How much does an ADU really cost in California?

The honest answer is "it depends" — but the things it depends on are knowable. Here is how an ADU budget actually breaks down in California, and where the real money goes.

Soft costs vs. hard costs

Soft costs are everything before a shovel hits the ground: design, structural engineering, Title 24 energy calcs, and city permit fees. On most projects these land somewhere between 10% and 20% of the total. They feel invisible, but skipping or rushing them is where the worst overruns start.

Hard costs are the build itself — foundation, framing, plumbing, electrical, finishes. This is the bulk of the number, and it swings the most based on what kind of unit you're building.

The single biggest variable: converting or building new?

A garage conversion reuses an existing slab, walls, and roof, so you're mostly paying for what happens inside. That's why conversions can start around $26k–$78k.

A detached ADU is a new structure with its own foundation and utilities, which is why those start closer to $145k for a studio and climb with size.

Where budgets actually move

• Kitchens and bathrooms — fixtures and cabinetry are cost-dense. One bath is very different from two.
• Utility connections — a new sewer line or an electrical panel upgrade can be a five-figure line item on its own.
• Finishes — the same floor plan can vary 20% on finishes alone.

The number that matters isn't the average — it's the one for your lot and your finishes. That's exactly what a free in-home visit is for.