Septic systems are sized according to the number of bedrooms in a house. The more bedrooms, the bigger the tank size and more area required for the drain field. If a property has a permit for a 3 bedroom home, and you wish to have a 4 or 5 bedroom home, you will need to apply for a new improvement permit and construction authorization in order to increase the size of the septic system.
It is important to note that for the purpose of sizing septic systems that a bedroom is considered to be any room in the home that can reasonably function as a bedroom, regardless of the current use of the room. A typical 4-bedroom septic system and repair area will need about 1/4 to 1/3 of an acre of acceptable soil.