How big can a Boulder detached ADU be?
✓ Verified against code · August 19, 2026
800 sq ft — among the tightest detached caps in our Colorado corpus, though Aurora (650 sq ft) and Arvada's smallest-lot tier (600 sq ft) go lower — unless you commit to affordable rent (75% AMI, recorded restriction) or own a designated historic property, which raises it to 1,000 sq ft.
Citation: B.R.C. 9-6-3(n)(1)(A)(ii), Table 6-3
How current is this?
Boulder’s ADU rules run under B.R.C. 9-6-3(n) (Boulder Revised Code) (Ordinance 8650 (building on Ord 8571)). Boulder spent two decades as one of the country's most ADU-restrictive cities and unwound it in stages: the per-block saturation caps died in September 2023 (Ord 8571), and owner-occupancy ended March 8, 2025 (Ord 8650). Older Boulder ADU lore — caps, waitlists, owner-occupancy — describes a code that no longer exists, but the size limits remain among the region's tightest. We verified this answer against primary sources on August 19, 2026, and every change we catch is logged in the changelog.
More Boulder ADU questions
- Did Boulder really drop its ADU saturation caps and owner-occupancy rule?
- Can I Airbnb a Boulder ADU?
- Do I need parking for a Boulder ADU?
All Boulder ADU rules → · Compare Colorado cities →
Not legal advice — lot-specific overlays can change the answer for your parcel. The citation above points at the governing code section; check it, or ask the city to confirm.