provide the following documentation to prove residency in the City of Longmont: i. A valid Colorado driver's license or a valid Colorado state identification card. ii. At least two of the following additional documents: [...] Exception: Proof of residency is not required when the ADU is being constructed simultaneously with a new primary dwelling unit. |
| Short-term rental | Noverified September 2026Longmont Land Development Code §15.02.080the ordinance textAccessory dwelling units shall not be used as a short-term rental. |
| Extra parking | None beyond the main houseverified September 2026Longmont Land Development Code §15.02.080the ordinance textAn accessory dwelling unit shall not provide more than two off-street parking spaces and is not required to provide any off-street parking. |
| Setbacks | Rear 5; side 5 (new detached ADUs and attached ADUs); ADUs placed in an existing detached structure are exempt from these setbacks unless the structure is expandedverified September 2026Longmont Land Development Code §15.02.080the ordinance textNew detached ADU: (B) Rear: five feet. (C) Side: five feet. Attached ADU: (B) Rear: five feet. (C) Side: five feet. ADU located in existing detached structure. Accessory dwelling units located in existing detached structures shall not be required to meet new setback standards unless such structure is expanded. |
Source: Longmont Land Development Code, §15.02.080 · ordinance text retrieved 2026-08-25, re-verified October 2026 · open “the ordinance text” under any figure to read the exact sentence it comes from.
Longmont’s size cap is conditional rather than a single number: 800 sq ft standard cap (or up to 50% of the primary dwelling's above-ground floor area if larger); no size limit applies if the ADU is located in a basement of the principal dwelling — worth reading closely before sizing a unit.
Short-term rental of the ADU is not permitted, so the model here is long-term tenancy or family use. No extra parking is required for the ADU.
And Longmont is one of only 30 Colorado jurisdictions certified ADU Supportive, which makes residents eligible for CHFA’s state-backed ADU financing — an interest-rate buydown and credit enhancement offered through participating lenders. Most of the Front Range doesn’t have this lever.
What an ADU adds to the tax bill in Longmont, and the federal benchmark for what it rents for.
Levies: Boulder County Assessor - Mill Levy Changes 2024 to 2025 (xlsx), sheets 'Changes_by_Tax_Area' and 'Changes_by_Authority' — example: Boulder County tax area 0680; levies differ between tax areas inside Longmont; check your parcel with the county assessor. Rates: Colorado DOLA, TY2026 — the 10% reduction on the first $700,000 of value applies to the local-government portion, once per property. Rents: HUD Fair Market Rents, huduser.gov.
Two ways to build the same unit in Longmont. Both figures are all-in — design, permits, foundation, utility connections and installation.
Published Front Range ranges for a comparable one-off build, 2025–26
≈ $2,100–$2,600 / month fully financed
In Longmont, the 490 is below Longmont's 500 sq ft minimum ADU size (the minimum does not apply to a basement ADU)
≈ $2,800–$3,500 / month fully financed
Fits Longmont’s size rule
Olerra ranges: published all-in project pricing, September 2026 — site preparation, foundation, utility connections and permitting included.
Permit, utility and tax charges for an ADU depend on your parcel, so we name the agency and link its official schedule rather than guess a number. Olerra’s all-in price already includes them; the calculator below itemizes any amount that has been confirmed.
| Building permit & plan review | Longmont Building ServicesThe fee for a factory-built ADU is not confirmed yet; this agency sets it for your parcel.building permit fee schedule (FY2026) |
| Water & sewer connection | City of Longmont Water DepartmentTap and connection charges for an ADU are not confirmed yet; they differ if the ADU shares the main home’s service.tap / connection fees |
| Construction use tax | Longmont taxes building materialsWhether it applies to a factory-built ADU, and its rate and taxable basis, are not confirmed yet.use tax information |
Official sources checked October 25, 2026. Water, sewer and tap charges also depend on whether the ADU shares the main home’s service — the provider confirms which applies.
Free and specific to you. Want to go further?
Yes. The Longmont Land Development Code permits accessory dwelling units (§15.02.080), subject to the size, height, setback and use rules quoted above from the ordinance text. Longmont is also an HB24-1152 subject jurisdiction, so since June 30, 2025 state law requires it to allow an ADU wherever it allows a single-family home, with administrative approval; the local standards above still apply. Zoning district, lot size and setbacks still decide what fits on a specific lot — the free property check confirms yours.
Yes. An ADU is a new dwelling unit, so Longmont requires a building permit (plus any zoning or site-plan review the code calls for) before construction starts. Building permits for Longmont addresses are handled by Longmont Building Services; water and sewer service come from City of Longmont Water Department (the local fees table above links their official schedules). Olerra's all-in price includes permitting, and factory-built units are inspected to the same building code as a site-built home.
Per the Longmont Land Development Code: 800 sq ft standard cap (or up to 50% of the primary dwelling's above-ground floor area if larger); no size limit applies if the ADU is located in a basement of the principal dwelling. The Olerra Flex Flat 490 is below Longmont's 500 sq ft minimum ADU size (the minimum does not apply to a basement ADU); the 735 fits under this rule. The free property check confirms what fits on your lot.
No — the ordinance does not permit short-term rental of an ADU here, so plan on long-term tenancy or family use.
Per the ordinance: No ongoing owner-occupancy rule; when you apply, you must show proof that you live in Longmont (not required when the ADU is built at the same time as a new main home).
A one-off site build with a general contractor typically runs about $500,000–$650,000 for a ~490 sq ft unit. Olerra's factory-built Flex Flats run $300,000–$365,000 (490) and $420,000–$465,000 (735) all-in — site preparation, foundation, utility connections and permitting included.
Roughly $678 per year for every $100,000 of value the ADU adds (for a home already worth $700,000 or more; a little less below that, because 10% of the first $700,000 of value is exempt from the local portion), in the example tax area shown above: Colorado's TY2026 school (7.05%) and local-government (6.80%) assessment rates on the latest certified (TY2025) levy, a mixed-year estimate. Levies differ between tax areas, so check your parcel with the county assessor.
Same method, each city’s own ordinance. The Colorado hub lists every city, and the statewide ADU calculator covers any Colorado address.
Guides and real Colorado numbers from our team — a fresh pick every couple of weeks, or search for exactly what you need.
How to hire an ADU contractor in Colorado, the questions to ask, the red flags to avoid, how licensing works here, and modular versus site-built compared.
Latest articles
Planning departments and journalists are welcome to reference this data.
Olerra ADU Cost Calculator — Longmont, CO. Rules last verified October 29, 2026 against the codified ordinance; tax example: tax year 2025 levy, collected 2026 (Boulder County tax area 0680); page released October 29, 2026. https://www.olerra.com/adu-calculator/longmont/ <a href="https://www.olerra.com/adu-calculator/longmont/">Longmont ADU rules & costs</a> Page released October 29, 2026 · rules last verified October 29, 2026 · the oldest dated figure on the page is from August 25, 2026.
Planning estimates, not quotes or legal advice. Rules, fees, levies and rents change — confirm with the city and your lender before committing. Rules data: codified ordinance as of August 25, 2026, re-verified October 2026. Olerra pricing: published all-in project ranges, September 2026. Corrections: sales@olerra.com.
Built for your location and what you want to do.
We'll be in touch before then — a confirmation is on its way to your email.
Thanks — we’ll reply to your email, usually the same day.
Our team is already on it — here’s what happens next.
