Recently, we ran into a situation involving the size of a septic tank on a property, and felt that the information we gathered could be of use to Buyers and Sellers that are involved in the sale or purchase of a home with a septic system. Missoula County requires that a septic system is permitted, which requires certain specifications, based on the number of bedrooms in a home. If a home has more bedrooms than the permitting allows, adjustments to the system may be needed. 
The permitting of septic systems (every on-site wastewater treatment system) in Missoula County has been required since the late 1960’s. The size of the septic tank and drainfields on residential structures is determined by the number of bedrooms that a home includes. This, however, is the part that becomes difficult. A “bedroom” is considered to be any space that could easily be used for sleeping and has a reasonable expectation of privacy (so this could include offices, exercise rooms, game rooms, etc.). In real estate, we generally consider a “bedroom” to be a room with legal egress and a closet. This, however, is not the same specification when a bedroom count is done for septic sizing purposes.
Why would a room that is obviously being used as an office or game room be considered a bedroom? Wouldn’t it be more accurate to count the number of people using the home and septic system?
The Health Department counts the number of rooms that could reasonably be used as bedrooms, rather than the actual current usage, because upon sale of a home, that home could have vastly different usage on the septic system. If the Health Department were to strictly compute based on usage and how many people occupied the home, they would, potentially, have to adjust the system upon every sale of the home. This, of course, would not make sense.
What if a septic system in a home was permitted, but then bedrooms or additions were made without a building permit?
Homes with permitted septic systems are usually only for the number of bedrooms that were in existence at the time of the permit. Sometimes, an additional bedroom was added to the count if there was an unfinished basement or other unfinished portion. If there are any additions made, you should definitely check with the Health Department, as they would be able to tell you if any changes are needed to the septic system. If additions were made without a building permit, and without checking with the Health Department, then the home could very likely be out of accordance with the permit, and would likely require changes and or upgrades.
So, what would be required if a home has usage beyond what the permit allows?
If a home contains more “bedrooms” than a permit allows, there are a number of different possibilities for correction. Corrections and/or upgrades to the system depend a lot on what the soil is like in the area. For example, a home in sand or gravel soils that went from three bedrooms to four bedrooms many only have to add fifty feet of drainfield to the existing system. However, if the same home were located in clay soil, the upgrades could include increasing the septic tank size, and adding a significant amount of lineal feet to the drainfield. It could also include changing the system from a gravity system to a pressurized system. The biggest circumstances that determine the changes to be made are (1) the type of soils in the area; (2) how far the homeowner has deviated from the original permit; and (3) the conditions of the subdivision approval.
There may also be some instances where increased usage is simply not allowed. This could include homes located in the floodplain, or in the Missoula Wastewater Treatment Plant Service Area.
We have received this information from the Missoula County Health Department, and they would strongly encourage anyone who is considering a change to their home to contact the Health Department prior to modifying the home. This will ensure that the permit is still valid, and if not, what needs to be done. It also will verify that the wastewater coming from you and your home is adequately treated before entering the aquifer that we all drink from.
There is a sanitarian available at the Missoula County Health Department from 8:00 AM to 5:00 PM, Monday through Friday (with the exception of the Noon hour). The contact phone number for the Health Department is: (406) 258-4755.