Longmont allows a detached accessory dwelling unit by right on most single-family lots. Here is exactly what the code permits — size, setbacks, parking, and owner-occupancy — each point cited to the adopted regulations.
Every figure below is drawn from Longmont's adopted ADU regulations. The exact setbacks for your parcel depend on its zoning — the planner reads them off your lot.
Permitted by right
Yes
Approved through the normal building-permit process — no discretionary hearing.
Maximum size
Up to 750 sq ft
Detached ADU max = the greater of 750 sf or 50% of the principal dwelling's floor area, further capped so all accessory structures total ≤ 75% of the dwelling (site-verify existing garages/sheds). LDC §15.04.040 / §15.02.080.E.
Owner-occupancy
Not required
You don't have to live on-site — Colorado's HB24-1152 bars owner-occupancy mandates for subject jurisdictions.
Off-street parking
None required
No off-street parking required (max 2 allowed). One space is required only where the lot has no existing off-street parking AND either has no public street frontage, takes vehicular access from an alley, or on-street parking is prohibited (e.g. for emergency access). Longmont ADU Guide (July 2025).
Setbacks
5 ft side · 5 ft rear
ADU may not sit closer to the front property line than the principal dwelling. City of Longmont ADU regulations (LDC §15.03 / ADU standards)
Source
Longmont LDC §15.04.040 (ADU = permitted accessory use) / §15.02.080.E (ADU standards); Longmont ADU Guide (July 2025); owner-occupancy + parking per Colorado HB24-1152 (eff. June 30, 2025)
This is a plain-language summary for planning, not a legal determination. Codes are amended; confirm the current requirements with Longmont and a design professional before you rely on them.
A subject jurisdiction must allow at least one ADU by right (administrative — no discretionary review or public hearing) on any lot where a single-unit detached dwelling is an allowed use, and may not impose owner-occupancy mandates, ADU-specific minimum lot size, parking minimums (narrow exceptions only), or design standards stricter than the primary dwelling. It took effect June 30, 2025. That is why cities like Longmont now approve a backyard home administratively — and why owner-occupancy and parking mandates are largely gone.
We keep cited ADU rules for every city below. Switch to yours.
Draw your lot and house on the satellite map, drop an ADU, and see the setbacks and a budget range on your own property — in about a minute.
Keep exploring
ADU rules by city
Cited, city-by-city ADU rules across the Front Range.
Rental income potential
Real numbers on what ADUs earn.
ADUs for family
Aging parents, multigenerational living, childcare.
How to finance an ADU
HELOC, construction loans, and CHFA options.
Process & timeline
What to expect from contract to move-in.
Floor plans & pricing
Three sizes, three tiers, two styles.
About Addwell
Who we are and how we build.