Boulder ADU rules
✓ Verified against code · August 19, 2026
Current rules under B.R.C. 9-6-3(n) (Boulder Revised Code), adopted by Ordinance 8650 (building on Ord 8571), in effect since March 8, 2025. 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 the region's tightest.
Common questions, answered from the code
Did Boulder really drop its ADU saturation caps and owner-occupancy rule?
Yes, in stages: the per-block saturation caps ended September 1, 2023, and owner-occupancy ended March 8, 2025 under the state ADU law. If a guide mentions caps, waitlists, or living on site, it's describing the old regime.
Ord 8571 (2023); Ord 8650 (2025); city ADU page · Permalink
How big can a Boulder detached ADU be?
800 sq ft — the tightest detached cap in our Colorado corpus — unless you commit to affordable rent (75% AMI, recorded restriction) or own a designated historic property, which raises it to 1,000 sq ft.
B.R.C. 9-6-3(n)(1)(A)(ii), Table 6-3 · Permalink
Can I Airbnb a Boulder ADU?
No — short-term rental of either the ADU or the main house on an ADU property is prohibited, with only a narrow pre-February-2019 grandfather. Long-term rentals require a city rental license.
B.R.C. 9-6-3(n)(1)(A)(iv); 10-3-19 · Permalink
Do I need parking for a Boulder ADU?
No — Boulder's parking chapter sets no vehicle-parking minimums for ADUs at all.
B.R.C. 9-9-6 · Permalink
The rules at a glance
| ADUs allowed per lot | One ADU per lot — attached or detached, not both — on a lot with one detached home, by right in all zoning districts except RH-6, MH, BMS, BCS, IS-1, and IM.B.R.C. 9-6-3(n)(1)(A)(i); 9-6-1 Table 6-1Previously: Per-block saturation caps limited how many ADUs a neighborhood could hold — changed by Ord 8571, effective Sept 1, 2023 |
| Maximum size | Attached: the lesser of half the house's floor area or 1,000 sq ft (750 if the house is under 1,500). Detached: 800 sq ft. Affordable ADUs (rent capped at 75% AMI) and designated historic properties get more: attached to 1,200, detached to 1,000. Areas under 6-ft ceilings, mechanical rooms, and below-grade parking don't count.B.R.C. 9-6-3(n)(1)(A)(ii), Table 6-3 |
| Detached ADU height | 20 ft for a detached ADU; the manager may allow 25 ft with a roof pitch of 8:12 or steeper. Conversions of legal existing accessory buildings may keep their height and roof form.B.R.C. 9-6-3(n)(3)(A) |
| Detached ADU setbacks | Accessory-building setbacks: interior side and rear 3 ft; front 55 ft or behind the house's rear wall; 6 ft of separation from any other building. An attached ADU can extend into the rear setback under conditions (ADU-only space, 5 ft off the rear line, ≤500 sq ft, ≤20 ft tall).B.R.C. 9-7-1 Table 7-1; 9-6-3(n)(2)(B) |
| Off-street parking | None — the parking chapter contains no minimum vehicle-parking requirements for ADUs (or anything else; only maximums in two districts).B.R.C. 9-9-6 |
| Owner-occupancy | Not required since March 8, 2025 — the city's page says so explicitly, and no owner-occupancy standard survives in the code.City ADU page; B.R.C. 9-6-3(n) (absence verified)Previously: Owner occupancy was required — changed by Ord 8650, effective Mar 8, 2025 |
| Lot coverage / floor area | The lot must still meet district FAR and building-coverage limits, and accessory buildings between the house's rear setback and the rear lot line are capped at 500 sq ft of cumulative coverage in the RR/RE/RL/RMX-1 districts.B.R.C. 9-7-1 Table 7-1; 9-7-8(a) |
| Permit path & timeline | Building permit only — the prior administrative-review step was eliminated in September 2023. ADUs in old PUD areas are approved as minor modifications, and prior PUD ADU bans are void. No published timeline.City ADU page; B.R.C. 9-6-3(n)(1)(A)(viii) |
Worth knowing
- The affordable-ADU trade: cap the rent at 75% AMI (recorded Declaration of Use) and the size limits jump — 800→1,000 detached, 1,000→1,200 attached.
- Renting either unit requires a City of Boulder rental license, and detached ADU rentals must meet SmartRegs efficiency standards.
- Short-term rental of the ADU or the main house is prohibited (narrow pre-2019 grandfather).
- Detached ADUs need fire sprinklers and an In-Property Wet Utility Permit — real cost lines.
- No separate sale of the ADU; the saturation caps and waitlists of Boulder legend died in September 2023.
On our watchlist
Ordinance 8745 (2026) re-amended section 9-6-3 as part of a broad Title 9 package; our text reflects the post-8745 codified version, and we characterize 8745's specific changes on the next pass.
Check your parcel
The rules above are citywide — what you can build depends on your lot’s zone, size, and setbacks. Look yours up on the official viewers:
Primary sources
Other Colorado cities
Arvada · Aurora · Colorado Springs · Denver · Fort Collins · Greeley · Lakewood · Longmont · Pueblo · Thornton · Westminster
Not legal advice. Every fact above cites the code section it comes from — check the cite, or ask your city’s planning department to confirm your specific lot. Spotted an error or a new ordinance? See about to reach us; corrections are logged in the changelog.