After countless hours of planning (thanks to everyone here for helping me out with that!) I had a licensed plumber complete the rough in of a bathroom in my basement this weekend. It proceeded without problem except it took longer to find the existing drain lines buried under the ground than I thought it would.

When the plumber finished the rough-in and replaced all the concrete, he left a small area (maybe 4 inch square) unfilled, around the new tub drain rough in. He said this was standard, that the space would be needed to make all the connections, and (this surprised me) that it would never get filled in at all, just left open the way it is.

It seems to me that's an easy way to get water in, possibly. Or even bugs. Who knows.

Is this normal?

Yup! That's called a tub dap out... very normal to keep unfilled as may need to access connections to trap in the future!

Hi James, Back again I see.

When the plumber finished the rough-in and replaced all the concrete, he left a small area (maybe 4 inch square) unfilled, around the new tub drain rough in. He said this was standard, that the space would be needed to make all the connections, and (this surprised me) that it would never get filled in at all, just left open the way it is.
It seems to me that's an easy way to get water in, possibly. Or even bugs. Who knows.
Is this normal?
This is call a "Tub Dap-Out" It's necessary because the tub waste connects to the trap raiser under the floor line. As a rule we leave them open with no complaints but If this opening bothers you can cover the dirt with roof cement and seal it that way.
Good luck, Tom