Garage conversion vs detached ADU — which is right?
If you have an existing detached or attached garage in good structural condition, garage conversion is usually the most cost-effective ADU path — 30-50% cheaper and 1-2 months faster than ground-up detached construction. If your lot has no garage, or the existing garage is too small or too compromised, detached is the right call. Many homeowners debate this for weeks; the lot check usually resolves it in 30 minutes.
Cost comparison
| Path | Typical cost (Standard finish) | Timeline | Per-sqft |
|---|---|---|---|
| Garage conversion (small) | $60k-$90k | 3 months | $140-$200 |
| Garage conversion (large) | $100k-$140k | 3-4 months | $200-$280 |
| Detached Goldfinch (280 sf) | $130k | 5-7 months | $460 |
| Detached Kingfisher (491 sf) | $200k | 5-7 months | $405 |
| Detached Kestrel (695 sf) | $285k | 5-7 months | $410 |
Conversion's lower cost comes from reusing the foundation, exterior walls, and roof. The trade-off is that you're locked to the garage's existing footprint — typically 440-600 sq ft for a 2-car garage — and constrained to studio or 1-bedroom layouts.
When conversion wins
- Existing detached or attached garage in good structural condition
- Garage already has separate egress or can easily get one
- Lot doesn't accommodate a separate detached ADU footprint
- Budget caps below ~$140k turnkey
- Owner-occupied use case (office, guest space) where 400-500 sq ft is enough
- Cap-rate rental strategy — yield per dollar invested favors conversion
When detached wins
- No existing garage or garage is structurally unsuitable
- Need a 2-bedroom unit (Kestrel, 695 sq ft) — conversion can't fit one
- Owner wants new construction with full design freedom
- Rental strategy targets the premium 1-2 BR market (rent ceiling matters more than cap rate)
- Resale upside matters (detached appraises higher per square foot)
- Garage is alley-loaded and you want to keep parking
Rental yield math
A converted 440 sq ft studio rents in the $1,200-$1,500/mo range in Boise. On a $90k Standard-finish conversion, that's roughly 16-20% gross yield — among the highest of any ADU configuration. A detached 491 sq ft Kingfisher rents for $1,500-$1,800/mo. On a $200k Standard-finish build, that's roughly 9-11% gross yield. Both are strong for residential real estate, but conversion's per-dollar yield is meaningfully better for cap-rate plays.
Total dollar income flips at scale: the Kingfisher's $1,800 beats the converted studio's $1,500 month over month. If absolute rental income matters more than cap rate (most landlords), detached is the better play. If maximizing return on capital matters more (smaller investors, cash-strapped homeowners), conversion is.
Code requirements differ
Conversion has a code-upgrade list that detached doesn't: R-21 wall insulation, R-49 attic, egress windows in every bedroom, 7'6" minimum ceiling height, 1-hour fire separation if attached to the main house, and sometimes dedicated HVAC. The garage conversion page has the full upgrade list.
What we usually recommend
If the lot already has a detached garage in good shape, conversion is almost always the right starting recommendation. If the garage is gone (already converted to living space), the Sandpiper or Osprey plans rebuild both the garage and add the rental in one project. If there's no existing garage and no plan to build one, detached is usually the cleaner answer.
Where to next
For the full feature matrix, scenario-by-scenario decision tree, and cost deep dive, see the Detached vs Garage Conversion money page. Both paths feed into the same builder team. For the detached track, see Boise's six pre-approved plans and the backyard cottage builder page. For end-to-end project structure regardless of path, the design-build contractor overview explains how feasibility, design, permits, and construction fit under one contract. Confirm fit on your specific lot with the free lot check.