When your house is on a septic tank and not connected to the public sewer system, there are limitations on what you can do when adding to the house. If the house was built in the 1960’s or earlier (and even in more recently built houses) any addition of any size will likely trigger the requirement of a new septic tank or an additional septic tank.
In Miami-Dade County a new septic tank must be 100′ away from any water well, whether it be yours, your neighbor’s, or a public well. If your existing house is near the water wells of your neighbors’, you must make sure that the new septic tank is at the required distance from those existing water wells. Imagine the size of the property you must have in order to accommodate these distances. In order to install a new septic tank you must have a drainfield and a reserve drainfield. These together take up a great deal of space on any site. I worked on an addition to a house on an acre site which had a septic tank and a water well, but it was not easy to accommodate the additional septic tank, the 2 new drain fields, and keep them at the required 100 feet from the water well of the neighbors. If you cannot meet the state of Florida requirements, you must submit to a variance from the Florida Health Department. There is no guarantee that the variance will be granted. The Florida Health Department reviews all septic tank applications as well as variances.
If your existing house is on septic tank and a new sewer line has been recently located in the street in front of your house or very close by, any substantial addition will require that you hook up to that sewer line. This could add substantial costs to the addition depending on the distance from the house to the connection and whether the connection is in the middle of a public street. In the case of the same house above, the county had just installed a new water line in front of the property in the front swale, so as part of the construction project, we included the connection to the water line.
Knowing this information about the property could save you headaches down the road. Get all the information possible prior to buying the house.