Total Cost of an ADU

3 Replies

Going to build an ADU (800 total square feet) next to a house that has a basement and a second floor.

Question is, which costs more:

1) Building a second floor (roughly 400 sqft foundation and 400 sqft second floor).


2) Build a basement (roughly 400 and 400 again).

I realize there are many factors (sewer slope in basement, soil removal, etc.), I was just curious about general cost (2nd floor wood vs basement concrete).

Any thoughts on the matter are greatly appreciated!

@Doug Kienzle - you have an expert right in Portland, OR. Check out if you haven’t done so yet. It might help you estimate costs.
If you are looking for more info than that, there are some other sources like (California specific) that offer ADU consulting services to answer these types of questions. Good luck with your search and your future project.

@Doug Kienzle

Don't forget to take property taxes into account

Around hear( Cook County, IL) anything below grade would not be factored into assessed value. Thus lower your tax basis.