ADU rules in Fort Collins

Detached ADUs (a free-standing backyard cottage or a conversion of an existing accessory building) are allowed anywhere a detached house is allowed or already exists. Ordinance 009, 2025 rewrote the Land Use Code for Colorado HB24-1152 and added Accessory Dwelling Unit to the Table of Primary Uses in every zone district where a detached house is or can be built, effective February 14, 2025. The city's own ADU page states that state law HB 24-1152 now allows ADUs in all Fort Collins zone districts as long as the property meets city requirements. An ADU may be accessory to a detached house, mobile home, duplex or triplex on the same lot, and Article 2 caps each lot at one ADU.

Attached and internal ADUs are allowed on the same terms as detached ones; the Table of Primary Uses lists a single Accessory Dwelling Unit use covering both categories. Article 3 defines an attached ADU as one that shares at minimum one common wall with the primary dwelling, and Article 7 adds that it may have a separate exterior entrance or an entrance into an internal common area. Basement conversions are explicitly contemplated: the definition exempts an ADU in an existing basement from the rule that the ADU be smaller than the primary dwelling, and the size table gives basement ADUs a separate, more generous cap.

New detached ADUs are capped at 750 sq ft when the primary building is smaller than 1,667 sq ft, and at 1,000 sq ft or 45 percent of the primary dwelling unit (whichever is less) when it is larger; converting an existing accessory structure that was already legal when the Land Use Code took effect allows up to 800 sq ft, not counting non-habitable space. An attached ADU at or above grade may be the greater of 750 sq ft or 45 percent of the primary dwelling, and a below-grade (basement) ADU may be the greater of 750 sq ft or 100 percent of that floor level. A detached ADU is limited to 1.5 stories and 28 ft, or the zone district standard if it is stricter, and several zones (UE, RF, MH) cap the ADU at no taller than the primary house or mobile home on the same lot; an attached ADU may be no taller than the primary dwelling. In the RL district the lot area test for the detached house excludes the ADU's floor area.

There is no owner-occupancy requirement. The only subordination rule in the Land Use Code is that the ADU be subordinate to a primary dwelling unit on an undivided lot; nothing in Article 3, Article 4 or the city's ADU page requires the owner to live on the property, and Ordinance 009, 2025 was adopted specifically to strip the pre-2025 ADU restrictions. Colorado HB24-1152 also forbids it: C.R.S. 29-35-103(2)(b) bars a subject jurisdiction from requiring an ADU, or any other dwelling on the same lot, to be owner-occupied (it may only ask an applicant to show they reside on the parcel when applying to build or convert an ADU that is not being built at the same time as a new primary dwelling, or when applying for a short-term rental license). Fort Collins is a subject jurisdiction: it has well over 1,000 residents and is one of the fifteen member governments of the North Front Range MPO.

Adding an ADU triggers no additional off-street parking. The Land Use Code's residential parking list in Section 5.9.1(K)(1) sets minimums for a detached house (one space if the lot has more than 40 ft of street frontage, two spaces if 40 ft or less), duplexes and rowhouses, manufactured homes, and short-term rentals, but has no entry for an ADU; Ordinance 009, 2025 rewrote that list and left ADUs out. The city's ADU page says the same in plain terms: no extra parking is required beyond what the main home already needs. Where a parking space is required for the lot, Section 5.9.1(D)(2) lets one tandem space count toward the minimum. Colorado HB24-1152 independently bars a subject jurisdiction from requiring a new off-street space for an ADU except in narrow cases (no existing driveway, garage or tandem space, a zone that required parking for the primary dwelling as of January 1, 2024, and a block where on-street parking is prohibited).

ADUs are approved administratively. The Table of Primary Uses assigns Accessory Dwelling Unit the Basic Development Review (BDR) level of review in every district where it is listed, which Ordinance 009, 2025 changed from the prior heavier review. BDR is a staff process under Article 6 Division 6.4: the decision is made by the Planning Manager or Community Development Director with no neighborhood meeting and no public hearing, though notice is posted or mailed. Section 6.4.3(L) removes the appeal step entirely for ADU applications, and Section 6.3.12(C) likewise exempts ADU decisions from appeals to the Planning and Zoning Commission. After development review approval a building permit is required. The city quotes a flat $6,925 BDR fee for ADU projects and total fees of roughly $20,000 to $25,000 once utility, transportation and building permit fees are added. A free, optional Conceptual Design Review with staff is available first.

Renting an ADU for 30 days or more is allowed, and there is no owner-occupancy condition on doing so. Since January 1, 2025 most long-term rentals in Fort Collins must be registered with the City every year and self-certified as meeting minimum housing standards, at a base fee of $37 per property plus $10 per additional unit; owner-occupied units are exempt, so an owner living in the house who rents out the ADU should confirm with the City how the exemption is applied to that unit. Short-term rental (under 30 consecutive days) of an ADU is barred: Land Use Code Section 4.3.1(B)(1) and 4.3.1(L)(3)(a) prohibit short term primary and non-primary rentals in accessory buildings and ADUs for any unit that applied for a building permit on or after January 1, 2024, and the city's STR FAQ states flatly that an ADU may not be used as an STR. The only exception is grandfathering: ADUs holding a short-term rental license issued before January 1, 2024 may keep renewing it, and pre-March 2017 legacy uses may still license under City Code Section 15-646. Any lawful STR needs a $500 annual city license plus sales and lodging tax licenses.