ADU Cost in Colorado in 2026

ADU costs in Colorado in 2026 range from $80,000 for a basic garage conversion in Pueblo to $410,000+ for a high-end detached ADU in Boulder. The most common build — a 600 sq ft detached mid-range ADU in Denver — costs $190,000 to $260,000. Boulder runs 10–15% higher than Denver. Colorado Springs and Pueblo are 25–35% less than Denver. All Colorado ADUs in new construction require a radon mitigation rough-in per building code.

Colorado House Bill 24-1152, effective June 30, 2025, prohibits local governments from banning ADUs in residential zones and requires administrative approval for code-compliant applications — no discretionary hearings, no neighbor votes required. The law also stripped HOAs in Subject Jurisdictions of the ability to ban ADUs outright, though aesthetic standards still apply. Denver, Boulder, Fort Collins, Aurora, and Colorado Springs are all covered Subject Jurisdictions.

Colorado has some of the highest naturally occurring radon levels in the United States — 47% of Colorado homes exceed the EPA's 4.0 pCi/L action level. Building code requires radon mitigation rough-in (a passive pipe system) in all new Colorado residential construction including ADUs. The rough-in adds approximately $500–$1,500 during construction. If post-construction testing confirms elevated levels, an active mitigation fan is added for $1,200–$2,500. Budget both — testing is mandatory before occupancy in most Colorado jurisdictions.

WUI (Wildland-Urban Interface) zones are areas where developed land meets or intermingles with undeveloped wildland. Approximately 40% of Colorado homes fall within WUI zone classifications — with highest risk in Boulder, Jefferson, Larimer, and Douglas counties. ADUs built in WUI zones require Class A roof assemblies, fire-resistant exterior cladding, enclosed eaves, and defensible space clearance of 5–30 feet. These requirements add $5,000–$25,000 to construction costs. Check the Colorado Wildfire Risk Map (CO-WRAP) before committing to a WUI-area project.

A mid-range 600 sq ft ADU in Denver costs $200,000 to $270,000 all-in and generates $1,500 to $2,500 per month in rental income. Cash-on-cash ROI runs 6–8%, with a cash payback period of 12–18 years. Basement ADUs offer the best Denver ROI — build cost around $100,000 with comparable rental income to detached units. ADUs add a 20–35% resale premium to Denver properties, which materially improves the total investment return even with longer cash payback periods.

Yes — and this is one of Denver's most commonly overlooked ADU costs. If a garage conversion eliminates off-street parking, Denver code requires the parking to be replaced elsewhere on the lot. Replacing a parking space through driveway expansion, a carport, or a new parking pad costs $10,000–$30,000 depending on site conditions. Most other Colorado Front Range cities (Fort Collins, Boulder, Aurora) do not have this requirement for ADUs — it is specific to Denver.