ADUs in the City of Boulder
Settled and permissive. ADUs are allowed by right in most zones, with a building permit and no separate approval step.
The City of Boulder rewrote its accessory dwelling unit rules in 2023 and again in 2025, and the state ADU law applies here. If your property is inside city limits, these are the rules that govern it.
Last reviewed September 18, 2026
The short version
What you need to know first.
How an ADU is reviewed
Building permit only since September 1, 2023; no separate approval step, and no city pre-review service
Maximum size, detached ADU
800 sq ft
Maximum size, attached ADU
The lesser of half the principal dwelling’s floor area or 1,000 sq ft; 750 sq ft if the principal dwelling is under 1,500 sq ft
Owner occupancy
Not required since March 8, 2025
Neighborhood saturation limits
None since September 1, 2023
Long-term rental
Allowed with a city rental license
Every rule
The rules, grouped.
Each row links the official page it came from and the date we last read it. Where a rule is still being confirmed, we say so rather than guess.
Where and how
| Rule | What applies | Source |
|---|---|---|
| Where ADUs are allowed | Being confirmed with City of BoulderBeing confirmed Status: being confirmed. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| How an ADU is reviewed | Building permit only since September 1, 2023; no separate approval step, and no city pre-review service Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Neighborhood saturation limits | None since September 1, 2023 Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Planned unit developments and site reviews | Prior PUD and site-review bans on ADUs are void; a minor modification is required before the building permit Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| HOA restrictions | An HOA in a state-covered jurisdiction cannot prohibit ADUs; reasonable restrictions are allowed Status: verified. | State of Colorado: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Establishing an ADU without construction | A permit is needed to establish an ADU even when no construction happens Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Deed-restricted affordable homes | Homes with permanent-affordability deed restrictions are not eligible Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Permit fees and review times | Being confirmed with City of BoulderBeing confirmed Status: being confirmed. | City of Boulder: Planning and Development Services 2026 Schedule of Fees (opens in a new tab)Reviewed September 18, 2026 |
Size and shape
| Rule | What applies | Source |
|---|---|---|
| Maximum size, attached ADU | The lesser of half the principal dwelling’s floor area or 1,000 sq ft; 750 sq ft if the principal dwelling is under 1,500 sq ft Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Maximum size, detached ADU | 800 sq ft Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Larger sizes for affordable and historic properties | Attached up to the lesser of two-thirds of the principal dwelling or 1,200 sq ft (750 sq ft if the principal is under 1,125 sq ft); detached up to 1,000 sq ftThis applies to affordable ADUs and to designated historic properties. Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Height, detached ADU | 20 ft, or up to 25 ft with a roof pitch of 8:12 or steeper Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Separation from other buildings | At least 6 ft from any other building Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Rear setback, attached ADU | 5 ft minimum, subject to height and coverage standardsA setback is the required distance between a building and a property line. Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Other standards that still apply | Zone setbacks, floor area ratio, solar access, building coverage, side-yard bulk plane, wall articulation and landscaping Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
Occupancy and rentals
| Rule | What applies | Source |
|---|---|---|
| Owner occupancy | Not required since March 8, 2025 Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Long-term rental | Allowed with a city rental license Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Short-term rental | Banned for both the ADU and the main house, unless the ADU and its rental license predate February 1, 2019 Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Rental license fee | Being confirmed with City of BoulderBeing confirmed Status: being confirmed. | City of Boulder: Planning and Development Services 2026 Schedule of Fees (opens in a new tab)Reviewed September 18, 2026 |
| SmartRegs energy requirements for rentals | Detached ADUs must meet SmartRegs when rented; attached ADUs are exempt; the main home must comply when it is rented Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Terms for an affordable ADU | Rent capped at 75% of area median income, with a Declaration of Use recorded against the property Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
Utilities and safety
| Rule | What applies | Source |
|---|---|---|
| Fire sprinklers | Detached ADUs need automatic sprinklers on a dedicated line through a bypass meter; equivalent mist systems can be approved Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| All-electric requirement | New homes and additions must be all-electric, with no residential exceptions, under the code effective December 1, 2024 Status: verified. | City of Boulder: 2024 City of Boulder Energy Code Update: Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Energy compliance path for small ADUs | ADUs of 1,000 sq ft or less may use the prescriptive path instead of an energy model Status: verified. | City of Boulder: 2024 City of Boulder Energy Code Update: Frequently Asked Questions (opens in a new tab)Reviewed September 18, 2026 |
| Wet utility permit | Detached ADUs need an In-Property Wet Utility Permit Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Plant investment fees | Being confirmed with City of BoulderBeing confirmed Status: being confirmed. | City of Boulder: Planning and Development Services 2026 Schedule of Fees (opens in a new tab)Reviewed September 18, 2026 |
Other
| Rule | What applies | Source |
|---|---|---|
| Selling the ADU separately | The ADU cannot be sold separately from the main home Status: verified. | City of Boulder: Accessory Dwelling Unit (ADU) Guide (opens in a new tab)Reviewed September 18, 2026 |
| Factory-built and modular ADUs | A state-approved modular ADU still needs a city building permit and must meet local codes and zoning Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| How the state’s 2026 factory-built rules interact with city code | Being confirmed with State of ColoradoBeing confirmed Status: being confirmed. | State of Colorado: Factory-Built Structures (opens in a new tab)Reviewed September 18, 2026 |
| Legalizing an unpermitted ADU | Unpermitted ADUs can be legalized with stamped plans, finishes opened for inspection, and sprinklers for detached units Status: verified. | City of Boulder: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
| Certified ADU Supportive Jurisdiction status | Being confirmed with State of ColoradoBeing confirmed Status: being confirmed. | State of Colorado: Accessory Dwelling Units (opens in a new tab)Reviewed September 18, 2026 |
It depends
What no page can answer for you.
These are the variables that decide what you can actually build. They differ lot by lot, and City of Boulder decides them, not us.
Go deeper
Permitting an ADU in the City of Boulder
The permit path for a City of Boulder ADU, step by step.
What drives ADU cost in the City of Boulder
The components that make one ADU cost more than another, and the fees the city publishes.
Renting an ADU in the City of Boulder
Long-term rules, the short-term ban, licensing and SmartRegs for a City of Boulder ADU.
What changed recently
March 8, 2025
Updated ADU rules apply to new proposals
The city’s updated ADU rules apply to ADUs proposed on or after this date, including the end of the owner-occupancy requirement.
December 1, 2024
All-electric energy code takes effect for new construction
The 2024 city energy code requires new homes and additions to be all-electric, with no residential exceptions.
Not your jurisdiction? See the Unincorporated County rules or work out which applies to you.
Sources
- Accessory Dwelling Unit (ADU) GuideCity of Boulder
- Accessory Dwelling UnitsCity of Boulder
- Accessory Dwelling UnitsState of Colorado
- Planning and Development Services 2026 Schedule of FeesCity of Boulder
- 2024 City of Boulder Energy Code Update: Frequently Asked QuestionsCity of Boulder
- Factory-Built StructuresState of Colorado
Last reviewed September 18, 2026.
This page summarizes public rules for general information. It isn’t legal advice or a permit determination. Confirm requirements for your property with the City of Boulder before you design or build.
Get City of Boulder updates
We email when a rule changes in the City of Boulder, with the source. Nothing else.