Garage Conversion vs Detached ADU
Two paths to the same destination — one cheaper and faster, one bigger and more flexible. Here is how to choose.
Quick Answer
Garage conversion is cheaper and faster ($80k-$150k, 2-4 months) because the foundation, framing, roof, and siding already exist. Detached new build offers more size and design flexibility ($150k-$350k+, 4-8 months) but costs $50k-$100k more. Both rent in the same Twin Falls market (about $800–$1,400/month for a 1-bedroom — an estimate that varies by city). Choose garage conversion if you have a usable garage and want the lowest cost. Choose detached if you want larger size, do not want to lose garage, or your existing garage has design or structural issues.
Side-by-Side Comparison
| Factor | Garage Conversion | Detached New Build |
|---|---|---|
| Total cost (Twin Falls) | $80,000 - $150,000 | $150,000 - $350,000+ |
| Construction timeline | 2-4 months | 4-8 months |
| Typical size delivered | 400-600 sq ft (existing garage size) | 450-1,200 sq ft (up to 45% rule limit) |
| Design flexibility | Limited to existing footprint + structure | Unlimited layout + placement |
| Site work required | Minimal — structure exists | Significant — foundation, utility trenching, grading |
| Property value impact | Appraised as income property | Appraised as income property (higher rent ceiling) |
| Rental income potential | $800–$1,100/mo (estimate) | $1,000–$1,400/mo (2-bedroom, estimate) |
| Lose garage parking? | Yes | No |
| Permit complexity | Same as detached — $250 ZUP + building permit + trades | Same — $250 ZUP + building permit + trades |
| Best for | Lowest cost, fastest timeline, existing usable garage | Maximum size, design freedom, preserve garage |
When Garage Conversion Is Clearly the Right Call
- Your existing garage is in good structural condition (no foundation cracks, sound framing, intact roof)
- You rarely use the garage for vehicles — it's already a storage / workshop space
- Budget is hard-capped at $150,000 all-in
- You want the project done quickly (under 4 months construction)
- Your garage is attached to the home (sharing utility infrastructure makes the conversion even easier)
- You don't need more than 600 sq ft for the intended occupant (aging parent, adult child, single tenant)
When Detached New Build Is Worth the Extra Cost
- You actually use your garage for cars and don't want to lose it
- You want 800+ sq ft or a 2-bedroom layout
- Your existing garage has structural issues that would cost as much to fix as building new
- Your lot has room for a separate structure that meets setback requirements
- You want maximum rental income potential (larger detached units rent for $200-$400/mo more)
- Design matters to you — kitchen layout, ceiling height, window placement, vaulted spaces
Hybrid Approach: Garage Conversion + Addition
In some cases the best path is converting the existing garage AND adding a small bump-out to gain extra square footage. This typically costs $100,000-$200,000 — between the two extremes — and works well when:
- Your garage is 400 sq ft but you really want 600 sq ft
- You want to keep the foundation, framing, and roof savings of conversion BUT add a master bedroom suite
- The lot supports a small extension that maintains setback compliance
We evaluate all three paths (conversion / detached / hybrid) during the Feasibility Check based on your specific lot, existing garage condition, and goals.
Why do national guides quote less for a garage conversion than our published range? See both ranges side by side, with sources — plus basements, above-garage units and outbuildings.
Not Sure Which Path Fits Your Property?
Our $499 Feasibility Check — applied to your construction contract by the contractor — evaluates garage conversion, detached, and hybrid options for your specific lot, then gives you real cost comparisons before you commit. Or start with a free Readiness Call.