Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Boulder ADU cost →Building an ADU in Boulder costs between $200,000-$450,000 for new construction, with per-square-foot costs averaging $400-$700. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $100,000-$250,000, while prefab/modular ADUs fall in between at $200,000-$425,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | 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. |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. We pass your request to up to 5 local builders serving Boulder, where we have them — free, no obligation.
Get Boulder builder quotes →Permit fees in Boulder are 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.. The review timeline is 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., with 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.. Impact fees are 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.. Pre-approved plans: None. The city publishes no pre-approved ADU plan set (Sept. 2026); the ADU Guide only lists the required drawings and forms..
| 1-Bedroom Rent | $1,900-$2,400/month |
| 2-Bedroom Rent | $2,300-$2,900/month |
| Annual Rental Income (est.) | $25,800/year |
| Payback Period (est.) | 12.6 years |
| Short-Term Rental | No. Short-term rental (under 30 days) of the ADU, or of the main house on a lot with an ADU, is prohibited (9-6-3(n) and BRC 10-3-19) unless both the ADU and the STR licence were legally established before Feb. |
| Overall ROI Estimate | 5-7% gross rental yield |
A typical ADU in Boulder generates approximately $25,800 per year in rental income based on average 1-bedroom rents. At an average construction cost of $325,000, the estimated payback period is 12.6 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Boulder and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Boulder: $100,000-$250,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $200,000-$425,000.
3. Pre-approved plans — available in Boulder, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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