New house, water line issue

[Amber Joyce]

Hello, I just bought a newly constructed house 4 months ago and I have a water line leak in my yard right outside the home (which means warranty won’t cover). Come to find out, the builder didn’t have a plumbing permit and built the new house on top of the existing slab and existing 60 year old water lines. I had a plumber come and estimate the cost to fix it the water issue. He said it would be $15,000 due to the house being on a slab and not being able to access the lines easily. Then he said he might not be able to fix it anyway due to the water lines going over into the neighboring lot (the builder split the lot and hasn’t built anything on the other part yet). So essentially the builder didn’t separate the water lines even though he owned both lots  before parcelling. I think this issue falls on the builder due to negligence. Does anyone think this would stand up in court or have any advice on how to proceed? Thank you!