They pay your to rent space on top of the soil while owning their home.

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).Or2) 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!

IN our market in the NW we have very few slabs and pretty stable soils  so to have a foundation issue is going to be a very old house.  

Since the sprinkler system runs throughout the soil in the garden wall and the soil is also basically a moderately inclined "hill", it might not be too easy to demolish the part of the wall that surrounds the ADU and fence it off, but my friend is fairly new to the rehab process.Here is a link to a diagram that explains it visually: ADU diagram (for some reason picture attachment isn't working)

Also, look at Sandy Springs to live.

Your architectural plan creation timeline varies greatly with the locale, municipality, architect's/engineer's workload & expertise, lot size, site utilities, soil conditions, age of current home, etc...

@Dieggo Goncalves I'm in Sandy Springs. 

The development process starts with site selection appropriate for the number of units you would like to build, then you hire a civil engineer to do surveys, site planning, soils and environmental in order to begin the entitlement process, then you get an architect to design the product, then you get bids from multiple contractors.

@Sandi L WilliamsThe closest program that I am aware of to what you are asking is the investor cash flow program.

The property is totally undeveloped, so water run-off, foundation work, soil testing all would need to be done.