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Longmont, CO Outdoor Lighting: Dark Sky Rules (2026)

Some Restrictions
Compiled from the official code textEditor Martyn O'NeillLast verified August 2026

Key Facts

Fixture standard
Full cutoff for all new/replacement lights
Exempt sources
Under 500 lumens (150 per string light)
Color temperature cap
3000K residential, 4000K elsewhere
Prohibited lamp types
Low-pressure sodium, mercury vapor
Photometric plan required
For new development/redevelopment
Existing lighting
Still bound by § 15.05.140.F and G

Summary

Longmont requires every new or replacement outdoor light fixture to be full cutoff, keeping light from escaping above the fixture, under Municipal Code § 15.05.140.G.2. Small fixtures are exempt: individual lights under 500 lumens, string lights under 150 lumens per bulb, and fixtures no warmer than 3000 Kelvin don't have to meet the full-cutoff standard.

2.All new and replacement light fixtures shall be full cutoff, except that light sources not exceeding 500 lumens for individual light fixtures, 150 lumens for individual lights in a light string (e.g., patio lights), and 3000 Kelvin in color temperature are not required to be full cutoff.

View official code

Full Breakdown

3). 2, requires that 'all new and replacement light fixtures shall be full cutoff'; the only carve-out is for small sources, individual fixtures under 500 lumens, string lights (patio lights) under 150 lumens per bulb, and anything no warmer than 3000 Kelvin. 12). 1 keeps every existing property subject to the general regulations in F and the design standards in G even without a new lighting plan. D).

Violations & Fines

The director can require post-installation modifications to any fixture that does not comply with § 15.05.140.G, under § 15.05.140.F.4, and noncompliant lighting is otherwise a development-code violation under § 15.09.030. Enforcement under § 15.09.050 lets the city withhold or deny permits and certificates of occupancy, revoke an issued permit, or seek a court injunction until the fixtures are brought into compliance.

Frequently Asked Questions

What is a 'full cutoff' fixture under this rule?
The code ties it to the fixture cutting off light at the horizontal plane: fixtures must be mounted with a luminaire angle of zero degrees under G.4 and shielded so sources aren't visible or aimed toward property lines under G.3 and G.5, in addition to meeting the full-cutoff mandate in G.2.
Does my existing porch light have to be replaced?
Only if it's new or a replacement fixture; § 15.05.140.G.2 applies going forward to 'new and replacement' fixtures. Existing installations remain subject to the general regulations in subsection F, such as the ban on blinking or color-changing lights, even without a plan on file.
Are string lights and small patio lights allowed?
Yes. Section 15.05.140.G.2 exempts individual light sources under 500 lumens and individual lights in a string, such as patio lights, under 150 lumens, along with any source at or below 3000 Kelvin, from the full-cutoff requirement.
Who has to submit a lighting plan before installing exterior lights?
New development and redevelopment projects must submit a photometric site plan designed by a licensed engineer or architect under § 15.05.140.C; single-family detached homes are exempt from that plan requirement but their lighting still must meet the design standards in subsections F and G.

Sources & Official References

Other rules in Longmont

Light Trespass & ShieldingSome RestrictionsNoise Ordinances: Amplified Music & Outdoor SoundSome RestrictionsNoise Ordinances: Aircraft NoiseLight RestrictionsParking Rules: Driveway RulesSome RestrictionsParking Rules: Abandoned VehiclesSignificant RestrictionsParking Rules: Loading ZonesSome RestrictionsFence Regulations: Fence Permit RequirementsSome Restrictions

All Longmont rules

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