new house's runoff draining onto our property

A new house was built and is in the final inspection stages with the City. It was built on the 50’ x 100’ corner lot behind our 50’ x 100’ corner lot. Both houses’ driveways run along the rear property line, accessible from the side street. Our 1920s home’s masonry garage rests on the property line. The new home’s garage is set back approximately 30". But its slab was poured more than 12" higher than the existing grade. I flagged this to the builder as soon as I saw the slab, which was months ago. He said to give them time and it would be all right.

Fast forward to now, the garage is built and the grading done, and their grade is at least 9" higher than ours. We hired a civil engineer