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: rents floored at $1,795/mo. 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, Colorado law guarantees one ADU by right on any lot with a single-family home here.
| 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. |
| Size relative to the main house | 50%verified September 2026Longmont Land Development Code §15.02.080the ordinance textMaximum 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. |
| 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 September 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.
The cap is also relative: an ADU here may not exceed 50% of the main house, so a smaller primary home means a smaller buildable ADU.
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 conservative floor on what it rents for.
Levies: Boulder County Assessor - Mill Levy Changes 2024 to 2025 (xlsx), sheets 'Changes_by_Tax_Area' and 'Changes_by_Authority'. 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
≈ $2,800–$3,500 / month fully financed
Olerra ranges: published all-in project pricing, September 2026 — site preparation, foundation, utility connections and permitting included.
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 one ADU to be allowed on any lot with a single-family home. 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. 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. Both Olerra Flex Flats — the 490 and the 735 — are designed to fit typical Colorado caps; check the rules table above for the exact ordinance text.
No — the ordinance does not permit short-term rental of an ADU here, so plan on long-term tenancy or family use.
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, using the local mill levy split across Colorado's TY2026 school (7.05%) and local-government (6.80%) assessment rates.
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.
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Planning departments and journalists are welcome to reference this data.
Olerra ADU Cost Calculator — Longmont, CO. Data updated August 25, 2026. https://www.olerra.com/adu-calculator/longmont/ <a href="https://www.olerra.com/adu-calculator/longmont/">Longmont ADU rules & costs</a> Data as of August 25, 2026, the oldest figure on the page.
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 September 2026. Olerra pricing: published all-in project ranges, September 2026. Corrections: sales@olerra.com.
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