From an email

The plumbing inspector reviewed our underslab work yesterday, but wants a check valve installed [under the slab] just for the bsmt fixtures.  I have our plumber reviewing the changes and I’ll forward a cost proposal once I receive their info.  It’ll involve some add’l bsmt floor slab removal, some underslab plumbing rework, and some above slab proposed plumbing re-routing.

-William Shakespeare “Much Ado About Plumbing”