Serving Idaho's Treasure Valley
Two core counties, a dozen city codes, and one fast-growing rental market. Here's how a backyard ADU pencils out from Boise to Caldwell — where the rules tighten, where the fees drop, and which pre-approved plan fits.
The Treasure Valley is the Boise metro — the federally defined Boise City MSA covers Ada, Canyon, Gem, and Owyhee counties, and the everyday valley spills into Payette and Elmore at the edges. The two cores, Ada and Canyon, held roughly 877,000 residents in 2025 (COMPASS estimate) after adding about 150,000 people in six years, which makes it one of the fastest-growing metros in the country. That growth is the whole reason an ADU pencils out here: deep, durable tenant demand and a road network straining to keep up.
The single biggest split in the valley is the county line. Ada County sits entirely inside the Ada County Highway District (ACHD) — one countywide road authority that charges a transportation impact fee of $5,803 per single-family dwelling effective March 1, 2026 (ordinance No. 254). Canyon County has no ACHD; it's served by four independent highway districts (Nampa, Notus-Parma, Golden Gate, and Canyon), each with its own approach and fee approach. The practical result is that the same plan often costs less to permit in Nampa or Caldwell than in Boise or Eagle, and Canyon County rents and lot prices run lower too.
The other thing that sets the valley apart is Boise's pre-approved ADU plan program — the City of Boise released a free catalog of permit-ready ADU designs in March 2026, the first program of its kind in Idaho, drawn up with a local architecture firm. Those plans are engineered to clear Boise's rules, which were rewritten in December 2023 to drop owner-occupancy, scrap the parking minimum, and raise the maximum ADU size to 900 square feet. And Idaho Senate Bill 1354 — signed March 31, 2026, effective July 1, 2026 — preempts owner-occupancy mandates and bans outright ADU prohibitions for every Idaho city over 10,000 people, which sweeps in Boise, Meridian, Nampa, Caldwell, and more. The upshot for a homeowner: the question isn't whether you can build, it's which county and city rules apply to your parcel — and we verify that before anyone signs.
Cities + jurisdictions
The capital and the most ADU-friendly city in Idaho. Created the state's first pre-approved plan program; removed owner-occupancy and parking minimums in the 2023 rewrite; 900 sq ft cap. Sits in Ada County, so the ACHD fee applies.
Idaho's third-largest city and one of the fastest growing. Larger newer-subdivision lots, often faster plan-check than Boise core, and HOA design review in master-planned communities. Ada County — ACHD fee applies.
Canyon County's largest city and the valley's affordability anchor. No ACHD — roads run through Nampa Highway District No. 1 — so permit math and rents both come in below the Ada County cities.
Canyon County seat with lower lot prices and deeper backyards. Served by Canyon and Golden Gate highway districts rather than ACHD. A strong spot for the larger two-bedroom and garage-combo plans.
Affluent Ada County suburb with larger lots and an extra layer — architectural compatibility, design review, and HOA approval in communities like Banbury and Two Rivers. Confirm current owner-occupancy rules with the city.
Fast-growing Ada County bedroom community with larger lots and lower buy-in than Boise. Detached ADUs fit comfortably on most parcels; verify the current size cap with the city.
The fastest-growing city by percentage in the valley — up roughly 131% since 2020 (COMPASS). Mostly Ada County with a sliver in Canyon. Acreages and deep backyards suit the larger plans.
An independent Ada County municipality surrounded by Boise, with its own code and the Live-Work-Create district. Mixed-use overlays make ADU siting different from the rest of the valley.
How the rules vary
The Treasure Valley's ADU rules split first by county — Ada has one countywide highway district and impact fee, Canyon has four independent ones — and then by city. The county line is the biggest cost variable; everything else (size cap, design review, HOA approval) is set city by city.
Every Ada County city — Boise, Meridian, Eagle, Kuna, Star, Garden City — sits inside the Ada County Highway District. ACHD charges a transportation impact fee of $5,803 per single-family dwelling effective March 1, 2026 (ordinance No. 254), the same number whether you build in Boise or Star. Whether ACHD discounts that rate for an ADU specifically is worth a direct call before you finalize a budget.
Canyon County has no countywide equivalent of ACHD. Roads are run by four independent highway districts — Nampa Highway District No. 1, Notus-Parma No. 2, Golden Gate No. 3, and Canyon No. 4 — each with its own approach permit and fee approach. ADUs adding separate road access need highway-district approach approval. Pull the current fee schedule from the specific district that covers your parcel; do not assume the Ada County number.
The City of Boise's free pre-approved ADU catalog (released March 2026, the first in Idaho) is engineered to clear Boise's code. There's no valley-wide reciprocity — the plan set still goes through each city's own plan-check (Meridian Community Development, City of Caldwell, etc.). The advantage is that a pre-approved design is already engineered and detailed, so it moves through any city's review faster than a custom submittal.
Idaho Senate Bill 1354, signed March 31, 2026 and effective July 1, 2026, requires every Idaho city over 10,000 population to allow at least one ADU per single-family lot and bars city-level owner-occupancy mandates. That sweeps in Boise, Meridian, Nampa, Caldwell, Eagle, Kuna, and Star. Cities have until February 1, 2027 to amend their codes — so during the transition, confirm what your city currently enforces.
Boise and Meridian cap ADUs at 900 sq ft (or a share of the primary dwelling); other valley cities run their own ceilings — verify the current cap with the city. Rents and lot prices are lower in Canyon County than in Ada, which changes the return on the same build. The per-city cost pages carry the local rent and fee detail.
Why Treasure Valley
Ada and Canyon counties added roughly 150,000 residents in six years. That growth outpaces housing supply, keeps rental vacancy low, and means a code-compliant ADU leases up fast almost anywhere in the valley.
The City of Boise released the state's first free, permit-ready ADU plan catalog in March 2026. Starting from a pre-approved design skips weeks of design review and thousands in design fees — and every plan is engineered under Boise's 900 sq ft cap, so it fits valley lots widely.
Want the deepest Boise rents? Stay in Ada County. Want lower lot prices and lower permit math? Cross into Canyon County (Nampa, Caldwell). We build in both and run the numbers each way so you pick the parcel that actually pencils.
SB 1354 (effective July 1, 2026) preempts owner-occupancy mandates and outright ADU bans for every city over 10,000 across the valley. The regulatory direction is toward easier ADUs, not harder — a tailwind for building now.
FAQ
The biggest difference is the highway impact fee. Every Ada County city sits inside the Ada County Highway District (ACHD), which charges $5,803 per single-family dwelling effective March 1, 2026 (ordinance No. 254). Canyon County has no ACHD — it's served by four independent highway districts (Nampa, Notus-Parma, Golden Gate, and Canyon), each with its own approach permits and fees. In practice that, plus lower lot prices and rents, makes Canyon County cities like Nampa and Caldwell cheaper to build and permit, while Ada County cities like Boise and Meridian carry higher rents. Confirm the exact fees for your parcel's jurisdiction before budgeting.
The federally defined Boise City Metropolitan Statistical Area covers four counties — Ada, Canyon, Gem, and Owyhee (per the OMB's 2023 delineation). The everyday "Treasure Valley" also reaches into Payette and Elmore at the edges. The two population cores are Ada County (Boise, Meridian, Eagle, Kuna, Star, Garden City) and Canyon County (Nampa, Caldwell, and others), which together held about 877,000 residents in 2025 per COMPASS estimates.
The plans are engineered to clear the City of Boise's code, and because they're built under the 900 sq ft cap they physically fit a wide range of valley lots. But there's no valley-wide plan reciprocity — your plan set still goes through the individual city's plan-check (Meridian Community Development, City of Caldwell, City of Eagle, and so on), and Canyon County cities run different highway-district approvals. The benefit is that a pre-approved design is already engineered and detailed, so it moves through any city's review faster than a custom submittal.
It's changing. Boise removed its owner-occupancy requirement in December 2023, so there you can rent both the house and the ADU. Idaho Senate Bill 1354, signed March 31, 2026 and effective July 1, 2026, preempts city-level owner-occupancy mandates for every Idaho city over 10,000 people — that includes Boise, Meridian, Nampa, Caldwell, Eagle, Kuna, and Star. Cities have until February 1, 2027 to update their codes, so during the transition confirm what your specific city currently enforces before you count on renting both units.
Build cost per square foot is roughly flat across the valley because the contractor and material pool is regional — Essential around $340/sqft, Standard around $405/sqft, Premium around $480/sqft. A turnkey ADU runs roughly $115k–$345k depending on plan size and finish. What changes is the permit and impact fees (higher in Ada County because of ACHD), HOA review costs, lot prices, and the rent ceiling (higher in Boise and Meridian, lower in Nampa and Caldwell). See the per-city cost pages for Boise, Meridian, Nampa, Caldwell, and others for the local detail.
It depends on your goal. For the deepest, most durable rents and the easiest code, Boise and Meridian (both Ada County) lead. For the lowest build-and-permit cost — no ACHD fee, cheaper lots — Nampa and Caldwell in Canyon County win, with rents that have been climbing as the valley grows. Star and Kuna sit in between: lower buy-in than Boise core, big enough lots for the larger plans, and fast population growth supporting demand. We run the math both ways on your specific parcel so the decision is based on real numbers, not averages.
Nearby
Sources
Last reviewed 2026-06-06. ADU rules and fees vary by jurisdiction and change over time — verify the specifics against the city or county record before any build commitment.
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