Septic Systems are designed differently depending on what Province or State you reside in, although a common element to designing a septic system is based on the amount of bedrooms that are located in a home.  The general rule is each bedroom constitutes a hydraulic load or production of effluent since you are housing more individuals. Due to this the septic systems must be made larger or use an engineered system to ensure the effluent (lets say dirty water) has enough area to disperse into the ground and be treated properly I just had the expereince of testifying at a court case where the above paragraph weighed heavily on the courts decision,  Once the decision is made public I will post the link The case involved a recent new homeowner suing the Municipality(me), the old home owner, ... 04/08/2009