Boulder vs. Longmont ADU rules
Boulder and Longmont both ban short-term rentals of an ADU, but they part ways on who can build one and how big. Boulder dropped owner-occupancy entirely in March 2025 (Ordinance 8650) and caps a detached ADU at a strict 800 sq ft under B.R.C. 9-6-3(n)(1)(A)(ii), rising to 1,000 only for affordable or designated-historic units. Longmont allows a smaller 500 sq ft floor but lets larger homes build up to 50% of their floor area (basements are uncapped), while requiring proof of City of Longmont residency at application under LMC 15.02.080.E.2 — a broader condition than Boulder imposes.
✓ Verified against code · August 19, 2026
Boulder rules run 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.
✓ Verified against code · August 19, 2026
Longmont rules run under LMC 15.02.080.E (Longmont Municipal Code), adopted by Ordinance O-2025-35 (amended by O-2025-83), in effect since June 3, 2025.
Rule by rule
| Rule | Boulder | Longmont |
|---|---|---|
| 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 | One ADU per lot, accessory to a single-family detached home only — integrated, attached, or in a detached accessory structure (a garage works). No mobile homes, RVs, or trailers as ADUs.LMC 15.02.080.E.3.a–b |
| 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 | 500–800 sq ft regardless of the home's size — and larger is allowed up to 50% of the home's finished above-ground floor area, so 800 isn't a hard cap on bigger houses. Basement ADUs have no size limit at all.LMC 15.02.080.E.3.b.ii–iv |
| 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) | A detached ADU may not exceed the height of the principal structure.LMC 15.02.080.E.3.c.i |
| 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) | Rear 5 ft, side 5 ft; no closer to the front line than the house (corner and multi-frontage lots get flexibility). An ADU inside an existing detached structure keeps its setbacks unless expanded.LMC 15.02.080.E.3.d |
| 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 | None required, and no more than two spaces may be provided. (The city's guide describes a statute-shaped one-space exception for lots with no off-street parking and no on-street option — not yet visible in the codified text.)LMC 15.02.080.E.3.f |
| 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 | No ongoing requirement — but the applicant must prove City of Longmont residency at application (Colorado ID plus two supporting documents), waived when the ADU is built together with a new home. Note the statute's carve-out is residency on the parcel; Longmont's city-wide version reads more broadly.LMC 15.02.080.E.2; C.R.S. 29-35-403(2)(b) |
| 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) | All accessory structures combined may not exceed 75% of the home's floor area — which can force removing or shrinking existing sheds and garages to fit an ADU. ADUs are excluded from density calculations.LMC 15.04.040.B.1; 15.03.070.B.3.e |
| 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) | Building permit through the Chief Building Official — a five-step checklist process with multi-department plan check. No published timeline; typical city fees run $5,000–$10,000, plus school fees.LMC 15.02.080.E.1; city ADU guide |
Boulder: 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.
Boulder: 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.
Longmont: worth knowing
- Short-term rental of an ADU is prohibited outright — no rentals under 30 days, ever.
- No ADUs in the 100-year floodplain or floodway, full stop.
- Architectural matching standards apply: matching eaves and colors, corner-lot façade features, and no fluorescent or glossy finishes.
- A 3-ft all-weather sidewalk to the ADU entrance, wayfinding signage, and a separate address are required.
- Post-2025 PUDs can't restrict ADUs beyond the code, and HOAs can't prohibit them (state law).
Longmont: on our watchlist
The city guide's one-space parking exception (statute-shaped) isn't in the codified text — 17 ordinances were pending codification at our last check; we watch for the amendment.
Whether the city-wide (vs. on-parcel) residency-proof reading draws a state compliance challenge.
Full detail: Boulder · Longmont · back to the Colorado hub. 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.