Garage to ADU: The 'Cheapest' Way to Build? (Real Costs & Layouts)

Let's be honest: looking at that detached garage filled with old boxes and thinking, "I could rent this out for $2,000 a month," is a classic California homeowner moment.

And you aren't wrong. Converting a garage to an ADU is widely considered the entry-level path to becoming a landlord. But is it as simple as throwing up some drywall and a kitchenette? Absolutely not.

We’re going to break down exactly how to do a garage addition the right way, answering the big questions about structural integrity, costs, and whether you should build in it or over it.

The Big Question: Can I actually do this?

In short: Yes. State laws (specifically in California) have made it incredibly difficult for cities to say "no" to garage conversions.

• Can I turn my garage into an ADU? Yes. This is a "conversion."
• Can I build an ADU over my garage? Yes, but this is an "addition."
• Can an ADU have a garage? Yes, you can keep the parking and build the unit elsewhere (or above).

But here is the catch: Just because you can, doesn't mean the existing structure is ready for it.

Most garages were built to hold cars, not people. They have thin slabs, no moisture barriers, and 2x4 framing that might be eaten by termites. When you ask "can i build an adu in my garage," the real question is: "Is my garage foundation worth saving?" Sometimes, demolition and rebuilding from scratch is actually cheaper than retrofitting a crumbling 1950s box.

Cost Reality Check: Conversion vs. Above-Garage

This is where budgets go to die. You need to know the difference between a conversion and a second-story addition.

Scenario A: The Conversion (The "Budget" Play)

How much does it cost to convert garage to ADU? Realistically? You are looking at $90,000 to $160,000 for a turnkey job. People hear "conversion" and think $30k. That’s a shed, not a legal dwelling. You still need:

• New sewer lines (trenching through the driveway).
• Upgrading your main electrical panel (often $3k-$5k).
• Leveling the floor (garages slope towards the door; living rooms shouldn't).
• Insulation (Title 24 energy standards are strict).

Scenario B: Building Above (The "Pro" Play)

How much to build ADU over garage? If you want to keep your parking spots and build a unit upstairs, brace yourself. Can you build an ADU on top of a garage? Yes, but you usually can't put a second story on an old garage. The walls won't hold it. This project is essentially a tear-down and rebuild. You are building a brand new two-story structure.

• Cost: $250,000 - $400,000+.
• Why? Heavy structural engineering, exterior stairs, fire-rated floor separation, and sheer construction mass.

Design Ideas: Making a Box Feel Like Home

A standard 2-car garage is about 400 sq ft (20' x 20'). That’s tight. If you don't plan it well, it feels like... well, a garage.

What is ADU garage conversion design done right?

1. Kill the Driveway Vibe: Don't just leave the garage door opening as a wall. Replace it with a large slider or french doors. Create a private patio away from the main driveway so tenants don't feel like they are living in a parking lot.
2. Vault the Ceilings: Since there is no second floor, rip out the rafter ties (with an engineer's help) and vault the ceiling. It makes 400 sq ft feel like 600.
3. The "Wet Wall": Keep the kitchen and bathroom on the same wall (often the one shared with the main house or nearest the sewer line) to save thousands on plumbing.

Final Verdict: Is it worth it?

If you have a healthy structure, converting a garage to an ADU is the highest ROI project you can do. You get the rental income of a detached unit for half the construction cost.

But if you need the storage? Can you add a garage to an ADU? Yes. If you have the yard space, building a new detached ADU with a garage included might be smarter than sacrificing your current storage.