ADU Rules in Boulder, CO (2026)
ADU rules, permit requirements, size limits, and fees for Boulder, CO. Source-cited from official municipal code.
Yes, ADUs are allowed in Boulder, CO.
Boulder, CO permits accessory dwelling units (ADUs) on residential properties. ADUs — also called granny flats, in-law units, backyard cottages, or secondary suites — are secondary housing units on a single-family or multifamily lot. State and local law actively preempts local restrictions, making ADU approval a streamlined ministerial process. In Boulder, the Boulder Planning and Development Services — Building Safety Division handles ADU permit applications. Below you will find the current size limits, setbacks, fees, and timeline for building an ADU in Boulder.
ADU Size Limits
ADU Size Limits in Boulder: Detached ADU: up to 800 sq ft (max height: 20 feet). Attached ADU: One-half of principal structure floor area or 1,000 square feet, whichever is less; 750 square feet if principal is less than 1,500 square feet. Colorado does not use the California JADU concept. Boulder BRC limits ADUs to 1,000 sqft. ADU floor area limited to 50% of primary dwelling's floor area in some zones. Boulder's high land values make ADUs for rental income compelling — studio ADUs near CU Boulder campus rent for $1,800–$2,800/month. BRC Section 9-8-5.
| Type | Max Size | Max Height |
|---|---|---|
| Detached ADU | 800 sq ft · Verified June 1, 2026 · Source | 20 feet · Verified June 1, 2026 · Source |
| Attached ADU | One-half of principal structure floor area or 1,000 square feet, whichever is less; 750 square feet if principal is less than 1,500 square feet · Verified June 1, 2026 · Source | 20 feet maximum height for any portion of building located within the minimum rear yard setback area · Verified June 1, 2026 · Source |
Permit Fees
ADU Permit Fees in Boulder: Plan check fee: $1,000–$5,000. Building permit fee: $1,500–$5,000. Impact fees: Boulder infrastructure use fees may apply — varies by project type. School fees: Boulder Valley School District impact fees may apply — typically $3,000–$6,000 for new units. Total estimated permit cost: Permit fees apply per Chapter 10-3 rental license requirements; impact fees permitted per CRS §29-35-403(3)(f). Boulder permit fees are among Colorado's highest given premium service levels and complex review process. Full fee schedule: https://bouldercolorado.gov/planning/building-permits/fees. Boulder Affordable Housing program offers fee reductions for deed-restricted ADUs.
| Fee Type | Amount |
|---|---|
| Plan Check | $1,000–$5,000 |
| Building Permit | $1,500–$5,000 |
| Impact Fees | Boulder infrastructure use fees may apply — varies by project type |
| School Fees | Boulder Valley School District impact fees may apply — typically $3,000–$6,000 for new units |
| Total Estimate | See official source |
Permit Timeline
Boulder ADU Permit Timeline: Subject to administrative approval process per CRS §29-35-403; Boulder Planning and Development Services processes permits Apply through the Boulder Planning and Development Services — Building Safety Division online permit portal. Phone: (303) 441-3200.
Setback Requirements
ADU Setback Requirements in Boulder: Front setback: Zone-specific (no ADU-specific front setback stated; follows zone standards). Side setback: Zone-specific accessory building setback standard (no ADU-specific side setback stated in 9-6-3(n)). Rear setback: 5 feet. Colorado HB 24-1175 prohibits minimum lot size requirements that effectively ban ADUs. Boulder's BRC sets 5-foot side and rear setbacks for detached ADUs compliant with state law. Front setback follows zone standard — RL zones typically require 25 ft front setback for primary dwellings. Corner lots must meet both street frontage setbacks.
Parking Requirements
Parking Requirements: Require the construction of a new off street parking space in connection with construction or conversion of an accessory dwelling unit is prohibited
Restrictions & Requirements
- ADU must comply with Boulder Revised Code Section 9-8-5 objective standards
- ADU is limited to 1,000 sqft and 50% of primary dwelling floor area
- Open Space Mountain Parks boundary (Blue Line) defines Boulde's growth limits — no ADUs outside UGB
- Boulder has voluntary deed restriction program for affordable ADUs — reduces fees and creates long-term affordability
- Short-term rental of ADU requires Boulder STR license
- ADU must have permanent foundation
- Properties with deed restrictions from prior affordable housing agreements may have additional limitations
How Many ADUs Are Allowed?
Number of ADUs Allowed in Boulder: Single-family lots: 1 ADU per single-family lot (state minimum per HB 24-1175); Boulder local ordinance may allow additional ADU types. Multifamily properties: Additional units per Boulder RMX and RH zone standards — consult Boulder Planning for zone-specific details. Colorado HB 24-1175 sets floor of 1 ADU per single-family lot. Boulder's Accessory Unit (AU) program has been in place since the 1980s — the city has extensive experience with ADU administration. CU Boulder proximity creates one of Colorado's strongest ADU rental markets.
Single-Family Lots
1 ADU per single-family lot (state minimum per HB 24-1175); Boulder local ordinance may allow additional ADU types
Multifamily Properties
Additional units per Boulder RMX and RH zone standards — consult Boulder Planning for zone-specific details
State Law & Preemption
State Preemption: Colorado House Bill 24-1175 (effective August 7, 2024) requires all Colorado municipalities with population over 1,000 to allow at least one ADU per single-family residential lot by-right. The law removes minimum lot size requirements targeting ADU prohibition, limits parking requirements, and creates state enforcement through Department of Local Affairs (DOLA). Boulder already had an ADU ordinance in place before HB 24-1175 and has updated BRC Section 9-8-5 to comply.
Historic District Considerations
Historic District Requirements: Boulder has 5 historic districts where additional design review may be required for ADU construction. Design review approval is required before obtaining a building permit for properties in historic overlay zones. Boulder has 5 local historic districts: Mapleton Hill Historic District (late 19th and early 20th century Victorian/Craftsman homes), Chautauqua National Historic Landmark (1898 Colorado Chautauqua grounds), Downtown Boulder Mall area, University Hill, and Whittier Historic District. The Landmarks Board reviews exterior alterations in these districts. Chautauqua is one of only three continuously operating Chautauqua sites in the US and has significant National Historic Landmark protections. Boulder has 240+ individually-designated landmarks. Look up historic district boundaries at: https://bouldercolorado.gov/planning/historic-preservation
Frequently Asked Questions
Can I build an ADU in Boulder?
Yes. Boulder, CO allows accessory dwelling units. On a single-family lot you can build 1 ADU per single-family lot (state minimum per HB 24-1175); Boulder local ordinance may allow additional ADU types.
How big can an ADU be in Boulder?
The maximum size for a detached ADU in Boulder is 800 square feet.
How long does it take to get an ADU permit in Boulder?
The typical ADU permit timeline in Boulder is Subject to administrative approval process per CRS §29-35-403; Boulder Planning and Development Services processes permits. Total estimated permit fees are Permit fees apply per Chapter 10-3 rental license requirements; impact fees permitted per CRS §29-35-403(3)(f).
Do I need extra parking for an ADU in Boulder?
Boulder parking rules for ADUs: Require the construction of a new off street parking space in connection with construction or conversion of an accessory dwelling unit is prohibited
Do I have to live on-site to have an ADU in Boulder?
Owner-occupancy requirement in Boulder: Subject jurisdiction may require property owner to demonstrate that property owner resides on parcel when application submitted; owner-occupied accessory dwelling unit requirement prohibited except at application
Source: Boulder Revised Code Title 9, Section 9-8-5 — Accessory Units; Colorado Revised Statutes Title 29 (HB 24-1175 codification). Last verified April 6, 2026. View source