Everything homeowners ask about building an ADU in Boulder — answered with 2026 data.
Yes, ADUs are allowed in All single-family and multi-family zones in Boulder, CO. The maximum size for a detached ADU is 800 sq ft (market-rate); 1,000 sq ft (affordable ADU program). Attached ADUs can be up to Up to 50% of principal structure or 1,000 sq ft max. Boulder has an ADU-friendliness score of 8/10. Check our full Boulder ADU rules page for complete zoning details.
ADU construction in Boulder typically costs $160,000-$320,000. The cost per square foot averages $140-$280. Garage conversions are more affordable at $80,000-$180,000, while prefab ADUs cost $120,000-$270,000. Permit fees add $1,900-$3,200 to your budget. See our detailed Boulder ADU cost guide.
Detached ADUs in Boulder can be up to 800 sq ft (market-rate); 1,000 sq ft (affordable ADU program). Attached ADUs are limited to Up to 50% of principal structure or 1,000 sq ft max. Junior ADUs (JADUs) have a cap of 800 sq ft. Maximum height is 20-25 ft (detached, depends on roof pitch); 35 ft (attached). You are allowed up to 1 ADU per lot on a qualifying lot.
Rear setback: 10 feet. Side setback: 3 feet. Front setback: 15 feet. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
No parking required (eliminated March 2025). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required (eliminated March 2025, per CO HB 24-1152). Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Boulder is typically 40-60 days. The plan check process takes approximately 2-3 weeks. Pre-approved plans availability: Available.
Average ADU rental income in Boulder: 1-bedroom units rent for $1,600-$2,100/mo/month, 2-bedroom units for $2,000-$2,800/mo/month. Short-term rentals: Limited restrictions. Expected ROI: 6-8% annual return.
The minimum lot size for an ADU in Boulder is 3,500 sq ft. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Boulder, a garage conversion typically costs $80,000-$180,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.