How Longmont Handles Outdoor Lighting: A Practical Guide
Longmont maintains 129 local ordinances across all categories, and 2 of those deal specifically with outdoor lighting. Here is a breakdown of what the city actually requires, what is prohibited, and where Longmont falls on the strict-to-permissive spectrum compared to other cities.
Dark Sky Rules
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.
Key details: 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.
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.
Light Trespass & Shielding
Longmont caps how much light can spill onto a neighboring residential lot, greenway or natural area at 0.1 footcandles at the property line under Municipal Code § 15.05.140.G.7. Fixture owners must also conceal or shield light sources to limit glare (G.3) and cut unnecessary security lighting after hours or motion-activate it (G.9).
Key details: Trespass limit: 0.1 footcandles at residential property line. Applies to: Residential use, greenway, natural area. Fixtures must be: Concealed or shielded, § 15.05.140.G.3. Aim rule: No light directed at property boundaries. Off-hours rule: Unneeded security lights sensored or off.
A light trespass complaint that shows more than 0.1 footcandles at a residential property line, or a fixture aimed at a neighboring lot, is a development-code violation under § 15.09.030. The director can order the fixture modified after installation under § 15.05.140.F.4, and under § 15.09.050 the city can withhold or deny building permits and certificates of occupancy, revoke an issued permit, or seek an injunction until the glare is corrected.
The Bottom Line
Longmont's outdoor lighting rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Longmont is broadly strict or permissive.
Keep in mind that Longmont can amend these rules at any council meeting. For the most current version of any rule mentioned here, check the specific ordinance page, where we track updates as they happen.