What an ADU actually costs (and what drives it)

An ADU is a major investment, usually running from around $200k up to about $600k. People often ask for a price per square foot when estimating their ADU project. But that can be the wrong place to start. Two 700-square-foot units on two different lots can differ by six figures!

Here's what actually moves an ADU budget:

• ADU Type. An internal ADU (converted from a basement, attic, or existing room) is typically much cheaper than a new detached ADU or a major addition to the existing building. In 2025-2026, we saw internal conversions pulling building permits with estimated costs of less than $200k. Building permits never show the full cost of development, but these (relatively) low cost estimates show that conversions can be (relatively) cheap.
• Wastewater If you don't have sewer, updating a septic system to accomodate an ADU can be a major cost-driver.
• Utility runs. How far is power, water, sewer (if you have it) from the new unit? Trenching across a yard adds up fast.
• Site access. Can a small excavator reach the backyard for sitework, or does everything come through a gate by wheelbarrow?
• Foundation & soils. Ledge, slopes, and high water tables all cost money.
• Finishes. This is the one you control. Same siting/layout/footprint, but very different bills.

Size matters less than you'd think once you're past the basics. A good feasibility study prices your conditions, not an average cost.