An ADU builder in Boulder is a licensed contractor who designs and constructs accessory dwelling units — detached backyard units, garage conversions, attached additions, and junior ADUs (JADUs). Because an ADU is a complete second dwelling, the builder coordinates foundation, framing, plumbing, electrical, and HVAC work and pulls it through Boulder's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in Colorado.
What ADU Builders Cost in Boulder [11][12]
Based on our Boulder cost data, expect the following ranges from local ADU builders. Quotes for the same scope typically vary 20-40% between contractors, so getting at least three bids is the single best way to control cost.
| New Detached ADU (all-in) | $200,000-$450,000 |
| Cost per Sq Ft | $400-$700 |
| Garage Conversion | $100,000-$250,000 |
| Prefab / Modular ADU (installed) | $200,000-$425,000 |
| City Permit & Plan Review | Valuation-based (2026 schedule): $1,059 + $5.95 per $1,000 over $100,000, plus 25% plan check and $83.90 energy fee: about $1,700 for an 800 sq ft unit at the city's $170.80/sq ft floor, about $2,900 on a $300,000 contract. Use tax, trade permits, impact fees and excise tax are extra. |
| Impact Fees | Charged per new dwelling unit by size (2026 schedule, BRC 4-20-62): capital facility impact fees $4,599 (799 sq ft and below) or $5,784 (800-999 sq ft), plus the Transportation Development Excise Tax of $4,826.22 per detached unit or $3,501.50 per attached unit, paid before final inspection. Water and wastewater plant investment fees apply only if demand increases (ADUs share the house meter unless a separate hookup is approved, Ord. 8745). Credits and exemptions are in BRC 8-9-3; confirm your total with the city's Housing, DET and Impact Fee Worksheet. |
For a full line-item breakdown, see our Boulder ADU cost guide. To confirm what you're allowed to build before you hire, review the Boulder ADU rules (max detached size: 800 sq ft of floor area (BRC 9-6-3(n), Table 6-3); 1,000 sq ft for an affordable ADU (rent capped at 75% of area median income under a recorded declaration of use) or on a designated historic property. Floor area is measured to the outside of the exterior walls per Ch. 9-16 and includes stairways; space under a 6 ft ceiling is excluded.; owner-occupancy: Not required. Ord. 8650 (adopted Feb. 6, 2025, effective March 8, 2025) deleted the owner-occupancy standard from 9-6-3(n), and the city's ADU page answers 'NO' as of March 8, 2025. The ADU may not be sold separately from the house, and both units need a city rental licence if rented.).
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How to Vet an ADU Builder in Boulder
- License: Confirm a current contractor's license in Colorado and that it's in good standing. Ask for the license number and verify it with the state board.
- ADU-specific experience: Ask how many ADUs they've completed in or near Boulder and request addresses or photos. ADU permitting and setbacks differ from a kitchen remodel.
- Portfolio & references: Get references from recent ADU clients and, if possible, visit a finished unit.
- Insurance: Verify general liability and workers' compensation coverage. Ask for a certificate of insurance.
- Written, detailed bid: Insist on an itemized, fixed-price or clearly-scoped bid — not a verbal estimate. It should state whether permits (Valuation-based (2026 schedule): $1,059 + $5.95 per $1,000 over $100,000, plus 25% plan check and $83.90 energy fee: about $1,700 for an 800 sq ft unit at the city's $170.80/sq ft floor, about $2,900 on a $300,000 contract. Use tax, trade permits, impact fees and excise tax are extra.), plan check (25% of the building permit fee for single-family residential, paid at application and nonrefundable (2026 schedule, Sec. 4-20-4(f)7); the completeness check and plan-check invoice take about 6 business days.), and impact fees (Charged per new dwelling unit by size (2026 schedule, BRC 4-20-62): capital facility impact fees $4,599 (799 sq ft and below) or $5,784 (800-999 sq ft), plus the Transportation Development Excise Tax of $4,826.22 per detached unit or $3,501.50 per attached unit, paid before final inspection. Water and wastewater plant investment fees apply only if demand increases (ADUs share the house meter unless a separate hookup is approved, Ord. 8745). Credits and exemptions are in BRC 8-9-3; confirm your total with the city's Housing, DET and Impact Fee Worksheet.) are included.
- Permit handling: Confirm they manage Boulder's permit process ("Building permit only: since Sept. 1, 2023 (Ord. 8571) there is no separate ADU or administrative review. Apply in the Customer Self-Service Portal with the ADU Application Form, stamped plans by a licensed design professional, the roof pitch (detached) or lockable separation and house floor plans (attached). Detached units need an automatic fire sprinkler on a bypass meter and an In-Property Wet Utility Permit; affordable ADUs record a Declaration of Use; lots under an old PUD or site review need a minor modification first.", typically Not fixed. P&DS posts 6 business days for the completeness check, then 7 weeks of review for new construction (4 weeks for other scopes) and 'full review up to 9 weeks' (Sept. 2026); a permit application expires after 180 days without progress.) and who is responsible if revisions are required.
- Payment schedule: Avoid large upfront payments. A reasonable schedule ties payments to completed milestones.
Questions to Ask Before You Hire
- How many ADUs have you built in Boulder in the last two years?
- Is your bid fixed-price, and what would trigger a change order?
- Are permits, plan check, and utility connection fees included?
- What's a realistic timeline, including permit review (Not fixed. P&DS posts 6 business days for the completeness check, then 7 weeks of review for new construction (4 weeks for other scopes) and 'full review up to 9 weeks' (Sept. 2026); a permit application expires after 180 days without progress.) and construction?
- Who is my point of contact and how are delays communicated?
- What warranty do you provide on workmanship?
- Can you share three recent Colorado ADU client references?
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