Caldwell · ADU Cost 2026
Updated: May 2026
Real 2026 Caldwell ADU pricing — $115k–$280k turnkey for plans that fit Caldwell's 700 sq ft / 1-bedroom cap. Lower Canyon County buy-in, College of Idaho rental demand, no ACHD transportation impact fee.
A Caldwell ADU costs $115k–$280k turnkey for plans that fit Caldwell's 700 sq ft / 1-bedroom cap (Goldfinch, Waxwing, Kingfisher, Sandpiper, Osprey). Per-square-foot cost is the same as Boise — what's different in Caldwell is the city ADU size cap (700 sq ft as of May 2026, with a proposed amendment to 900 sq ft pending), the separate-entrance rule, lower buy-in property prices, and the Star Fire impact fee structure (Caldwell is not in ACHD jurisdiction). Caldwell 1BR median rent is $1,200/month and 2BR is $1,445/month per Apartment List and Zumper, May 2026 — meaningfully below Boise but stretched by a much lower property entry price.
Source notes: City of Caldwell ADU size + entrance rules per Caldwell city code (Article 12); proposed amendment per BoiseDev March 2025 reporting; Star Fire and ACHD figures per Ada County Development Services Appendix E (FY26); rent comps per Zumper and Apartment List, accessed May 2026.
This is the single most important fact for a Caldwell ADU budget. Today, Caldwell allows one ADU per parcel, up to 700 sq ft of habitable space, with a maximum of one bedroom. That's materially more restrictive than Boise (900 sq ft, 2BR allowed), Meridian (900 sq ft, 2BR allowed), and Eagle (~900 sq ft, zone-dependent). It's larger than Nampa's 500 sq ft cap.
In March 2025, the City of Caldwell introduced a proposed amendment to raise the ADU cap from 700 to 900 sq ft and allow ADUs in the historic district at 75% of the original home size with matching design elements. As of May 2026 (this page's update date), that amendment has not been confirmed as adopted in primary city sources we can verify. The proposal is real and the direction of travel favors a higher cap — but plan for 700 sq ft today and don't assume an upgrade until the city confirms adoption.
Practical takeaway
Design your Caldwell ADU project around the Goldfinch, Waxwing, Kingfisher, Sandpiper, or Osprey — all five fit comfortably under 700 sq ft of habitable space with a 1-bedroom-or-less layout. The Kestrel 2BR is currently blocked. If the amendment lands during your project, you gain the option to scale up; if it doesn't, you've already designed inside the legal envelope.
Five of the six pre-approved Boise plans fit under Caldwell's 700 sq ft / 1BR rule. Caldwell does not have a formal reciprocity agreement with Boise — every Caldwell ADU goes through its own plan-check submittal at Caldwell Community Development — but the same plan sets and engineering work as starting points. The garage square footage on the Sandpiper and Osprey doesn't count toward the 700 sq ft habitable cap.
| Plan | Type | Size | Beds | Fits current cap? | Note |
|---|---|---|---|---|---|
| Goldfinch | Type A studio | 280 sqft | Studio | Yes | Best fit under the current 700 / 1BR rule — strongest cap rate on Caldwell rents. |
| Waxwing | Type B studio | 396 sqft | Studio | Yes | Defined sleeping + living zones; fits under 700 with room to spare. |
| Kingfisher | Type C 1-bedroom | 491 sqft | 1BR | Yes | Real walled bedroom — matches Caldwell's 1-bedroom cap exactly. Top pick for College of Idaho rentals. |
| Sandpiper | Type G-1 studio + garage | 396 sqft | Studio | Yes | Studio + garage on one pad. Garage square footage doesn't count toward Caldwell's 700 sq ft habitable cap. |
| Osprey | Type G-2 studio + garage | 376 sqft | Studio | Yes | Sister plan to Sandpiper. Fits narrow Caldwell side-yards. Garage excluded from habitable count. |
| Kestrel | Type D 2-bedroom | 695 sqft | 2BR | No (1BR cap) | 695 sq ft is under 700 — but the 2-bedroom layout exceeds Caldwell's current 1-bedroom cap. Blocked until the proposed amendment is adopted. |
For full per-finish pricing on each plan, see the Boise ADU Cost page — the same square-foot rates apply in Caldwell.
Caldwell's ADU permit + impact fee profile is genuinely different from the Ada County cities. The cost drivers worth getting right before you lock a budget:
Parcels inside city limits go through Caldwell Community Development (Planning & Zoning + Building Safety). Parcels in unincorporated areas — Sunnyslope, NW Caldwell rural — fall under Canyon County Development Services, with different setback, septic, and well rules. Verify jurisdiction by parcel before budgeting.
Caldwell-area parcels covered by Star Fire Protection District pay $2,152 per single-family dwelling unit for 2026 (per Ada County Development Services Appendix E). Most Caldwell city parcels sit inside Caldwell Fire's service area instead — confirm per address.
Ada County Highway District jurisdiction is statutory — Ada County only. Caldwell sits in Canyon County, so Caldwell ADU builders do not pay the $5,803 ACHD transportation impact fee per dwelling unit that Meridian and Eagle pay (effective March 1, 2026). That's a meaningful structural savings vs. Ada County.
Caldwell publishes its FY26 building fee schedule via Resolution 300-25, hosted as a PDF on cityofcaldwell.org. Per-tier permit dollar amounts and plan-review percentages need to be pulled from the city directly — we do that as part of every Caldwell estimate so the number you see matches the city's current schedule.
The headline rent numbers in Caldwell are lower than Boise or Meridian, but the math on a backyard ADU is not just about rent — it's rent divided by the buy-in. Caldwell has structural advantages that don't show up in a rent comparison table.
Caldwell median home values run in the $330k–$380k zone vs. Boise's $475k+. When you add a Goldfinch or Kingfisher to a Caldwell lot, your total invested basis is materially lower than the same plan on a Boise lot — and the rent ratio becomes the strongest cap rate in the valley.
C of I has been operating since 1891 and produces a steady stream of upperclassmen, grad students, and visiting faculty looking for quiet off-campus housing. A 1BR Kingfisher near campus rents quickly at $1,100–$1,300 on a 9- or 12-month lease — and student tenants self-select for longer stays the further from undergrad dorms they get.
Bitner, Sawtooth, and the rest of the Snake River Wine Trail draw visitors year-round. A small detached unit on a Sunnyslope or Vineyard Estates lot is a credible agritourism stay where zoning allows — verify Caldwell or Canyon County STR rules per parcel before underwriting STR income.
No ACHD = no $5,803-per-dwelling transportation fee. Star Fire's $2,152 (where it applies) is below ACHD by about $3,650 on its own. Caldwell's combined permit + impact fee burden trends below Meridian and Eagle even before factoring in the lower land basis.
Caldwell isn't always the right answer. Three situations where we'd point a homeowner back across the county line:
Caldwell requires every ADU to have a separate exterior entrance — or an entrance into an internal common area that's itself accessible from outside without passing through the primary dwelling's living space. The intent is simple: an ADU is a separate household and needs independent access for tenants, family members, or guests.
All six of Boise's pre-approved plans (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey) ship with dedicated exterior entrances by design — they're standalone or attached units with their own front door. On garage-combo plans (Sandpiper, Osprey), the dwelling entrance is separate from the garage door. No design changes needed to clear Caldwell's entrance rule.
A pre-approved Caldwell ADU runs $115k–$280k turnkey in 2026 — same per-square-foot cost as Boise. The 280 sq ft Goldfinch studio starts around $115k; a 491 sq ft Kingfisher 1BR runs $170k–$220k at Standard finish. Caldwell's 700 sq ft / 1-bedroom cap means the larger Kestrel 2BR plan isn't currently buildable. Total project cost runs slightly lower than Boise because Caldwell sits in Canyon County (no ACHD transportation impact fee — a savings of roughly $5,803 vs. Meridian or Eagle).
Not currently. As of May 2026, Caldwell caps ADUs at 700 sq ft of habitable space with a 1-bedroom maximum (one ADU per parcel). A city amendment proposed in March 2025 would raise the cap to 900 sq ft and allow 2-bedroom ADUs, but it has not been confirmed adopted as of this update. Plan around the 700 / 1BR rule today — the Kingfisher 1BR (491 sq ft) is the largest pre-approved plan that fits. If you need 2 bedrooms, Boise, Meridian, and Eagle all allow up to 900 sq ft.
Caldwell rents are lower than Boise but the buy-in is meaningfully lower too. Per Apartment List and Zumper (May 2026 data), Caldwell 1BR median rent runs about $1,200/month and 2BR about $1,445/month. Boise is closer to $1,479–$1,500 for a 1BR. The cap rate math typically lands in Caldwell's favor on a Goldfinch or Kingfisher because the project cost is similar but the property entry price for the underlying parcel is materially lower.
Probably yes, but the exact per-tier dollar amounts aren't published in a scrapable format — Caldwell's FY26 building fee schedule (Resolution 300-25) is hosted as a PDF on cityofcaldwell.org and the rate sheet needs to be pulled from the city directly before budgeting. What we can confirm: Caldwell sits outside ACHD jurisdiction, which saves the $5,803-per-dwelling Ada County transportation impact fee that Meridian and Eagle pay. Some Caldwell-area parcels fall under Star Fire Protection District ($2,152 per single-family unit for 2026); most are inside Caldwell Fire and follow city fees instead.
Yes, today. Caldwell currently requires the owner to occupy either the primary home or the ADU. That changes on July 1, 2026, when Idaho Senate Bill 1354 preempts city-level owner-occupancy rules statewide for cities over 10,000 population (Caldwell qualifies). Cities have until February 1, 2027 to update local codes. If your plan is a pure investment ADU with no owner on site, the math improves materially after July 2026.
Sunnyslope, NW Caldwell rural, and other unincorporated parcels fall under Canyon County Development Services, not the City of Caldwell. Canyon County has its own zoning, setback, and septic/well rules — typically larger setbacks, one-acre minimums in ag-residential zones, and well + septic instead of city sewer/water. Permit timelines vary; ag-residential parcels tend to have more placement flexibility but more upfront site engineering. We run the jurisdictional check upfront on the free lot check so you know which counter your application is going to.
Cross-references for the Caldwell cost picture — baseline city, sibling per-city cost pages, the Caldwell service-area breakdown, and the tools that turn this page's numbers into a real budget.
Baseline
The canonical Treasure Valley cost page — same per-square-foot rates, no 700/1BR ceiling, full Kestrel pricing.
Sibling
900 sq ft cap, ACHD applies, higher rent ceiling than Caldwell.
Sibling
Canyon County neighbor with a 500 sq ft cap — the most restrictive size in the four-city set.
Sibling
Ada County, EASD design review adds cost and timeline vs. Caldwell.
Caldwell
Neighborhoods, lot types, zoning detail, and recommended plans for Caldwell parcels.
Pillar
Full-stack pre-approved plan + permit + ROI primer.
Tool
Square footage + finish × per-sqft rate. Same math we use to quote.
Tool
Layer Caldwell rent ($1,200 1BR) on any plan and see gross yield + payback.
Plan
Smallest pre-approved plan — strongest cap rate on Caldwell rents.
Plan
Largest plan that fits Caldwell's current cap — top pick for College of Idaho rentals.
Lot check
Parcel-specific feasibility — Caldwell city vs. Canyon County, setbacks, plan fit.
Free Caldwell Lot Check · 24-Hour Response
Start with a free lot check — city vs. county jurisdiction, plan fit under the 700 / 1BR rule, and the right cost path for your goals. Or call (208) 297-2036.
