My parents dug out their back yard kind of like this in Salt Lake City Utah to allow for full Southern sun exposure for a passive solar home. The house is not the lowest point and has a few steps going from the back door down to a patio. So there's a small very gentle slope from the patio up to the house.

However, there have been 2 incidents in the last 35 years where huge rainstorms combined with nearby irrigation canals have caused flooding. The second incident was just last summer and we had to get some emergency pumps to save their finished basement. Since then they have purchased some pumps to have on hand in case of emergency. If the lowest point and the patio had been even with the basement like your second photo, the water would have been 2ft deep and would have gone into the house.

Are you not allowed to build a two-story house? Why not just have an unfinished basement and build the patio at ground level?

My sister has that arrangement and has not had any problems that I know of in the 22 years she has lived there. Toronto. It makes the basement very pleasant. I don't know how it drains.

If a civil engineer who looked at your specific project said what you want is impractical, what suggestions are you looking for?

If you want natural light un the basement, could you do an egress window?

With a flat lot of clay soil, maybe forego a large below-grade patio, and just utilize more windows with a window well and a man door that exits directly to a stair up to grade. The footprint is much smaller, and drainage would connect to your footing drain tile which is handled by the interior sump pump. Less risk.

he said the drainage pipe would be 5ft below street level by the time it made it to the front of the lot (185ft from patio to sidewalk).

Is the clay soil so impermeable that drainage onto the lot through a system of underground piping would not be feasible? In our jurisdiction, by law you cannot connect to municipal storm drains, but must keep all runoff on the property.

Get a second opinion.

I took the civil engineer's statement as calculating for gravity drainage instead of an exterior sump pump because of the size of requested patio and freezing climate. So if it hits 5' below street level that could be optimal to connect to a storm sewer, so to nix that opportunity means they cannot connect and civil is trying to drain to daylight in the front because the lot slopes to the front. For ballpark elevations, say basement is 8' down, so for a level pipe to hit 5' below street level means the top of foundation is min 3' above street level.

All assumptions based on that one statement from someone who has site information we do not have. I would give creedence to a civil engineer, they figure elevations for storm daily.

The CE said there is no storm drainage to connect to and code says it will have to be brought to daiylight. i was hoping thatthere was a ”dry” work around , like a pump or design change…. other than ”not doing it” ( just to address the potential sarcasm from other commentors😊)