How big can a Longmont ADU be?
✓ Verified against code · August 19, 2026
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.
Citation: LMC 15.02.080.E.3.b
How current is this?
Longmont’s ADU rules run under LMC 15.02.080.E (Longmont Municipal Code) (Ordinance O-2025-35 (amended by O-2025-83)). 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. We verified this answer against primary sources on August 19, 2026, and every change we catch is logged in the changelog.
More Longmont ADU questions
- Do I have to live in Longmont to build an ADU?
- Can I Airbnb a Longmont ADU?
- What will a Longmont ADU permit cost?
All Longmont 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.