A common misconception is that the size of a house or number of bathrooms is what dictates the size of an on-site wastewater system. The other one is that “It will only be the two of us living here, so why does the system have to be so big?”

Septic systems have to be able to handle the likely occupancy at any time in its life. For example, if you sell the house to some who occupies every room possible.

Therefore the size of the wastewater system for residential homes is based solely on the number of bedrooms or rooms that could be used as bedrooms (such as a study, media or rumpus rooms) there is in a house. The guidelines that council requires we use stipulates the possible occupancy based on bedroom numbers:

   Number of Bedrooms.    Occupancy for Design Purposes.
   1    2
   2    4
   3    5
   4    6
   5    8
   6    9

The reason that council is strict with this rule is that it needs to be sure that the possible wastewater flows from a dwelling will be able to be treated, thus avoiding any public health or environmental risks of an overloaded wastewater system.