Fort Collins accessory dwelling units can create flexible space on a residential property, but “can I build one?” is only the first question. The useful questions are whether the parcel is eligible, how large the unit may be, where it can sit, which utilities must change and what reviews come before construction.
This guide reflects City guidance on November 12, 2026. Verify site-specific rules, fees and requirements with City staff.
Are ADUs allowed in Fort Collins?
The short answer is broadly yes, subject to property-specific requirements. The City of Fort Collins ADU page says state law has allowed ADUs in all city zone districts since February 2025. An ADU is a smaller home on the same property as a larger house, duplex or triplex. It may be attached, such as a basement unit or addition, or detached, such as a backyard cottage or garage apartment. It must include a kitchen, bathroom and living area.
The parcel still has to qualify. The City says the lot needs an existing—or permitted—house, duplex or triplex. No new units are allowed in the Poudre River 100-year floodplain. Most properties may add one ADU, although some zone districts may allow a second detached house under separate standards. Start with the City’s zoning and floodplain maps rather than assuming a neighboring project proves eligibility.
For a broader look at locations and housing choices, use the Fort Collins community guide.
Fort Collins ADU size and placement rules
For a detached ADU, the City currently lists a maximum of 750 square feet when the main home is smaller than 1,667 square feet. For a larger main home, the maximum is 1,000 square feet or 45% of the main home—whichever is less. An existing accessory building may be converted up to 800 square feet.
For an attached ADU above grade, the maximum is 750 square feet or 45% of the main home. A basement ADU may be up to 750 square feet or 100% of that floor. Because the published wording uses “up to,” the parcel’s zone, existing building and design can produce a lower practical limit.
A detached ADU must be behind the front wall of the primary home, comply with the zone’s setbacks and remain at least five feet from other buildings. Greater separation may be needed when fire-prevention construction requirements cannot be met. Its height cannot exceed the main building or the maximum allowed in the zone. A walkway must connect the ADU entrance to the street, and the address must be visible from the street.
Parking, utilities and stormwater can shape the design
The City says an ADU does not trigger extra parking beyond what the primary home already requires. Existing off-street spaces, access and site design still need to comply.
Utilities deserve attention before a designer finalizes the footprint. The City requires an ADU to have its own electric meter and notes that some projects may require transformer upgrades or trenching. Its published spacing guidance includes 10 feet between water and sewer lines, 10 feet between storm drains and other utilities, six feet between trees and water or sewer lines, and four feet between shrubs and those lines.
If a project creates more than 1,000 square feet of new impervious surface—such as roof or pavement—the City says stormwater-quality treatment is required. Map utility routes, mature trees, drainage and fire access before finalizing the design.
The Fort Collins ADU permit path
The City outlines two separate approval stages: development review, followed by a building permit.
A free Conceptual Design Review is optional but useful before paying for a complete application. It gives City staff a chance to flag zoning, utility, access and site issues. All ADUs then go through a mandatory Basic Development Review. Planning, Utilities, Fire, Stormwater and other departments may participate; the City says no neighborhood meeting is required, although notice is posted or mailed.
After development approval, the owner applies for a building permit. Building-code review can address fire separation, heating systems, kitchens, bathrooms, exits and other life-safety requirements. Construction should begin only after the required approvals and permits are issued, with inspections following the approved plans.
What should you budget for City fees?
The current City page lists a $6,925 flat fee for Basic Development Review. It also identifies water, sewer and electric capacity charges; Larimer County road-impact and City capital-expansion fees; and building-permit fees based on project value and details. The City says total ADU development fees may fall in the $20,000–$25,000 range.
That range is not a construction estimate or a quote for a specific parcel. Design, utilities, site work and construction are separate. Verify the latest fee schedules and request address-specific utility information before relying on a project budget.
ADU due diligence for buyers and sellers
For buyers, an attractive backyard does not establish ADU feasibility. Ask for the zoning, floodplain status, survey, easements, utility locations and any HOA or recorded-covenant documents. Then compare the concept with City standards. Your home search plan should also account for financing, inspections and the rest of the property; the Northern Colorado buyer resources can help organize those next steps.
If a property already has a second living area, request permits, approved plans and the certificate or final inspection documentation. A kitchenette or separate entrance does not by itself prove the space is a legal ADU. Sellers should describe existing and potential uses accurately and avoid promising future approval.
Fort Collins ADU FAQ
Can I convert an existing garage or accessory building?
Possibly. The City’s current size guidance allows conversion of an existing accessory building up to 800 square feet, but eligibility, setbacks, structure, utilities and building-code compliance still require review.
Does an ADU require another parking space?
The City says no additional parking is required beyond what the main home already needs. Confirm that the existing property meets its applicable parking and access standards.
What is the best first step?
Check zoning and floodplain status, sketch the site with buildings and utilities, and use the City’s free Conceptual Design Review before committing to detailed plans.
Considering a Fort Collins property with an existing ADU—or one where future flexibility matters? Ask Brandon about the specific property and pair the real-estate review with confirmation from City planning, qualified design professionals and your lender.