Broken basement sewer line?

We have had one of the rainiest years on record in NYC and now over the last few months I noticed a dampness in the basement  grow into downright flooding in what use to be a fairly dry cellar. With the heavy rains the horizontal sewage line appeared to be blocked and the water from the roof leader backed up and came out into a utility sink that is down there. My plumber came and snaked the sewer line after running some tests to see how quickly the water from sink or flushed toilet upstairs reached the trap that is closest to the street side. Since then it is worst. He seems to think that the line buried in the basement may have been cracked and is simply not draining properly and it needs to be replaced. THe flooding/dampness is precisely along where this line is located though it is about three feet from the neignbor’s party wall and I have not ruled out entirely that there may be another source for the flooding.I have asked the neighbors and they said their basement is dry.THankfully we are attached only on one side.THe rest of the basement is dry . I suspect it is getting worst sinc it is now damp regardless of whether it is raining or not.    Can anyone reccommend someone who can troubleshoot this correctly and give me a price? Do we need a permit to replace this? Any additional advice/comments are welcome.