How Much Does It Actually Cost to Build an ADU?

Let's do what most ADU websites won't: give you real numbers, with honest context about what they mean.

The planning benchmark

For new-construction ADUs built by a licensed general contractor, a realistic planning number in California starts around $350–$375 per square foot on the lower end — for straightforward projects on flat lots with reasonable utility access. Complex sites, high-end finishes, hillsides, and small units push well above that.

Run the math on common sizes (lower-end planning ranges):

• 400 SF: ~$140K–$150K+ construction
• 600 SF: ~$210K–$225K+
• 800 SF: ~$280K–$300K+
• 1,000 SF: ~$350K–$375K+

Read this correctly: these are planning ranges, not quotes. Your property, your design, and your market set the real number — that's what estimates are for. But if someone promises a new-construction ADU at $200/SF, you've learned something important about that someone (Module 11 covers what).

Where the money actually goes

A typical new-construction budget splits roughly like this:

• Site work & foundation: 10–15% — grading, trenching, concrete
• Structure: 20–25% — framing, roof, windows, doors
• Systems (MEP): 20–25% — plumbing, electrical, HVAC, utility connections
• Interior finishes: 25–30% — drywall, flooring, kitchen, bath, paint
• Soft costs: 10–15% — design, engineering, permits, fees (lesson 6.5)

Notice something: the parts you see in photos — the kitchen, the flooring — are only about a quarter of the budget. Most ADU money buys things that disappear into walls and ground. That's why two ADUs that look identical can differ by $100K, and why judging bids by the pretty parts misleads.

The three cost paths, compared

• Garage conversion: cheapest (lesson 6.3 has full numbers) — the structure exists.
• Attached ADU: moderate — shares a wall and shorter utility runs, offset by the complexity of tying into an occupied home.
• Detached new construction: the benchmark above — full scope, full cost, most value.