Thornton, CO Environmental Rules: Stormwater Management (2026)

City Code Section 22-115 requires a stormwater quality permit for any land disturbance activity exceeding one acre of surface area within Thornton, and applications cannot be processed until the permit fee is paid. Fees and reinspection charges are set by city council resolution and, once collected, are deposited into the city's stormwater enterprise fund. A stormwater quality permit stays valid for up to two years from issuance; if construction and revegetation are not finished by then, the permit holder must apply for renewal and pay the renewal fee under Section 22-134(b).

The code also limits how much ground can be torn up at once: no more than 40 acres of a development may be disturbed simultaneously, though the director may authorize a variance to exceed that cap. Design of the permanent stormwater system must follow the Mile High Flood District's Urban Storm Drainage Criteria Manual and the city's MS4 permit, adopted by reference under Section 22-181, and detention volumes and release rates are set by the city's Standards and Specifications under Section 22-182. Section 22-188 separately makes it unlawful to discharge any pollutant that is not entirely stormwater into the municipal separate storm sewer system, or to maintain an illicit connection to it, except for discharges covered by the current MS4 permit or resulting from emergency firefighting activities.

Projects disturbing more than one acre, or part of a larger common plan of development, cannot get a construction or building permit until the applicant shows proof of a state CDPS stormwater discharge permit application or an approved sediment control plan under Section 22-184.