In the City of Boulder, an ADU is a building permit. There is no separate land use approval, no hearing and no discretionary decision: Building permit only since September 1, 2023; no separate approval step, and no city pre-review service.
That is a genuine simplification compared with how this used to work, and compared with unincorporated county land today. It does not mean there is nothing to satisfy. It means everything you have to satisfy is written down.
Before you design anything#
Confirm you are in the city. A Boulder mailing address is not the same as being inside city limits. Check the property first.
Check your zone. Most zones allow an ADU by right on a lot with a detached dwelling, but not all of them: Being confirmed with City of Boulder.
Check for a PUD or site review on your property. Older approvals sometimes banned ADUs. Those bans no longer hold, but they leave a step behind: Prior PUD and site-review bans on ADUs are void; a minor modification is required before the building permit.
Check whether the home is deed restricted. Homes with permanent-affordability deed restrictions are not eligible.
The standards your design has to meet#
Size is the first constraint, and it differs by form. Attached: 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. Detached: 800 sq ft. If the unit will be affordable or the property is designated historic, more is allowed: 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 ft.
For a detached unit, height is capped at 20 ft, or up to 25 ft with a roof pitch of 8:12 or steeper, and it has to stand clear of other buildings: At least 6 ft from any other building.
An attached unit has its own rear setback: 5 ft minimum, subject to height and coverage standards.
None of that replaces the ordinary zoning envelope. Everything else still applies: Zone setbacks, floor area ratio, solar access, building coverage, side-yard bulk plane, wall articulation and landscaping.
Energy, fire and utilities#
The city's energy code applies to the new unit: New homes and additions must be all-electric, with no residential exceptions, under the code effective December 1, 2024. Smaller units have a simpler route to compliance: ADUs of 1,000 sq ft or less may use the prescriptive path instead of an energy model.
Detached units carry two requirements attached units do not: Detached ADUs need automatic sprinklers on a dedicated line through a bypass meter; equivalent mist systems can be approved, and Detached ADUs need an In-Property Wet Utility Permit.
Plant investment fees apply to the new connection: Being confirmed with City of Boulder.
Submitting and what it costs#
The city publishes a fee schedule, but not an ADU-specific line: Being confirmed with City of Boulder. Ask Planning and Development Services for a current figure for your scope of work rather than relying on any number you read online, including ours.
Two situations with their own path#
An ADU that already exists without a permit. It can be brought into compliance: Unpermitted ADUs can be legalized with stamped plans, finishes opened for inspection, and sprinklers for detached units.
A space that is already built but not a legal ADU. Even with no construction, you need a permit: A permit is needed to establish an ADU even when no construction happens.
After the permit#
Inspections run through the build. When the unit is finished and you intend to rent it, there is a separate licensing step, covered in renting an ADU.
Where to go next#
The full City of Boulder rules, or what drives ADU cost.