Your town's Health Code or ordinances govern this question - size of sewer lines, septic tanks, etc depending on the number living on the premises. If no town Code, then it is governed by your County. Go to those offices and get copies of those Codes. They should be clear enough for you to get the answers yourself, but if not, bring them to an attorney.
This comment does not create an attorney-client relationship. The law and its application by the courts is constantly evolving and changing. This discussion is not to be taken as a definitive guide, and should not be relied upon to determine all fact situations. Each set of facts must be examined separately with the current case and statutory law analyzed and applied accordingly.