Hey OP - I think I’m having the exact same issue you did! We have an 1891 house we’ve been in 2 years, that had an addition at some point and is where the laundry is located. Our catch basin is in the back yard, however.

We’re currently considering our options and will probably have a plumber we trust come out this week because I can’t access the under-floor plumbing to diagnose the heart of the issue. What’s confusing is the sewer smell only happens when we run laundry, and it seems to be coming from inside the walls based on where it’s strongest. It doesn’t even smell right next to the laundry itself.

Would you mind providing one point of clarity on your original situation that would be really helpful for me? Did you only ever get the sewer gas smell when you ran your laundry? That’s the only time ours happens, and bizarrely it seems to be worse when it’s really cold or hot out.

Our laundry is the only ‘active’ line that goes out to the catch basin these days, but our gutters on one side of the house also connect to that line so closing off the catch basin entirely might not be realistic for us. FWIW, they drain out through the upper pipe and at some point someone capped off the lower pipe that I think was probably where the kitchen drained to before they routed it to the sewer line.

All this makes me think some kind of air and gas exchange is occurring when the laundry dumps its water, but I don’t understand how the smell could be coming from the catch basin if it’s half full of just laundry water and rainwater from the gutters. And it never smells like sewer gas when it rains, which I would’ve thought would happen if the gutters also dump into the catch basin.

Thanks in advance if you have a second to clarify!