Why is there a large amount of water flowing for 1-3 seconds outside the house when the ground level toilet is flushed?
Without actually being able to check out your plumbing system this is only an educated guess toward the obvious. If the flow of water is definitely related to the flushing of the toilet, then you have an opening in the sewer drain line. It could be either a broken pipe, a missing cleanout plug or cap, or possibly a joint in the piping has seperated. j3h.