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 for the same Twin Falls market rate ($800-$1,400/month). 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 increase | +15-22% | +22-30% |
| Rental income potential | $800-$1,200/mo | $1,000-$1,400/mo |
| 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.
Not Sure Which Path Fits Your Property?
Our Feasibility Check evaluates garage conversion, detached, and hybrid options for your specific lot — then gives you real cost comparisons before you commit.