ADUs are allowed in Unincorporated BoulderTypes:DetachedInternal
Types:DetachedInternal
Accessory Dwelling Unit (ADU) Rules for Unincorporated Boulder, CO
Is your property eligible for an ADU?
# of Properties
23,160
Owner-occupied
58%
Zoning districts
44
Avg Home Value
$477,396
Median Lot Size
1.07 acres
Avg Income
$137,137
Residential Types:
23,160 housing units
Single Family
75%
Other
25%
Zoning Districts:
27,291 parcels · 44 districts
F
38%(10,395)
A
21%(5,685)
RR
19%(5,197)
SR
8%(2,130)
Other
14%(3,884)
ADU Rules
What can you build in Unincorporated Boulder, CO?
Unincorporated Boulder, CO follows state and local rules for Accessory Dwelling Units. Here is a concise overview of the local rules.
Eligibility
- –Internal / Attached ADU: By Right
- –Detached ADU: By Right
Detached ADU Size Limits
- –Maximum size of 700 sq ft for a detached Family Care accessory dwelling
- –Maximum size of 1,800 sq ft for a detached Agricultural Worker accessory dwelling
- –Maximum size of 900 sq ft for a detached Disaster Recovery accessory dwelling
Max Units
- –Disaster Recovery ADU: 1 per lot
Owner Occupancy
- –Owner must live on the property.
Parking
- –1 space per ADU (baseline); NRCD I requires 3 total spaces for a 2-unit lot; parking must be on-site on the same lot; reductions possible for units <600 sq ft and up to 40% via Parking Reduction Plan
Rental Restrictions
- –STR: Historic ADUs only; other ADUs ineligible.
- –Vacation Rentals: not allowed for ADUs.
- –ADUs may only be used for approved purposes (no general rental); Marshall Fire: the principal dwelling is not eligible for Vacation Rental or Secondary Short-Term Rental if an ADU exists.
ADU State Law
What can you build in Colorado?
- –ADUs must be approved based on objective standards; no discretionary hearings allowed.
- –Historic properties: Local historic review may still apply in designated historic districts.
- –Exempt parcels: ADUs are not required to be allowed on parcels without adequate water/sewer, in floodways, or on certain historic properties.
- –HOAs cannot prohibit ADUs: HOA rules that conflict with the state ADU law are void.
- –Parking exceptions exist: In rare cases, one parking space may be required if there is no existing off-street parking and on-street parking is prohibited.
- –Local rules still matter: Building code, fire code, utility capacity, and life-safety standards still apply.
- –More permissive is allowed: Municipalities may allow larger ADUs, multiple ADUs, or additional housing types beyond what the state requires.
- –Local zoning controls whether ADUs are allowed at all.
- –HOA rules may still apply.
- –More permissive local rules are allowed.
What's allowed on your lot?
These are the municipality-wide rules. What's allowed on a specific property also depends on state laws, setbacks, floor area ratio, lot coverage, environmental constraints, topography, and other property-specific factors. Search your address on FutureLot to see what's allowed on your property.
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ADU rules in other Colorado municipalities
Rules change at the town line. Check the municipality your property is actually in.
Boulder County