Longmont ADU rules
✓ Verified against code · August 19, 2026
Current rules 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. Longmont adopted its state-compliance rewrite in June 2025 and amended it again in December. Its distinctive features: proof of City of Longmont residency at application (a broader reading than the statute's resides-on-the-parcel check), a total ban on short-term-renting ADUs, and a flat prohibition in the 100-year floodplain.
Common questions, answered from the code
How big can a Longmont ADU be?
The baseline band is 500–800 sq ft — but it's not a hard ceiling: an ADU may exceed 800 sq ft up to half the home's finished above-ground floor area, and basement ADUs have no size limit at all.
LMC 15.02.080.E.3.b · Permalink
Do I have to live in Longmont to build an ADU?
At application, yes — the city requires proof of Longmont residency (Colorado ID plus two documents like a utility bill or voter registration), waived if the ADU is built with a new home. There's no ongoing live-on-site requirement afterward. Note the state statute's carve-out is written as residency on the parcel; Longmont's city-wide version is a broader reading.
LMC 15.02.080.E.2 · Permalink
Can I Airbnb a Longmont ADU?
No — the code prohibits short-term rental of ADUs entirely; nothing under 30 days.
LMC 15.02.080.D · Permalink
What will a Longmont ADU permit cost?
Budget $5,000–$10,000 in city fees (plan review, permit, taxes, community investment fees) plus school district fees — before construction costs. The process is a five-step checklist through the building department.
City ADU guide (Jan 2026) · Permalink
The rules at a glance
| ADUs allowed per lot | 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 | 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 | A detached ADU may not exceed the height of the principal structure.LMC 15.02.080.E.3.c.i |
| Detached ADU setbacks | 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 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 | 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 | 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 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 |
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).
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.
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
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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.