Module 3 · Lesson 1
Detached vs. Attached vs. Garage Conversion
Cost, privacy, difficulty, size, utilities, and yard impact — the three main ADU strategies compared honestly so you can pick the right one.
Most homeowners end up choosing between three paths. Here's how they honestly compare.
| Detached ADU | Attached ADU | Garage Conversion | |
|---|---|---|---|
| Cost | Highest — full new construction | Moderate-high — saves a wall, shorter utility runs | Lowest — structure exists |
| Privacy | Best — real separation between homes | Requires careful design (sound, entries) | Depends on garage location |
| Construction difficulty | Full build, but self-contained | Tying into an occupied house adds complexity | Simplest — unless foundation issues appear |
| Size potential | Up to 1,200 SF detached in most cases | Up to 50% of house size (min. 800 SF allowed) | Limited to the garage footprint (~400 SF for 2-car) |
| Utilities | Longest trenching runs | Shortest runs — house is right there | Moderate; panel capacity is the usual question |
| Yard impact | Takes real yard space | Takes side/rear yard next to house | Zero new footprint |
How to read this table
Choose detached when privacy and long-term value are the priority and the budget supports it. Tenants pay more for standalone units, and families appreciate the separation.
Choose attached when the lot is tight, the budget is mid-range, or local height rules make an attached second story attractive (remember: attached ADUs must be allowed up to 25 feet).
Choose garage conversion when the garage is structurally decent and your goal is the most unit for the least money. You give up size and parking; you gain speed and savings.
The mistake to avoid
Don't pick your path by cost alone. A $120K garage conversion that rents for $1,700 and a $300K detached unit that rents for $2,900 are different investments, not a cheap and expensive version of the same thing. Match the path to the goal you defined in Module 1.
Key takeaways
- Detached = most privacy and value, highest cost. Attached = middle path with height advantages. Garage conversion = cheapest and fastest, smallest.
- Attached ADUs can go up to 50% of the main house's size, and cities must allow at least 800 SF.
- Garage conversions add zero new footprint — the yard stays untouched.
- Choose based on your goal and property, not on cost alone.