Serving Ada County, Idaho
Six cities, one highway district, and ADU rules that change at every city line. Here's how a backyard ADU pencils out in Boise, Meridian, Eagle, Kuna, Star, and Garden City — and where the rules actually differ.
Ada County is the center of gravity for ADUs in Idaho. It holds the state capital, more than half a million residents, and the only city in Idaho — Boise — that publishes a set of pre-approved ADU plans. When the City of Boise rewrote its zoning code in December 2023, it removed the owner-occupancy requirement, scrapped the off-street parking minimum, and raised the maximum ADU size to 900 square feet. ADU permit applications jumped roughly nine-fold, and the rest of the county has been moving in the same direction ever since.
What makes Ada County tricky is that the rules change at every city line. Boise runs the most permissive code in the state. Meridian's Unified Development Code permits faster but is stricter on owner-occupancy in some zones. Eagle layers design review and architectural compatibility on top of the base code. Kuna, Star, and Garden City each have their own quirks. The one constant is the Ada County Highway District (ACHD), the single countywide road authority whose transportation impact fee applies to every new dwelling in the county — $5,803 per single-family dwelling effective March 1, 2026, per ACHD ordinance No. 254.
For a homeowner, that means the question isn't really "can I build an ADU in Ada County" — it's "which set of rules applies to my specific parcel, and which pre-approved plan fits." Every City of Boise pre-approved plan (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey) is engineered to stay under the 900 sq ft cap, which means it works in every Ada County jurisdiction that allows ADUs — but each city still runs its own plan-check. We pull permits in all six Ada County cities and verify the parcel-specific rules before anyone signs.
Cities + jurisdictions
The capital and the most ADU-friendly jurisdiction in Idaho. Created the pre-approved plan program; removed owner-occupancy and parking minimums in the 2023 rewrite; 900 sq ft cap.
Idaho's third-largest city. Faster plan-check than Boise core, larger newer-subdivision lots, and HOA design review in master-planned communities. 900 sq ft cap.
Affluent suburb with larger lots and an extra layer — architectural compatibility, design review, and HOA approval in communities like Banbury and Two Rivers. Owner-occupancy still applies.
Fast-growing Ada County bedroom community with larger lots and lower buy-in. Detached ADUs fit comfortably on most parcels; verify the current size cap with the city.
The fastest-growing city by percentage in the county — acreages, equestrian parcels, and deep backyards that suit the larger Kestrel and garage-combo plans.
An independent 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 county.
How the rules vary
Every Ada County city sits inside the Ada County Highway District, so the transportation impact fee is the one cost that's consistent countywide. Almost everything else — size cap, owner-occupancy, parking, design review — is set by the individual city.
ACHD is the single road authority for all of Ada County. Its residential transportation impact fee is $5,803 per single-family dwelling effective March 1, 2026 (ordinance No. 254). Whether ACHD discounts the rate for an ADU is worth a direct call before you finalize a budget — confirm the current ADU treatment with ACHD.
Boise removed its owner-occupancy requirement in December 2023 — you can rent both the house and the ADU. Eagle generally still requires it. Meridian keeps it in certain zones. Idaho Senate Bill 1354 (effective July 1, 2026) preempts owner-occupancy mandates for cities over 10,000 population, which will pull the larger cities into line, but verify your city's current code.
Boise and Meridian both cap ADUs at 900 sq ft (or 50% of the primary dwelling). Eagle runs a similar ceiling. Every City of Boise pre-approved plan is engineered under that cap, so plan fit is rarely the constraint in Ada County — lot setbacks and HOA review are.
There's no single "Ada County" ADU permit for incorporated parcels — your permit goes through the city (Boise, Meridian, Eagle, Kuna, Star, or Garden City). Unincorporated Ada County parcels go through Ada County Development Services. We confirm which office owns your parcel before drawing anything.
Why Ada County
Ada County anchors the Boise metro labor market — St. Luke's, Saint Alphonsus, Micron, HP, the state government, and Boise State all draw tenants who want a short commute. ADU vacancy is low and lease-up is fast across the county.
The pre-approved plan program was created by the City of Boise specifically for this market. Using one skips 4-8 weeks of design review and roughly $8-15k in design fees — and every plan already fits the county's 900 sq ft cap.
From 5,000 sq ft North End historic lots to Star acreages, Ada County has a lot type for every plan — compact studios for tight Boise Bench parcels, the two-bedroom Kestrel for deep Meridian and Eagle backyards.
Because ACHD is countywide, the biggest impact fee is the same number whether you build in Boise or Star — no surprise transportation charge that changes at the city line.
FAQ
It depends on the city. Boise removed its owner-occupancy requirement in December 2023, so you can rent both the primary home and the ADU. Eagle generally still requires the owner to live on-site, and Meridian keeps it in certain zones. Idaho Senate Bill 1354 preempts owner-occupancy mandates for cities over 10,000 population effective July 1, 2026, which will eventually standardize the larger cities — but confirm your specific city and parcel before you count on renting both units.
The Ada County Highway District charges a transportation impact fee of $5,803 per single-family dwelling, effective March 1, 2026 (ACHD ordinance No. 254). ACHD is the single road authority for the whole county, so this fee applies in Boise, Meridian, Eagle, Kuna, Star, and Garden City alike. Whether ACHD applies a reduced rate to an ADU specifically is worth confirming directly with ACHD before you lock your budget — we include the confirmed pass-through fees in the fixed-price contract once your parcel is verified.
Boise has the most permissive code — no owner-occupancy, no parking minimum, a 900 sq ft cap, and six pre-approved plans that skip design review. Meridian often issues permits faster but is stricter on owner-occupancy in some zones and adds HOA review in newer subdivisions. Eagle is the most involved because of architectural compatibility and design review. For most homeowners the easiest path is a Boise or Meridian parcel using a pre-approved plan.
The plans physically fit everywhere in the county because they're all engineered under the 900 sq ft cap that Boise, Meridian, and Eagle share. But there's no countywide plan reciprocity — your plan set still goes through the individual city's plan-check (Meridian Community Development, City of Eagle, 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.
Build cost per square foot is essentially flat across Ada County — Essential around $340/sqft, Standard around $405/sqft, Premium around $480/sqft — because the contractor pool is regional. A turnkey ADU runs roughly $115k-$345k depending on plan size and finish. What changes city to city is permit and impact fees, HOA review costs, and the rent ceiling. See the per-city cost pages for Boise, Meridian, Eagle, and others for the local detail.
Parcels in unincorporated Ada County are governed by Ada County Development Services rather than a city, and the rules differ from any of the incorporated cities. If your address isn't inside Boise, Meridian, Eagle, Kuna, Star, or Garden City limits, the county handles your permit — confirm the parcel's jurisdiction and the applicable zoning before assuming city rules apply.
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.
Ada County, Idaho · Free Lot Check
Free 24-hour lot check — zoning, plan fit, and a real number for your specific Ada County parcel. Or call (208) 297-2036.
