I just completed a masterbathroom addition to my 1952, 2/1 home. I had a new septic tank and drainfield put in. Recently, I've detected a sewage odor coming from the old bathroom. There's no smell outside the house. And the odor seems to occur after a heavy rainstorm.
This problem never happened prior to the new addition. Thanks for your help!

The entire house including the new addition is one story. The existing bathroom floor elevation is 4" lower than the new addition.
Thanks for your response.
Don