To determine the size of the private sewerage system you need to visit the local engineers/board of health. They will have the plans & layout on any private sewerage system. Typically a 3 bedroom house has around a 1000 gallon tank for septic but it can vary widely depending on local laws, soil conditions and age of the system. That's just one of the reasons prior to transfer of ownership a Title V is required for most situations. There are exceptions though and you need the proper profesionals advising you.
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