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.
Boulder’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 Boulder Revised Code itself — the operative legal text, not a website summary. Boulder 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 | Detached: 800 sq ft (1,000 sq ft if affordable). Attached: up to 1,000 sq ft (1,200 if affordable) or half the main home's floor area, whichever is less, with a 750 sq ft floor for small homesverified September 2026Boulder Revised Code §9-6-3the ordinance textTable 6-3: Maximum Floor Area / Accessory Dwelling Unit / Affordable Accessory Dwelling Unit / Designated Historic Property / Attached / "One-half of the total floor area of the principal structure or 1,000 square feet, whichever is less, except that if the principal structure has less than 1,500 square feet of floor area, the maximum is 750 square feet." / "Two-thirds of the total floor area of the principal structure or 1,200 square feet, whichever is less, except that if the principal structure has less than 1,125 square feet of floor area, the maximum is 750 square feet." / Detached / "800 sq. ft." / "1,000 sq. ft." -- preceded by: "Maximum Floor Area: The accessory dwelling unit shall be limited to the maximum floor area set forth in Table 6-3. The board of zoning adjustment may grant a variance to this floor area requirement pursuant to Section 9-2-3, 'Variances and Interpretations,' B.R.C. 1981." |
| Size relative to the main house | Attached ADUs: 50% of the main home's floor area (two-thirds if affordable), subject to the caps above; detached ADUs use flat capsverified September 2026Boulder Revised Code §9-6-3the ordinance textTable 6-3, Attached row: "One-half of the total floor area of the principal structure or 1,000 square feet, whichever is less..." (market-rate) and "Two-thirds of the total floor area of the principal structure or 1,200 square feet, whichever is less..." (affordable). |
| Height limit | Detached: 20 ft, or 25 ft with a roof pitch of 8:12 or steeper; an attached ADU built into the rear setback area is capped at 20 ftverified September 2026Boulder Revised Code §9-6-3the ordinance text"Maximum Height: The maximum height of accessory buildings with a detached accessory dwelling unit shall not be greater than twenty feet. The city manager may modify this height standard if the building meets one of the following: (i) If the roof pitch is 8:12 or steeper, provided the building height does not exceed 25 feet; or (ii) If a legal existing accessory building is converted to a detached accessory dwelling unit, provided that no changes are proposed to the existing accessory building's height, floor area, or roof form." / "(iv) The maximum height of any portion of the building located within the area of the minimum rear yard setback established in Table 7-1 is 20 feet. The height of this portion of the building shall be measured to the uppermost point of the portion of the building with the accessory dwelling unit consistent with Section 9-7-5, 'Building Height,' B.R.C. 1981." |
| Short-term rental | Only for ADUs established before February 1, 2019 that hold a valid STR license, capped at 120 days per year — newer ADUs may not be short-term rentedverified September 2026Boulder Revised Code §10-3-6the ordinance textAn accessory dwelling unit or a principal dwelling unit on a detached dwelling unit lot or parcel with an accessory unit may not be rented as a short-term rental unless all the following requirements are met: (1) Both the accessory dwelling unit and the principal dwelling unit were legally established by February 1, 2019; (2) A current and valid short-term rental license exists for the unit; ... (5) An accessory dwelling unit may not be rented as a short-term rental for more than one hundred twenty days in any calendar year ... After February 1, 2019, no application for a new short-term rental license may be accepted. |
| Setbacks | Attached ADUs may extend into the rear yard to within 5 ft of the rear lot line (up to 500 sq ft of coverage in RR/RE/RL/RMX-1 zones); detached ADUs follow standard accessory-structure setbacksverified September 2026Boulder Revised Code §9-6-3the ordinance text"Rear Yard Setback: The minimum rear yard setback for principal buildings established in Table 7-1 does not apply to a principal building with an attached accessory dwelling unit provided the following standards are met: (i) Any floor area of the building located within the area of the minimum rear yard setback established in Table 7-1 is solely floor area of the accessory dwelling unit; (ii) The attached accessory dwelling unit portion of the building has a rear yard setback of five feet or more; (iii) In the RR, RE, RL, and RMX-1 districts, the maximum building coverage of the attached accessory dwelling unit portion of the building and any accessory buildings or structures within the area of the minimum rear yard setback established in Table 7-1 is 500 square feet; and (iv) The maximum height of any portion of the building located within the area of the minimum rear yard setback established in Table 7-1 is 20 feet." |
Source: Boulder Revised Code, §10-3-6 · 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.
Boulder’s size cap is conditional rather than a single number: Detached: 800 sq ft (1,000 sq ft if affordable). Attached: up to 1,000 sq ft (1,200 if affordable) or half the main home's floor area, whichever is less, with a 750 sq ft floor for small homes — worth reading closely before sizing a unit.
Short-term rental is conditional: Only for ADUs established before February 1, 2019 that hold a valid STR license, capped at 120 days per year — newer ADUs may not be short-term rented.
And Boulder 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 Boulder, 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 Boulder. 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 Boulder Revised Code permits accessory dwelling units (§10-3-6), subject to the size, height, setback and use rules quoted above from the ordinance text. Boulder 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 Boulder 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 Boulder Revised Code: Detached: 800 sq ft (1,000 sq ft if affordable). Attached: up to 1,000 sq ft (1,200 if affordable) or half the main home's floor area, whichever is less, with a 750 sq ft floor for small homes. 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.
It depends: Only for ADUs established before February 1, 2019 that hold a valid STR license, capped at 120 days per year — newer ADUs may not be short-term rented.
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 $619 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 — Boulder, CO. Data updated August 25, 2026. https://www.olerra.com/adu-calculator/boulder/ <a href="https://www.olerra.com/adu-calculator/boulder/">Boulder 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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