ADU permits in Boulder trigger building permit fees, plan review fees, Boulder water and sewer plant investment fees, and the Development Excise Tax under the Planning and Development Services Schedule of Fees. Permit fees for a typical ADU run roughly $1,900-$3,200. Critically, City Council's Ordinance 8712 (adopted November 6, 2025) established a new citywide affordable housing impact fee of $11 per square foot on replacement homes and substantial additions — but explicitly EXEMPTS the area of a new ADU from this fee. The exemption applies to complete permit applications submitted on or after January 31, 2026.
Boulder's ADU fee structure stacks several charges, all governed by the annual Planning and Development Services Schedule of Fees and BRC Title 4 (Licenses, Permits, and Taxes). The base building permit fee scales with valuation under the standard residential schedule; expect roughly $1,900-$3,200 for a 600-800 sf ADU. Plan review adds approximately 65% of the building permit fee. Boulder water and sewer Plant Investment Fees are the largest line items: a separate 3/4-inch tap can run $15,000-$22,000 for water plus a similar sewer charge, while ADUs that share the principal dwelling's meter typically pay reduced or zero new tap fees. The Development Excise Tax under BRC Title 4 applies to new residential development and funds affordable housing, fire-rescue, and parks; rates are published annually. The newest piece is Ordinance 8712, adopted by City Council on November 6, 2025 and effective for permits applied for on or after January 31, 2026 — it imposes a flat $11/sf affordable housing impact fee on replacement homes and substantial additions to single-unit homes, but the ordinance expressly exempts the area of a new ADU. This exemption is a deliberate incentive aligned with Colorado HB24-1152's encouragement of small-unit fee waivers. School-district impact fees do not apply in Colorado because they are not statutorily authorized.
Permits will not issue until all fees and taxes are paid in full. Construction without permit and payment is a municipal violation under BRC 5-2-4 with fines up to $1,000 per day and/or 90 days in jail. Unpaid Plant Investment Fees can result in service termination by Boulder Water Utilities. Unpaid Development Excise Tax accrues interest, penalties, and a lien against the property under BRC Title 4. After-the-fact permits are charged at double the standard fee.
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See how Boulder's adu impact fees rules stack up against other locations.
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