Side-by-side: a detached ADU in a Boise backyard next to a converted garage ADU

Comparison · Boise ADUs

Detached ADU vs garage conversion — which one wins in Boise?

Honest side-by-side on cost, timeline, rent ceiling, and resale. Built and priced by the same team — no thumb on the scale. The answer depends on whether you already have a usable garage.

Get a Free ADU Lot Check Check feasibility on your lot

The honest answer up front

If you already have an existing detached garage in solid shape, conversion is almost always the cheaper starting recommendation — $80–150k for the conversion vs $180–280k for a comparable detached 1BR. If you don't, detached new-build wins on rent ceiling, resale story, and longevity. The decision tree most Boise homeowners land on after a lot walk: existing garage in good condition, convert it; no garage, or the garage is already lost to living space, build detached. A third path — the Sandpiper or Osprey pre-approved plans — gives you both an ADU and a new garage on a single foundation, which is where many landlords end up.

Disclosure: Our builder partner Iron Crest Remodel (Idaho contractor license RCE-6681702) builds both detached ADUs and garage conversions — this comparison isn't trying to push you one direction. Each path wins specific scenarios; we spell out which below.

Side-by-side: detached vs garage conversion

Eight factors that move the decision. Cost and speed favor conversion; permit speed, rent ceiling, privacy, and resale favor detached. Most lots have a clear winner once you walk the property.

Factor	Garage conversion	Detached new build	Edge
Turnkey cost (Iron Crest grid)	$80k–$150k	$180k–$280k (1BR Kingfisher band)	Conversion
Permit timeline	Custom review (each garage is unique)	3–5 weeks on a pre-approved plan	Detached
Build time	3–5 months	5–8 months	Conversion
Rent ceiling	Studio / 1BR — lower end of the band	1BR / 2BR — top of the band	Detached
Privacy (both directions)	Good if detached garage; mixed if attached	Best — separate building, separate yard	Detached
Resale lift	Improves house but reads as a remodel	Appraises as a standalone permitted dwelling	Detached
What happens to the garage	Gone — that's the trade	Stays (or rebuilds with Sandpiper / Osprey hybrid)	Detached
Pre-approved plan available	No — each garage is one-of-one	Yes — 6 city plans skip design review	Detached

Cost ranges sourced from Iron Crest Remodel's published ADU pricing grid. Build times and permit timelines reflect typical Boise jobs after the 2023 Modern Zoning Code update.

When garage conversion wins

Six scenarios where conversion is the right call. Most reduce to one question: do you already have a usable garage on the right part of the lot?

You already have a sound detached garage

A 22'x22' detached garage on a usable slab, with at least 7'6" of clearance and a roof that doesn't need replacing, is a candidate for an $80–120k conversion — the cheapest legal ADU path in Boise.

You're optimizing cap rate, not gross rent

A $90k converted studio at $1,200/mo is roughly 16% gross yield. A $200k detached 1BR at $1,500 is closer to 9%. If yield per dollar invested matters more than total dollars, conversion wins by a meaningful margin.

Budget caps below ~$150k turnkey

Hard ceiling at $150k? Detached new construction won't fit — even the 280 sqft Goldfinch clears that on plan and finish alone. Conversion is the only path that respects the ceiling without cutting scope.

Your lot can't accommodate a second footprint

Narrow infill lots, mature trees you don't want to lose, or buildable areas that fail 5'/10' setbacks for a new detached unit. If the lot can't carry a second building, the existing garage is the only place the ADU goes.

Owner-occupied use case (office, in-laws, guest)

If the ADU is primarily for family or a home office, the privacy premium of a separate building matters less. 400–500 sqft in the converted garage covers the use case at half the cost.

Alley-loaded detached garage

North End, East End, and parts of the Bench have alley-loaded detached garages already set back from the main house with their own access. These convert cleanly: separate entry, no front-street parking impact.

When detached new-build wins

Six scenarios where detached is the right call. The common thread: you're optimizing for rent ceiling, resale, or layout freedom rather than the lowest upfront number.

You're targeting top-of-band rent

A detached 1BR or 2BR rents higher than any converted studio in Boise. Tenants pay a meaningful premium for a separate building. If rent ceiling is the goal, detached delivers — conversion can't.

Resale is part of the math

Ada County appraisers treat a permitted detached ADU as a full, separate structure. A converted garage lifts the parent house but reads as a remodel. Over a 5–10 year hold, detached is the cleaner asset.

You need a 2-bedroom unit

A typical 2-car garage caps at ~440–600 sqft — enough for a studio or compact 1BR, not a 2BR. If the use case needs two bedrooms, you're going detached on a Kestrel-style plan.

No existing garage, or the garage is gone

Plenty of Boise homes either never had a garage or already converted theirs to interior living space. No foundation to reuse means the cost advantage of conversion disappears.

You want design freedom

Conversion is constrained — same footprint, same roofline, same orientation. Detached lets you site the building for sun, privacy, and curb appeal, with six pre-approved plans as starting points.

You want a 3–5 week permit

Pre-approved detached plans skip design review entirely. Conversion permits run the full custom review cycle. If permit speed matters, the pre-approved path is the only one that clears in weeks.

The hybrid path: Sandpiper + Osprey

Two of the six City of Boise pre-approved plans pair a studio ADU with a new garage on the same foundation. They're the answer for the homeowner who wants the rental income of a detached ADU but doesn't want to lose covered parking — and they keep the 3–5 week pre-approved permit timeline. If you're torn between conversion (which takes the garage) and detached (which leaves the garage out), these two plans are the third option most people don't know exists.

Pre-approved · Studio + Garage · 396 sqft

Sandpiper

Studio ADU paired with a new 2-car garage on the same foundation. Pre-approved by the City of Boise. Gives you the rental and replaces the garage in one project.

See the Sandpiper plan →

Pre-approved · Studio + Garage · 376 sqft

Osprey

Compact studio ADU with a single-car garage attached on the same slab. Smaller footprint than Sandpiper — good fit for lots where setbacks make Sandpiper tight.

See the Osprey plan →

Cost deep dive — published grid vs per-sqft math

Two ways to triangulate cost. Iron Crest's published bands give you the turnkey range; the per-sqft math shows where each path actually lands when you back into a price from floor area. Both agree: conversion is the cheaper starting point if you have a garage to reuse.

Path	Published grid	Size	Per-sqft	Notes
Garage Conversion	$80k–$150k turnkey	~440–600 sqft typical	~$180–$340/sqft	Lower end: clean detached garage on a sound slab. Higher end: attached conversion with fire-rated separation, dedicated HVAC, or significant slab and roof work.
Detached Kingfisher (1BR)	$180k–$280k turnkey	491 sqft pre-approved plan	~$365–$570/sqft	Matches the cost calculator math: Standard finish ($405/sqft) lands the median quote in the middle of Iron Crest's published 1BR band.
Detached Studio (Goldfinch)	$120k–$200k turnkey (Iron Crest studio band)	280 sqft pre-approved plan	~$430–$715/sqft	Smaller plans carry a higher per-sqft rate because fixed costs (foundation, MEP rough-in, permits) don't shrink proportionally with floor area.

Small detached plans show a higher per-sqft number than 1BR or 2BR plans — fixed costs (foundation, MEP rough-in, permits) don't shrink proportionally with floor area. Larger plans usually deliver better cost-per-foot, but only if the size actually matches the rental use case.

Permit and code differences

Both paths require a full building permit plus electrical, plumbing, and mechanical permits, with inspections at framing, rough-in, insulation, and final. The 2023 Modern Zoning Code lifted the ADU size cap to 900 sq ft (raised from the older 700 sq ft figure) for both configurations, and removed the owner-occupancy requirement — meaning you can rent the ADU regardless of whether you live in the main house.

The meaningful difference is design review. Detached new construction can use the six City of Boise pre-approved plans (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey), which skip the 4–8 week design review step entirely — permits issue in roughly 3–5 weeks. Garage conversion can't use the pre-approved set because every garage is one-of-one in size, slab condition, and egress configuration. Conversion permits run the full custom design review cycle.

Conversion also carries a code-upgrade list detached doesn't: R-21 wall insulation, R-49 attic, full air sealing, egress windows in every bedroom, 7'6" minimum ceiling height, and a 1-hour fire-rated separation if the garage is attached to the main house. None of these are dealbreakers — they're priced into the conversion bands — but they explain why a $90k conversion isn't a $60k one.

Detached vs garage conversion — frequently asked

Is a garage conversion really cheaper than building detached?

Yes — almost always, if you have a sound existing garage. Iron Crest's published grid puts garage conversion at $80–150k and a detached 1BR at $180–280k. The gap comes from reusing the foundation, walls, and roof. Where conversion doesn't save: insulation, MEP, finishes, and permits all cost the same as a new build. If the existing garage is in rough shape, the gap narrows fast.

Can I do a garage conversion using a pre-approved plan?

No. The City of Boise's pre-approved set (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey) is for new detached construction only. Every garage is one-of-one — different dimensions, different slab condition, different egress paths — so each conversion goes through custom design review. The trade-off: a slower permit on conversion in exchange for the cost savings.

Which one rents for more in Boise?

Detached, by a meaningful margin. A separate building reads as a 'real' rental to most tenants — its own walls, its own outdoor area, no shared hallway with the main house. Detached 1BR units rent at the top of the Boise band. Converted studios rent at the bottom of the studio band. The exact spread depends on neighborhood, finish, and whether the conversion is detached-garage or attached-garage.

Will my garage's slab support a conversion?

Usually — but it's the first thing we check. Most Boise garage slabs built in the last 30 years are sound enough to reuse with a vapor barrier retrofit and edge insulation. Older slabs sometimes need cracks repaired, a leveling pour, or a moisture mitigation system. If the slab is severely compromised, the math flips: a new foundation costs $15–25k, which eats most of the conversion savings.

Do I lose my garage in the conversion?

Yes. The garage becomes the ADU. If you need to keep covered parking, the options are: build a new detached garage elsewhere on the lot (zoning permitting), use a carport, or pick the Sandpiper or Osprey pre-approved plan — both pair a studio ADU with a new garage on the same foundation. That's the hybrid path most Boise homeowners gravitate to once they realize the trade-off.

What if I want both — keep a garage AND have an ADU?

Two clean paths. First: build a detached ADU somewhere else on the lot and leave the existing garage standing. Setbacks and lot size determine whether this is possible — most lots over 6,500 sqft can carry both. Second: pick a Sandpiper or Osprey, which is a pre-approved studio plus garage in one combined building. You get the rental, you get the covered parking, and you stay on the pre-approved track for a 3–5 week permit.

Keep going

Money pages, hybrid plans, and informational deep dives that feed into the same decision.

Money page

Ready to pick a path?

Start with a free lot check — we'll walk both options for your specific address and recommend the one with the better economics. Or call (208) 297-2036 to talk it through.

Get a Free ADU Lot Check Call (208) 297-2036 Info@Boiseadu.com