Standard Boise infill lot
Rectangular lot, 50–80 ft frontage, conforming side and rear setbacks. About 70% of Boise lots clear this bar — pre-approved permits in 3–5 weeks against a known-good drawing set.
Comparison · 2026 Pre-Approved Program
Pre-approved vs custom ADU plans in Boise — when to pick which
Updated: May 2026
Pre-approved plans save 4–8 weeks of permitting and $8,000–$15,000 in design fees. Custom buys layout flexibility for atypical lots. About 70% of Boise lots fit at least one of the six pre-approved plans — for the other 30%, custom is the right call.
The short answer — which path should you pick?
Pre-approved plans skip 4–8 weeks of design review and save $8,000–$15,000 in design fees. Custom plans add time, design fees, and design freedom. About 70% of Boise lots can take at least one of the six pre-approved plans — for those owners, pre-approved is the right call almost every time. The 30% that can't (unusual lot shape, slope, historic overlay, specific accessibility needs, HOA architectural rules) genuinely need custom. There's no difference in construction quality between the two paths — the same crews, the same finish levels, the same Idaho residential code. The only real variables are the design fee, the permit clock, and which lots each path can serve.
How Boise's pre-approved program actually works
The City of Boise launched its pre-approved ADU plan catalog on March 17, 2026, designed by Parke Architecture (Eamonn Parke, AIA) under a contract with Planning and Development Services. The catalog currently lists eight plans — six are downloadable today (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey) and two mirrored variants (Sandpiper B, Osprey B) are marked "Coming Soon" for lots where the original layout reads the wrong way against the primary house.
The plans pre-clear Boise's planning, zoning, and building review as a complete package. Building one of them as-drawn drops the permit timeline from 7–12 weeks to 3–5 weeks and removes the $10,000-plus typical custom design fee the city cites as the program's headline savings. Boise's 2023 Modern Zoning Code raised the maximum ADU size from 700 to 900 sq ft (or 50% of the primary dwelling, whichever is less), so both pre-approved and custom builds share the same upper size cap. The largest pre-approved plan, the Kestrel, tops out at 695 sq ft by design choice — well under the legal ceiling.
Pre-approved vs custom — side-by-side
Ten factors that actually decide the call. Construction quality and cost-per-square-foot aren't on the list because they don't change between paths — same crews, same finishes, same code.
| Factor | Pre-Approved | Custom |
|---|---|---|
| Architectural design fee | $0 (city-published) | $8,000–$15,000 |
| City permit review timeline | 3–5 weeks | 7–12 weeks |
| Total contract-to-keys timeline | 5–7 months | 7–9 months |
| Lot flexibility | Standard rectangular Boise lots — ~70% fit | Any lot, including narrow, sloped, or oddly shaped |
| Finish customization | Full — cabinets, counters, fixtures, paint, flooring | Full |
| Footprint / elevation / roof pitch | Fixed — change any and you lose pre-approved status | Anything you and the engineer can stamp |
| Maximum ADU size | Up to 695 sq ft (Kestrel) — under the 900 sq ft cap | Up to 900 sq ft (or 50% of primary dwelling, whichever is less) |
| Build cost per square foot | Same as custom at matched finish | Same as pre-approved at matched finish |
| Plan IP ownership | City of Boise (free to use) | You (you paid for it) |
| Best use case | Rental, in-law suite, owner-use on a conforming lot | Atypical lot, specific accessibility, integrated whole-home remodel |
When pre-approved wins
Five scenarios where the pre-approved set is the obvious pick. If two or more describe your project, you're in the 70% — go pre-approved.
Cap-rate rental builds
Goldfinch or Waxwing studios or a Kingfisher 1-bedroom. The $8–15k design-fee savings drops straight to your basis — material at $1,600/month rents, that's roughly 6 months of payback on the design fee alone.
In-law suites and aging-parent units
The 491 sq ft Kingfisher and 695 sq ft Kestrel cover the use case cleanly. You can spec wider doors, lever hardware, and grab-bar blocking inside the existing plan without triggering re-review.
Owners on a permit-driven timeline
Need a C of O before a school year, lease end, or family arrival. Custom adds 4–8 weeks of design review at the city. Pre-approved skips it entirely.
Studio + garage builds on garage-less lots
Sandpiper (Type G-1) and Osprey (Type G-2) bundle a 376–396 sq ft studio with a single-car garage on one foundation. Useful when a prior conversion erased the original garage and the lot still needs covered parking.
When custom wins
Five scenarios where custom is the right call — and worth the $8–15k design fee and 4–8 extra weeks of permit time.
Unusual lot geometry
Narrow lots under 50 ft, deep flag lots, sloped sites (foothills, riverfront), and severe corner-setback envelopes where none of the six plan footprints fit between setbacks and easements.
Historic, hillside, or HOA overlays
North End historic-district streets, hillside overlays, and HOA architectural-committee rules (Banbury, Two Rivers, Spurwing) often require custom matching to the main house — pre-approved aesthetics don't always pass.
True ADA / accessibility layouts
Wheelchair turning radii, roll-in showers, single-level bedrooms with adjacent baths beyond what the Kestrel accommodates. The pre-approved plans handle aging-in-place; full ADA usually needs custom.
Two-story or above-garage configurations
All six pre-approved plans are single-story. If lot coverage forces you up rather than out, you're in custom territory — typically an above-garage unit or a stacked detached design.
Integrated whole-home projects
If the ADU is part of a primary-house remodel and you want the elevations, materials, and rooflines to read as one architectural piece, custom is the only path that lets the two structures be designed together.
The six pre-approved plans (eight on paper)
Six downloadable plans today, plus two mirrored variants in the catalog marked "Coming Soon." Each is a complete drawing set — structural, MEP, energy compliance — already cleared by Boise Planning, Zoning, and Building.
| Plan | Type | Size | Beds | Notes |
|---|---|---|---|---|
| Goldfinch | Type A | 280 sq ft | Studio | Smallest pre-approved plan. Cap-rate king. |
| Waxwing | Type B | 396 sq ft | Studio | Real living + sleeping zones in a single room. |
| Kingfisher | Type C | 491 sq ft | 1 bedroom | Walled-off bedroom — landlord favorite for long-term rental. |
| Kestrel | Type D | 695 sq ft | 2 bedroom | Largest pre-approved plan and the only true 2BR. |
| Sandpiper | Type G-1 | 396 sq ft + garage | Studio | Studio bundled with single-car garage on one pad. |
| Osprey | Type G-2 | 376 sq ft + garage | Studio | Sandpiper sibling — different layout for narrow side-yard lots. |
| Sandpiper B | Type G-1 (mirrored) | 396 sq ft + garage | Studio | Coming Soon — mirrored variant |
| Osprey B | Type G-2 (mirrored) | 376 sq ft + garage | Studio | Coming Soon — mirrored variant |
Full plan details, finish tiers, and turnkey pricing on the pre-approved plans overview or the floor plans hub.
What you can — and can't — change on a pre-approved plan
The freedom is in everything you can see and touch inside the unit. The lock is on anything the city reviewed as part of the approved package. Cross the second line and the plan is no longer pre-approved.
You can change
- Cabinet style, door profile, and hardware
- Counter material — laminate, quartz, granite, solid surface
- Flooring — LVP, engineered hardwood, tile, polished concrete
- Paint colors and interior trim profiles
- Plumbing fixtures, lighting, ceiling fans, smart switches
- Appliance package, including range type and refrigerator size
- Tile patterns, shower glass, and bath accessories
- HVAC equipment selection (heat pump vs mini-split) within the same loads
You can't change (without losing pre-approved status)
- Structural footprint — wall lines, foundation outline
- Exterior elevations — siding, trim layout, roofline geometry
- Window placement, size, and operation type
- Door placement and swing direction
- Roof pitch and overhang dimensions
- Siding type — switching panel-and-batten to lap siding, for example
- Plate height and ceiling configuration
- Energy code path — the approved package is locked
Common mistake: moving or adding a single window. Even a relocation kicks the plan out of pre-approved status and back into the 7–12 week design-review queue. If a change matters that much, go custom from day one instead of paying for both paths.
Cost math — pre-approved vs custom in real dollars
Pre-approved Boise ADUs run $115k–$345k turnkey across the six plans and three finish tiers. A Goldfinch at Essential finish starts the range; a Kestrel at Premium finish tops it. Custom builds typically land between $200k–$450k depending on size, layout complexity, and finish — the cost-per-square-foot is essentially identical to pre-approved at any matched finish level. What custom buys is design (an architect's $8–15k fee), additional engineering stamps ($3,000–$6,000 in structural and energy review), and the layout flexibility that lets the plan fit lots and use cases the pre-approved set can't reach.
The full plan grid lives on our partner builder Iron Crest Remodel's page — Iron Crest Remodel (Idaho contractor license RCE-6681702) publishes the same ranges we use to bid: $115k–$150k Goldfinch, $135k–$190k Waxwing, $170k–$235k Kingfisher, $245k–$345k Kestrel, $145k–$210k Sandpiper, $140k–$200k Osprey. Add 6–10 weeks of foregone rent on a custom path and the total premium over pre-approved usually shakes out to $13,000–$25,000 — worth it when the lot requires it, hard to justify when it doesn't.
Full cost breakdown on the Boise ADU cost page or model your specific build with the cost calculator.
Pre-approved vs custom — frequently asked
Is a pre-approved plan really cheaper than a custom design?
Yes — but the savings are in design fees and permit time, not construction. A pre-approved plan saves roughly $8,000–$15,000 in architectural fees because the City of Boise published the drawings for free, and it skips 4–8 weeks of design review at the city. Construction cost per square foot is the same as custom at matched finish level. Add 6–10 weeks of foregone rent on a rental project and total custom premium typically lands between $13,000 and $25,000 over the life of the project.
What can I change on a pre-approved plan?
Anything inside the envelope. Finishes, cabinets, counters, flooring, fixtures, paint, lighting, appliances, and HVAC equipment within the same load calculation are all fully customizable. What you cannot change without losing pre-approved status is the structural footprint, exterior elevations, window placement, door placement, roof pitch, plate height, or siding type. Modifying any of those kicks the plan into full design review and erases the timeline advantage.
What if my lot can't fit a pre-approved plan?
About 30% of Boise lots fall outside the pre-approved fit envelope — usually because of narrow widths under 50 feet, deep flag-lot shapes, sloped sites, severe corner setbacks, or historic and hillside overlays that require matching to the primary house. Custom is the fallback, and it solves those cases cleanly. The lot check confirms which path your specific parcel needs before any money changes hands.
How much faster is the permit on a pre-approved plan?
City of Boise pre-approved permits typically issue in 3–5 weeks from submittal. Custom permits run 7–12 weeks because they include a design-review stage that pre-approved plans skip. Construction itself takes the same amount of time on either path — the entire timeline advantage sits inside the permitting office.
Can I use a pre-approved plan on a garage conversion?
No. The six pre-approved plans are detached new-builds with their own foundation, framing, and envelope. A garage conversion reuses an existing structure and is inherently a site-specific project — different foundation conditions, different wall framing, different envelope upgrades. Garage conversions in Boise go through standard permit review as a remodel and don't qualify for the pre-approved path.
What's the largest pre-approved plan in Boise?
The Kestrel (Type D) at 695 sq ft and two bedrooms is the largest of Boise's six pre-approved plans. Boise's 2023 Modern Zoning Code allows ADUs up to 900 sq ft or 50% of the primary dwelling's living area (whichever is less), so the Kestrel sits about 200 sq ft under the legal cap. If you need the full 900 sq ft, you're in custom territory.
Keep reading
Boise pre-approved ADU plans overview
Full specs, finish tiers, and pricing for all six plans.
Boise ADU design-build contractor
Single-contract path from feasibility to final inspection.
All six pre-approved floor plans
Plan-by-plan detail pages with use-case fit and lot guidance.
Detached ADU vs garage conversion
Sibling comparison — when conversion wins vs when detached new-build wins.
Backyard cottage ADU vs tiny house
Legal status, financing access, and the IRC code threshold.
Resource article — deeper explainer
Real examples of when each path paid off — and when it didn't.
Boise ADU permit process
Step-by-step timeline — 3–5 weeks pre-approved vs 7–12 custom.
Boise ADU pillar guide
Zoning, costs, plans, ROI — the full foundation read.
Boise ADU cost — full 2026 pricing
$115k–$345k pre-approved, $200k–$450k custom — line by line.
Free Lot Check · 24-Hour Response
Want to know whether your lot is pre-approved or custom?
Submit your address. We'll run it against all six pre-approved plans and the buildable envelope, then tell you the cleanest path forward — pre-approved, custom, or garage conversion. Or call (208) 297-2036.