The ADU costs that don't show up in the first bid

Most ADU budget blowups aren't the contractor padding the bill; they're real costs that were never in the bid. The pattern repeats often enough that you can check for each one by name before you sign anything.

Utility service upgrades are the biggest recurring surprise. An ADU is a dwelling, so it needs dwelling-grade service: often a bigger electrical panel at the main house, sometimes a larger gas or water meter, and if your sewer main sits uphill of the new unit, an ejector pump. Ask every bidder which of these their number includes; the honest answer is specific.

Landscaping comes back twice. Trenching for utilities tears up the yard on the way in, and equipment access can take out fences and gates. The repair work after the build is real money that almost never appears in a construction bid, because it isn't construction.

Solar belongs in every detached new-build conversation: the state energy code requires a new solar system on newly constructed detached ADUs, while garage conversions and attached units are exempt. There's a separate guide on this site with the full rule and its exceptions.

Fees have thresholds worth designing around. State law waives development impact fees entirely for ADUs under 750 square feet, and school facility fees generally apply from 500 square feet up. A 749-square-foot plan and a 751-square-foot plan can be separated by thousands of dollars in fees; your designer should know exactly where your city draws these lines.

On top of all of it, carry a 10 to 20 percent reserve. Older homes surprise you when walls open: end-of-life sewer laterals, undersized panels, garage slabs that were never built to carry a dwelling. The free budget tool on this site itemizes what's in your range, lists what to budget separately, and sizes the reserve for you, so the first bid you compare is a complete one.