Boise, ID Accessory Structures: ADU Impact Fees (2026)

Idaho is more permissive than Virginia on municipal impact fees but more restrictive than California. The Idaho Development Impact Fee Act (Idaho Code §67-8201 through §67-8216) authorizes cities to collect impact fees only for specific categories (police, fire, parks/recreation, transportation) and only when supported by a capital improvements plan with documented rational nexus. Boise has adopted impact fees under this authority through Boise City Code provisions. ADU costs typically include: (1) Building permit fees per the Boise PDS Fee Schedule (scaled to construction valuation under the 2018 IRC); (2) Impact fees for police, fire, and parks — Idaho ADUs are typically assessed at a fraction of full single-family rates (often 40-50 percent) based on smaller anticipated household size, but the fee schedule is set annually by Boise City Council resolution; (3) Boise Public Works water and sewer connection fees if a new connection or meter is installed — shared utilities with the principal dwelling avoid these charges; (4) Stormwater fees if new impervious surface exceeds thresholds. Idaho Code §67-6511A (the 2023 ADU statute) does not preempt impact fees but limits unreasonable financial barriers. Many Idaho municipalities have reduced ADU impact fees post-2023 to comply with the statute's intent. School impact fees: Idaho law does not authorize school impact fees on residential development.