Thornton collects development impact fees for new dwelling units through the City Development Department and Thornton Water/Infrastructure Division. ADU permits typically trigger a building permit fee scaled to valuation under Chapter 10, plan-review fees, Thornton Water/sewer tap or System Development Fees, drainage and traffic impact fees per the annual Thornton Fee Schedule, and use-tax on construction materials at the Thornton sales tax rate. Thornton has not adopted a categorical ADU fee waiver, though HB24-1152 encourages waivers for small or affordable ADUs through the Colorado DLG ADU Supportive Jurisdiction program.
ADU fees in Thornton are layered. The building permit fee is scaled to valuation under the schedule referenced in Code of Ordinances Chapter 10, typically $800-$2,500 for a 600-800 sq ft ADU at current valuation multipliers. Plan-review fees add roughly 65% of the building permit fee. Thornton Water charges System Development Fees (water tap, sewer connection, and raw water dedication or cash-in-lieu) under the schedule published by the Thornton Infrastructure Department; a residential 3/4-inch service can run well over $20,000 if a separate meter is pulled, but ADUs sharing the primary dwelling's meter generally pay no new tap fee, only the additional service capacity charge. Drainage and traffic impact fees apply to net new dwelling units; rates are republished annually in the Thornton Fee Schedule. The Thornton sales-and-use tax rate (3.75%) applies to construction materials, collected at permit issuance or reconciled at final inspection. Colorado HB24-1152 (codified at C.R.S. 29-32-104 et seq.) preserves local impact-fee authority for ADUs but encourages waivers; the Colorado Division of Local Affairs operates an ADU Supportive Jurisdiction program that provides grant funding to cities adopting fee waivers for income-qualified or small ADUs. Thornton has not adopted a categorical ADU waiver as of 2026. School-district impact fees do not apply because Colorado does not authorize them.
Permits will not issue until fees are paid in full. Construction without payment is a violation of Code of Ordinances Chapter 10 prosecuted in Thornton Municipal Court with fines up to $2,650 per day under Section 38-1 of the Code. Unpaid use-tax is collected through liens. Connecting to Thornton Water without paying the System Development Fees can result in service termination and reconnection charges plus all unpaid fees.
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