Building ADUs across Canyon County

It depends on the city, and the caps are smaller than Boise's. Nampa limits accessory dwellings to roughly 500 square feet with one bedroom. Caldwell allows a larger unit, but its published size ceiling has shifted — figures from 700 toward 900 square feet have circulated — so the current cap has to be confirmed against the city's live zoning code. Middleton permits larger units through a conditional-use process with a minimum lot size. Because the limits vary this much, we start every Canyon County project by pulling the specific city's current ADU standards rather than assuming Boise's 900 sq ft number applies.

No. Ada County sits entirely inside the Ada County Highway District, which charges one consistent transportation impact fee everywhere. Canyon County has no single road authority — it's split among several independent highway districts (Nampa Highway District No. 1, Highway District No. 4 around Caldwell and Middleton, and others), plus streets that individual cities maintain inside their limits. So the transportation fee, if your build triggers one, depends on which district or city your parcel falls in. We confirm the jurisdiction and current fee schedule for your specific address before quoting a number — we never assume an Ada County figure carries over.

Several Canyon County cities still require owner-occupancy, but that is changing. Idaho Senate Bill 1354, signed March 31, 2026 and effective July 1, 2026, preempts owner-occupancy mandates and most parking requirements for cities over 10,000 population — which covers Nampa, Caldwell, and Middleton. Cities have until February 1, 2027 to update their codes, so the ordinance text on file may still show an owner-occupancy rule even after the state law overrides it. Confirm your city's current position before counting on renting both the house and the ADU.

Caldwell generally allows a larger unit than Nampa, which makes a two-bedroom or family-sized ADU more workable there — but verify its current size cap first, because it has moved. Nampa is the largest market and the deepest rental pool, but its roughly 500 sq ft cap limits you to a compact studio. Middleton adds a conditional-use permit and a minimum lot size, which is an extra step. For most homeowners the smoothest path is a Caldwell or Nampa parcel using a plan already engineered under the city's cap.

Build cost per square foot is similar across the Treasure Valley because the contractor pool is regional — the same crews work both counties. What's different in Canyon County is the inputs around the build: land costs less, the size caps are smaller (so the total build is often smaller and therefore cheaper), and rents are lower, which changes the return math rather than the per-foot price. See the per-city cost pages for Nampa and Caldwell for the local fee and rent detail.

Often yes, but the rules and the permitting office change. Parcels inside Nampa, Caldwell, Middleton, or Parma city limits go through that city. Parcels in unincorporated Canyon County go through Canyon County Development Services, and rural builds usually hinge on septic approval from Southwest District Health rather than a city sewer connection. Land is cheaper and lots are larger out here, which helps the math, but confirm the parcel's jurisdiction, zoning, and septic capacity before assuming you can build.