Napa County’s affordable-housing impact-fee FAQ lists a specific fee treatment for ADUs. The Board approved a tiered residential fee schedule in October 2025, with a three-year phase-in beginning January 16, 2026. For an ADU of 750 square feet or less, the County table lists a US$0 fee in 2026, 2027, and 2028.
For an ADU larger than 750 square feet, the same table says the fee is proportionate to the square footage of the primary house. It does not give a flat amount or enough detail to calculate a charge for a particular property. Do not infer a number from the 750-square-foot row or from the ADU’s own area.
Confirm which authority and charges apply
This is a County affordable-housing impact fee, not a blanket exemption from other development, utility, connection, permit, or capacity charges. The County’s schedule should not be applied automatically to the City of Napa or another incorporated jurisdiction. Confirm whether the parcel is subject to the County fee, which adopted schedule applies to the permit, and how the current formula treats the proposed unit.
The FAQ says revenue supports the County Affordable Housing Fund, including programs such as the County’s Affordable ADU Forgivable Loan. That relationship does not make a fee exemption automatic for a loan applicant and does not mean the same amount is due on every property.
Before relying on the published row, ask Napa County for its current adopted schedule and a written project-specific calculation. Also ask the relevant utility providers about separate connection and capacity costs. For the County’s affordable-unit construction support, see the Napa County ADU loan guide; for other project-linked costs, see the development and utility charge guide.