Type AThe Goldfinch
From $115k
280 sq ft · Studio · 1 ba
The smallest pre-approved Boise ADU plan — a 280 sq ft studio that fits the tightest lots and the cleanest budgets.
See the The Goldfinch
Boise · Foothills · Hillside Development Overlay
ADU builder in the Boise Foothills
Foothill ADUs trigger Boise's Hillside Development Overlay — 4–8 extra weeks of review and a geotechnical soils report on top of normal permitting. Smaller pre-approved plans on the flatter portion of the lot are the honest path here.
Boise Foothills · Boise, Idaho · zips 83702, 83712, 83714
What an ADU build looks like in Boise Foothills
The Boise Foothills wrap the northern and eastern edge of the city — hillside parcels above the North End, East End, and Northwest Boise across zips 83702, 83712, and 83714, including the Highlands, Highlands Cove, Crane Creek, and Hulls Gulch-edge subdivisions. Lots are larger, views are premium, and the regulatory overhead is the highest of any in-scope neighborhood because Boise's Hillside Development Overlay (HS-O) applies wherever slope exceeds 15%.
Foothill lots typically run a quarter to a half acre (~10,890 sq ft to ~21,780 sq ft), with luxury parcels reaching 2.5 acres or more. Lot size on paper is generous, but buildable area is constrained by slope, setbacks, and the engineering required to put a structure on grades over 15%. Housing stock ranges from 1970s ranch to current-decade custom builds; HOA and design-review covenants apply on most subdivisions.
Overlays + Review
Which Boise overlays apply in Boise Foothills
Hillside overlayHOA / CC&R review
Boise's Hillside Development Overlay applies wherever slope exceeds 15% or where slope-stability, expansion soils, high water table, springs, erosion, or sedimentation conditions are present. The overlay sorts projects into three categories: Category 1 (planning director approval, minor work on prepared pads or pre-approved-lot single-family construction), Category 2 (exterior additions, moderate grading, retaining walls > 4 ft exposed, driveways > 100 ft or > 15% grade), and Category 3 (PUDs, preliminary plats, major grading). Slopes over 25% are flagged as effectively undevelopable unless the project engineer can demonstrate the limitations can be overcome. Most foothill subdivisions also have HOA design review separate from city code.
ADU considerations
What actually matters on a Boise Foothills ADU project
Hillside review adds 4–8 weeks to the permitting calendar AND requires a geotechnical soils report — third-party labs typically quote $7K–$9K for a geotech study on a Boise foothill site, with 3 weeks to 2 months of lead time before the report is in hand.
Slopes over 25% are effectively undevelopable for an ADU under HS-O absent engineered demonstration that site limitations can be overcome. The honest read: if your proposed ADU footprint sits on grade greater than 25%, the project is not viable on that location.
Smaller pre-approved plans are strongly preferred — less cut and fill, easier to fit on the buildable portion of a sloped lot, and faster Hillside Category 1 review path. The 900 sq ft cap applies but you'll typically size well below it for foundation cost reasons.
Foundation scope on sloped sites adds material cost — post-tensioned slabs, pier-and-grade-beam systems, and engineered retaining walls all run materially more than a flat-lot footing. Plan for $20K–$50K of foundation premium over a Bench build at the same plan size.
Most foothill subdivisions have HOA architectural review on top of city code; review the CC&Rs for ADU restrictions, exterior material requirements, and any short-term-rental prohibitions before any bid.
Plan fit
Best pre-approved plans for Boise Foothills lots
The City of Boise's pre-approved plan set works across the whole city — but each neighborhood has plans that fit cleaner than others. These are our picks for Boise Foothills.
Type A
Type BThe Waxwing
From $135k
396 sq ft · Studio · 1 ba
A 396 sq ft studio with more breathing room — full living, kitchen, and bath in a thoughtfully laid-out single space.
See the The WaxwingPlans that typically don't fit Boise Foothills lots: The Kestrel. We're happy to walk through why on a free lot check.
Numbers
Home values + rent — what we can honestly say
Typical home value
$1,000,000 median sale; $1,150,000 current median; Highlands Cove new-builds ~$1,540,000
Homes.com — Foothills neighborhood guide, 2025–2026 sale data · 2026
Rent — verify per address
Foothills-specific rent comps are unreliable in public datasets — apartment data sources don't cover single-family / custom-home rentals well, and the foothills have very little purpose-built apartment inventory. Use the broader 83702 / 83712 1BR band ($1,500–$2,300 depending on dataset) as a directional proxy, but verify against current Zillow Rental Manager comps for the specific address. A premium-view detached ADU in the Foothills will generally rent above the Boise-wide 1BR median if it's well-built and well-sited.
Lifestyle drivers
What makes Boise Foothills rentable
Ridge to Rivers trail system (Hulls Gulch, Camel's Back Reserve)
Premium foothill views west across the Treasure Valley
Crane Creek + Bogus Basin road access
Highlands + Highlands Cove subdivisions
Quick drive to downtown Boise + the North End commercial spine
FAQ
Boise Foothills ADU questions, answered
Will my foothill lot trigger Hillside Development Overlay review?
If any portion of your proposed ADU footprint sits on a slope exceeding 15%, yes — Boise's HS-O is triggered. The overlay also applies where slope-stability, expansion soils, high water table, spring, erosion, or sedimentation conditions exist independent of slope. Most foothill parcels above the North End and East End are inside HS-O somewhere on the lot; the question is whether you can site the ADU on the flatter portion to keep the project in Category 1 (planning director approval) rather than Category 2 or 3.
How much does the geotechnical soils report cost in Boise?
Third-party geotech labs typically quote $7,000–$9,000 for a foothill ADU site in the Boise market, with a turnaround of 3 weeks to 2 months depending on the complexity of the borings and the lab's backlog. That cost is in addition to the ADU build budget and has to be paid before the building permit is issued. The lot check confirms whether your specific site needs a Category 1, 2, or 3 review and what that implies for the geotech scope.
Which pre-approved plan works best on a sloped foothill lot?
The Goldfinch (280 sq ft Type A studio) and Waxwing (396 sq ft Type B studio) are the strongest fits — smaller footprints mean less cut and fill, simpler foundations, and a cleaner Hillside Category 1 review path. Larger plans like the 695 sq ft Kestrel are workable on flatter portions of the lot but tend to push the project into Category 2 review with more grading and retaining-wall scope.
Can I build on a slope greater than 25%?
Boise's HS-O code flags slopes over 25% as effectively undevelopable unless the project engineer can demonstrate that the site limitations can be overcome. In practice that means a Category 3 hearing, substantial engineering scope, and a project that rarely pencils for a 280–500 sq ft ADU. Honest answer: if the only buildable area on your lot is on grade over 25%, the ADU project likely isn't viable on this site without a custom plan and a significant budget premium.
Does the 900 sq ft Boise ADU cap still apply?
Yes — the 2023 Modern Zoning Code cap of 900 sq ft or 50% of the primary dwelling, whichever is less, applies across Boise including the Foothills. In practice the binding constraint on a sloped lot is usually foundation cost and buildable area rather than the 900 sq ft hard ceiling.
Who actually builds the ADU?
Construction is performed by our licensed builder partner, Iron Crest Remodel (Idaho RCE-6681702), under the BoiseADU.com pre-approved-plan program. Same fixed-price contract, same Boise-based crews, same lot check before any bid.
Sources
Where the Boise Foothills facts come from
- City of Boise — Hillside Development Overlay
- Homes.com — Foothills neighborhood guide
- Build Idaho — Highlands Cove subdivision overview
- Alluvial Soil Lab — geotechnical report reference
- BoiseADU — Boise ADU permit process
Last reviewed 2026-05-24. Zoning rules referenced against City of Boise code; verify the specific overlay against the parcel record before any build commitment.
Boise Foothills · Boise, Idaho · Free Lot Check
Ready to build your Boise Foothills ADU?
Free 24-hour lot check — overlays, lot fit, plan recommendations, and a real number for your specific Boise Foothills address. Or call (208) 297-2036.