Everything homeowners ask about building an ADU in Boulder — answered with 2026 data.
Yes, ADUs are allowed in By right on a lot with one detached house in every residential district (RR-1, RR-2, RE, RL-1, RL-2, RM-1/2/3, RMX-1/2, RH-1/2/3/7), the MU-1 to MU-4, BT, BC, BR and DT-1 to DT-5 business and downtown districts, IG, A and P (Table 6-1 as amended by Ord. 8650; use standards in BRC 9-6-3(n)). Not allowed in RH-6, MH, BMS, BCS or IS-1 (the ADU Guide also excludes IM), on a lot without a single detached dwelling, or on a home with a permanent affordability deed restriction. in Boulder, CO. The maximum size for a detached ADU is 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. Attached ADUs can be up to Half the principal house's floor area or 1,000 sq ft, whichever is less; if the house is under 1,500 sq ft, the cap is 750 sq ft. Affordable ADU or historic property: two-thirds of the house or 1,200 sq ft, whichever is less. 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 $200,000-$450,000. The cost per square foot averages $400-$700. Garage conversions are more affordable at $100,000-$250,000, while prefab ADUs cost $200,000-$425,000. Permit fees add 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. to your budget. See our detailed Boulder ADU cost guide.
Detached ADUs in Boulder can be up to 800 sq ft of floor area. Attached ADUs are limited to Half the principal house's floor area or 1,000 sq ft, whichever is less; if the house is under 1,500 sq ft, the cap is 750 sq ft. Affordable ADU or historic property: two-thirds of the house or 1,200 sq ft, whichever is less. Junior ADUs (JADUs) have a cap of Boulder has no junior ADU category. A unit inside the house is an attached ADU under 9-6-3(n)(2): it must be separated from the main home by a wall or lockable door, and any existing interior connection must be kept. Maximum height is Detached ADU: 20 ft, or 25 ft if the roof pitch is 8:12 or steeper; a legal existing accessory building converted without changing its height, floor area or roof form keeps its height; dormers allowed above 20 ft within bulk-plane limits (9-6-3(n)(3)(A)). Attached ADU: the principal-building limit in Table 7-1, 35 ft in every district that allows ADUs, except that ADU floor area inside the rear setback is capped at 20 ft. You are allowed up to One ADU per lot, and only on a lot with one detached house on a qualifying lot.
Rear setback: Detached ADU (an accessory building) in RL-1, RL-2, RM-1, RM-2, RMX-1, RE and RR: 0 or 3 ft from the rear lot line (Table 7-1; a 0 ft setback needs a maintenance easement recorded with the neighbor unless the lot backs an alley or street), and 6 ft from any other building. Attached ADU: the principal rear setback can drop to 5 ft where only ADU floor area sits in the setback, that part is no taller than 20 ft and, in RR/RE/RL/RMX-1, covers no more than 500 sq ft (9-6-3(n)(2)(B), Ord. 8650). Side setback: Detached ADU: 0 or 3 ft from an interior side line in RL-1, RL-2, RM-1, RM-2 and RMX-1 (0 ft needs a recorded maintenance easement); 10 ft in RE and RR-2; 15 ft in RR-1. Street-side line on a corner lot: 12.5 ft (RL-1, RM-2, RMX-1), 10.5 ft (RL-2, RM-1), 25 ft (RR, RE). Walls within 5 ft of a lot line need fire-rated assemblies and eaves may come no closer than 2 ft. Attached ADUs use the principal-building side setbacks. Front setback: Detached ADU: at least 55 ft behind the front property line in RR-1, RR-2, RE, RL-1, RL-2, RM-1, RM-2 and RMX-1 (accessory-building front setback in Table 7-1), which in practice means the rear yard. Attached ADU: the principal-building front setback for the district. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None for the ADU. Ord. 8650 (effective March 8, 2025) struck the ADU parking table from 9-6-3(n) and set the Table 9-2 requirement for attached and detached ADUs to 0; the main house keeps whatever its district requires. This is Boulder's own rule: C.R.S. 29-35-103 (HB 24-1152) only bars requiring a new space and lets a city designate an existing one. 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. 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. 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 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. 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. Pre-approved plans availability: None. The city publishes no pre-approved ADU plan set (Sept. 2026); the ADU Guide only lists the required drawings and forms.
Average ADU rental income in Boulder: 1-bedroom units rent for $1,900-$2,400/month, 2-bedroom units for $2,300-$2,900/month. Short-term rentals: 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. Expected ROI: 5-7% gross rental yield.
The minimum lot size for an ADU in Boulder is None. Ord. 8650 removed the former 5,000 sq ft minimum; an ADU may be established on any lot with one detached dwelling unit (9-6-3(n)(1)(A)(i)). District lot coverage, floor area ratio, solar access and side-yard bulk-plane rules still apply. 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 $100,000-$250,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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