ADUs are allowed in ThorntonTypes:DetachedInternal
Types:DetachedInternal
Accessory Dwelling Unit (ADU) Rules for Thornton, CO
Is your property eligible for an ADU?
# of Properties
43,002
Owner-occupied
85%
Zoning districts
36
Avg Home Value
$605,362
Median Lot Size
0.16 acres
Avg Income
$122,292
Residential Types:
43,002 housing units
Single Family
10%
Other
90%
Zoning Districts:
49,325 parcels · 36 districts
PD-L
42%(20,605)
SFD-L
40%(19,688)
RM
6%(2,875)
RH
5%(2,468)
Other
7%(3,689)
ADU Rules
What can you build in Thornton, CO?
Thornton, 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
Interior / Attached ADU Size Limits
- –Maximum size is the greater of 1,000 sq ft or 50% of the primary residence’s gross floor area (excluding garages/porches).
Detached ADU Size Limits
- –Maximum size is the greater of 1,000 sq ft or 50% of the primary residence’s gross floor area.
Max Units
- –Single-family detached lots: 1 ADU per lot (detached, attached, or internal).
- –Duplex/2-unit attached; Townhouse/Rowhouse (3–8 units attached); Triplex/Quadplex; Multifamily: 0 ADUs per lot.
Owner Occupancy
- –Owner must occupy either the principal dwelling or the ADU.
Parking
- –1 space/ADU; max 8 spaces/lot for attached/detached residential; parking on the same lot, shown on site plan; outdoor ADU parking limited to driveway or parking pad; reductions up to 50% total (incl. up to 50% for transit within 0.25 miles); satellite parking prohibited.
Rental Restrictions
- –Short-term rentals are allowed only when the principal dwelling is the owner's primary residence and the ADU is owner-approved.
- –An ADU cannot be used for short-term rentals if any portion of the principal dwelling is used as a short-term rental.
- –STRs must be operated only at the licensee's primary residence.
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.
Weld County