A septic design is a reflection of the home. The larger the home (More Bedrooms/Bathrooms/Finished Floor Area) the larger the septic is. When someone finishes a basement without a building permit, they skip the investigation into the sizing issue. These questions are raised by the permit departments to ensure the house and septic match up. The septic may have only been designed for the initial finished portion of the home. Therefore adding bedrooms/bathrooms in the basement can make your septic undersized.