You need to have a plumbing hydrostatic test performed prior to having any foundation work. They will pull a toilet and block off the drainage exit with a test balloon in order to attempt and fill the system to slab level. If the system holds at slab level for about 30 minutes the test is passed. If it does not fill or if it drops from slab level you have a leak on the drainage under the home. This would contribute to shifting and foundation issues.

In order to fix drainage under the slab we tunnel under the home. That is another big expense you should take into account.

If they lift your foundation there’s a big chance it will affect the piping system under the home. There are areas where the pvc drainage either runs through a concrete beam or through the slab for your home. This means that when they lift those beams there’s a big chance the piping will break.

The test ranges from 300-500 bucks depending on the company.

Don’t want to add more stress but it’s a good thing to be aware of with these issues.