Yes: Boulder allows one ADU by right on any lot with a single detached house in every residential zone (BRC 9-6-3(n) and Table 6-1), with no owner-occupancy, parking or minimum-lot-size rule since Ordinance 8650 took effect March 8, 2025. A detached ADU may be 800 sq ft (1,000 sq ft if rent-capped at 75% of AMI or on a designated historic property) and 20 ft tall (25 ft with an 8:12 roof); an attached ADU may be half the house up to 1,000 sq ft. In RL, RM and RMX-1 zones a detached unit sits at least 55 ft back from the front lot line and 0 or 3 ft from rear and interior side lines, needs fire sprinklers, and cannot be short-term rented. Approval is an ordinary building permit: the fee is valuation-based (about $2,900 on a $300,000 unit, plus use tax and roughly $9,400-$10,600 in impact fees and excise tax), and the city posts 6 business days for completeness plus 7 weeks of review for new construction.
Zoning & Size Requirements [1][2][3][4][5][6]
Yes — Boulder allows ADUs. A detached ADU can typically be up to 800 sq ft under current Colorado and Boulder zoning rules.
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| Allowed Zones | 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. |
| Max Size (Detached) | 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. |
| Max Size (Attached) | 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 (750 sq ft if the house is under 1,125 sq ft). Table 6-3 in BRC 9-6-3(n). |
| Max Size (JADU) | 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. |
| Max Height | 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. |
| Max Units | One ADU per lot, and only on a lot with one detached house (9-6-3(n)(1)(A)(i), Ord. 8650). Not allowed with a duplex or multi-unit building. ADUs on lots with an older PUD, site review or use review approval are allowed after a minor modification (9-6-3(n)(1)(A)(viii)). |
Setback Requirements [1][2][3][4][5][6]
Boulder requires a rear setback of about 3 ft for a detached ADU, and no additional parking is required. Owner-occupancy is not required — you can rent it out without living on the property.
| 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. |
| Parking Required | 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. |
| 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. |
| Min Lot Size | 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. |
Permit Process & Fees [7][8][9][10]
An ADU permit in Boulder goes through a standard permit review. Review time: 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. Permit 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.
| Approval 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. |
| Review Timeline | 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. |
| Permit 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. |
| 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 | None. The city publishes no pre-approved ADU plan set (Sept. 2026); the ADU Guide only lists the required drawings and forms. |
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Construction Costs [11][12]
Building an ADU in Boulder typically costs $200,000-$450,000 ($400-$700 per sq ft); a garage conversion runs $100,000-$250,000.
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| Average Build Cost | $200,000-$450,000 |
| Cost per Sq Ft | $400-$700 |
| Garage Conversion | $100,000-$250,000 |
| Prefab ADU (Installed) | $200,000-$425,000 |
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.
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Rental Income Potential [13][14][15][16]
A 2-bedroom ADU in Boulder rents for around $2,300-$2,900/mo, an estimated 5-7% gross rental yield.
| Avg Rent (1 Bedroom) | $1,900-$2,400/mo |
| Avg Rent (2 Bedroom) | $2,300-$2,900/mo |
| 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. |
| Est. Annual ROI | 5-7% gross rental yield |
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Recent Law Changes
What's Changed Recently
- 2026-09-20: entry re-verified against Ordinance 8650, Ordinance 8745, the city ADU Guide, the Residential Accessory Structure Guide and the 2026 P&DS Schedule of Fees; allowed zones, setbacks, fees and impact fees rewritten from those sources and the affordable-ADU 'fee waiver' claim dropped (no such waiver in the code or fee schedule).
- June 20, 2026: Ordinance 8745 (Omnibus code amendment, adopted May 21, 2026) adds a utilities standard to 9-6-3(n): the ADU shares utility hookups and meters with the main house unless a separate hookup is approved under the Design and Construction Standards.
- Aug. 1, 2025: the 2024 International Codes with Boulder amendments (Ordinance 8684) and the 2024 Wildland Urban Interface Code (Ordinance 8695) apply to new ADU permits; AMPS Ordinance 8696 revised off-street parking standards from Aug. 23, 2025 (the ADU requirement was already 0).
- March 8, 2025: Ordinance 8650 removed the owner-occupancy rule, the ADU parking requirement, the 5,000 sq ft lot minimum, side-entrance screening and the 60 sq ft private open space rule, and extended the ADU use in Table 6-1 to align with HB 24-1152 (C.R.S. 29-35-101 et seq.).
- Sept. 1, 2023: Ordinance 8571 made ADUs a building-permit-only approval (no administrative review) and removed neighborhood saturation caps.
Tips for Building an ADU in Boulder
- Cost basis: build and prefab ranges span a Boulder modular builder's 2026 all-in figures ($200,000-$240,000 for 245 sq ft, about $341,000 for 490 sq ft, about $425,000 for 735 sq ft, stick-built 15-25% more) and a local custom builder's $250,000-$600,000 ADU/garage-conversion range (March 2025); rents start at HUD FY2026 Small Area FMRs for ZIPs 80301-80305 (1BR $1,910-$2,120, 2BR $2,260-$2,510).
- Worked fee example (2026 schedule): a $300,000 detached ADU pays a $2,249 building permit fee, $562 plan check and $83.90 energy fee, about $7,800 in prepaid city and county use tax (5.195% on half the valuation), then before final inspection $4,599-$5,784 in capital facility impact fees plus the $4,826.22 transportation excise tax; the plumbing new-construction fee is $68.85-$94 per unit.
- Affordable ADU option: agree to cap rent at 75% of area median income (the city posts the maximum-rent table each year), sign a Declaration of Use recorded with the Boulder County Clerk, and the size limit rises to 1,000 sq ft detached or 1,200 sq ft attached; changing the affordability status later needs a new building permit and an amended declaration.
- Detached ADUs must have an automatic fire sprinkler system on a dedicated sprinkler line through a bypass water meter (NFPA 13D or an approved equivalent mist system), and the water, sewer and other wet-utility runs from the house need an In-Property Wet Utility Permit; budget both before comparing quotes.
- Legalizing an existing unpermitted unit uses the same building permit: finishes come off for inspection of framing, insulation, plumbing and electrical, a licensed design professional stamps the plans, and a detached unit gets sprinklers; a 'change of use' permit ($81 analysis fee) is needed even if no construction is proposed.
- Boulder has no occupancy limit based on the number of unrelated people since Ordinance 8651 (March 2025), and HOA bans on ADUs are void under HB 24-1152; a deed restriction for permanent affordability, a historic district, a floodplain or a PUD approval can still add review steps.
- This page covers the City of Boulder. Homes in unincorporated Boulder County (Niwot, Gunbarrel outside city limits, mountain areas) follow the Boulder County Land Use Code, administered by Boulder County Community Planning & Permitting, with different size and process rules.
- Cost basis (2026-09-20 re-verification): short term allowed: 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. 1, 2019 (the STR licensing FAQ says Jan. 3, 2019). Long-term rental of either unit is allowed with a $190 rental licence per unit (four-year term); a detached ADU must be SmartRegs-compliant when rented.
ADU Builders in Boulder
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Accessory Dwelling Units (FAQ) — City of Boulder Planning & Development Services Planning Department — Accessed 2026-09-20
- Accessory Dwelling Unit (ADU) Guide — City of Boulder Planning Department — Accessed 2026-09-20
- Ordinance 8650 (adopted Feb. 6, 2025) amending BRC 9-6-1, 9-6-3(n), 9-7-1 and 9-9-6 — City of Boulder records ordinance — Accessed 2026-09-20
- Ordinance 8745, Omnibus Land Use Code amendments (adopted May 21, 2026, effective June 20, 2026) — City of Boulder records ordinance — Accessed 2026-09-20
- Residential Accessory Structure Guide (accessory-building setbacks by district) — City of Boulder Planning Department — Accessed 2026-09-20
- Planning & Development Code Changes (ordinance list) — City of Boulder Planning Department — Accessed 2026-09-20
- Planning and Development Services 2026 Schedule of Fees — City of Boulder fee_schedule — Accessed 2026-09-20
- Building Permits and Inspections — City of Boulder Planning Department — Accessed 2026-09-20
- Planning & Development Services current processing times — City of Boulder Planning Department — Accessed 2026-09-20
- Ordinance 8571 (adopted May 4, 2023): ADU process and standards — City of Boulder records ordinance — Accessed 2026-09-20
- ADU Construction Cost in Boulder: 2026 Homeowner's Guide — Olerra (modular builder) industry — Accessed 2026-09-20
- How Much Does an ADU or Garage Conversion Cost in Boulder, CO? — Petra Custom Builders (March 18, 2025) industry — Accessed 2026-09-20
- HUD FY2026 Fair Market Rents — Boulder, CO MSA Government Data — Accessed 2026-09-20
- HUD FY2026 Small Area FMRs — Boulder County, CO (ZIP 80301-80305) Government Data — Accessed 2026-09-20
- Rental Housing Licensing: Short-Term — City of Boulder Planning Department — Accessed 2026-09-20
- Rental Housing Licensing: Long-Term — City of Boulder Planning Department — Accessed 2026-09-20
- HB24-1152 Accessory Dwelling Units — Colorado General Assembly Government Data — Accessed 2026-09-20
Frequently Asked Questions
Can I build an ADU in Boulder?
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. 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.. Owner-occupancy is not required, so you can rent out the ADU.
How much does an ADU cost in Boulder?
ADU construction in Boulder typically costs $200,000-$450,000. Garage conversions run $100,000-$250,000, and prefab ADUs cost $200,000-$425,000.
How much can I rent an ADU for in Boulder?
Average ADU rents in Boulder are $1,900-$2,400/mo for a 1-bedroom and $2,300-$2,900/mo for a 2-bedroom unit.
Do I need parking for an ADU in Boulder?
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.. Check with Boulder planning department for the latest Colorado ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Boulder?
Boulder does not publish a standard ADU review timeline. 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. Permit 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.
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