ADU rules in Westminster

One ADU per lot is permitted with any single-family detached home in every zoning classification, including Westminster's many PUD districts, where single-family detached dwellings are a permitted use. A detached ADU (a separate building on the same lot) must sit on a permanent foundation; mobile homes and RVs cannot serve as ADUs. Rear setback is the accessory-structure setback for the district or 5 feet, whichever is greater; front and side setbacks and maximum height match the principal dwelling's standards.

ADUs may be created inside the principal dwelling (for example a basement or attic conversion) or attached to it as an addition. Interior and attached units must have their own entrance from the exterior or from a common hallway, and every ADU is assigned its own address. The ADU receives its certificate of occupancy only after the principal dwelling has one.

The base cap is 1,200 square feet or 50 percent of the principal dwelling's gross floor area, whichever is less. Ordinance No. 4335 (Councillor's Bill No. 5, enacted March 9, 2026, at the Colorado Department of Local Affairs' direction for full HB24-1152 compliance) added relief for small homes: where the principal dwelling is under 1,500 square feet, an ADU up to 750 square feet may be permitted, though never larger than the principal dwelling itself. Minimum ADU size is 190 square feet and no ADU may have more than two bedrooms. Height is capped at the principal dwelling's limit, and combined lot coverage of house plus ADU cannot exceed the coverage allowed for the principal dwelling.

Section 11-4-19 imposes no owner-occupancy requirement, and the city's ADU info sheet confirms the ADU may be rented out and that the owner may even live in the ADU and rent out the main house. State law reinforces this: C.R.S. 29-35-404(2)(b) (HB24-1152) bars Westminster, a subject jurisdiction, from requiring either unit to be owner-occupied; the only residency check allowed is a demonstration that the owner lives on the parcel when applying to build the ADU or for a short-term rental license. Westminster's STR license requires the applicant to be the property's fee owner but does not require living on site.

One new off-street space is required only when both conditions exist: the principal dwelling has no existing off-street space (driveway, garage, or tandem space) that could serve the ADU, and the home sits on a block where on-street parking is prohibited for any reason, including emergency access. If either condition is absent, no new parking is required. The city's info sheet adds that placing the ADU may not reduce on-site parking below the principal dwelling's own minimum. This mirrors the parking limits Colorado set for subject jurisdictions in C.R.S. 29-35-404(2)(a) and (3).

Approval is administrative by code mandate: Section 11-4-19(K) states ADU approval shall not require, and may not be elevated to require, a public hearing or a decision by an elected or appointed body or hearing officer when the unit meets the section's requirements. Only building and utility permits are needed, submitted through the city's eTRAKiT portal; staff review can take up to 4 weeks, and no ODP amendment is required even on PUD-zoned lots. Variances go to the Planning Commission under Section 2-2-8, with a simplified single-criterion variance available for the architectural-match rule.

The city's ADU info sheet states an ADU may be rented long term or short term and that the city does not regulate rent amounts. Long-term rentals fall under the citywide rental program: properties of three or fewer units must be registered as rental dwellings before leasing (WMC 11-12-10 and Title V, Chapter 12). Short-term rentals (under 30 days) are defined to expressly include accessory dwelling units and require a city STR license: the applicant must be the fee owner, each person may operate only one licensed STR premises citywide, licenses run two years and terminate when the property sells, occupancy is capped per WMC 11-12-4, and every advertisement must display an active license. The initial STR application is free; renewal costs $200 plus processing.