Are Shared Septic Systems a Good Idea?

Precautions and planning will help pumpers provide quality service and avoid getting in the middle of homeowner disputes.

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I occasionally get questions involving shared septic systems and other infrastructure. Most often these questions are connected to separating properties that were part of a resort or as a part of condominium or townhome developments. Other situations I have seen involve duplexes where one side will be used as a rental unit or individual residences where a family owns a property that is split to accommodate two related families. 

As service providers, you need to understand how each of these situations may impact system performance. From my perspective if you are involved in discussions with the parties about whether they should share a system, you may suggest considering separate systems if the property can accommodate them. However, you and the homeowners need to understand any and all requirements of the permitting authority before any design or installation work is done. 

In the case of breaking up an old resort, there is probably some infrastructure in place in the form of septic tanks and drainfields. My experience in these cases is that most often they were installed before the current regulations were in place and have been grandfathered in. When the property is split, the expectation is everything will be brought up to current code. The first step is determining what is there and the condition of system components to assess whether any of it is usable under current rules.

If the septic tanks are in good condition and up to code, they may not be usable for a shared system due to location or additional local requirements. Most situations I have encountered require a separate septic tank or tanks for each residence. Even if a shared tank would be allowed, separate tanks should be encouraged. Family-use patterns may vary widely. Separate tanks allow separate maintenance and pumping schedules based on individual usage without getting involved in potential disagreements about who is putting what into the system.


I was briefly involved in a lawsuit in Canada concerning joint use of septic tanks for a series of condominiums. The case involved one party flushing sanitary products that plugged the tank outlet even though part of the agreement was these products should not be flushed. Of course, the tank backed up, flooding the other residence’s basement. One question we would probably ask: Why there wasn’t a high-water alarm system to alert the owners before this became a problem? 

As the lawsuit progressed, the installer was called into court as well because there wasn’t an alarm system installed. There were additional tank infiltration issues due to installation problems revealed during the discovery phase of the lawsuit.

From my perspective there are a couple of takeaways. As a service provider or installer, you are not necessarily off the hook in shared system situations if one of the parties does something that impacts the other residences. Separate septic tanks equipped with effluent screens means the homeowners who flushed inappropriate materials are the ones who will suffer the back-up. No matter how specific the agreement/covenant is, it does not prevent problems from occurring. 

Access to all parts of the system for management and maintenance is important. Taking care of shared system access is as important as it is for an individual system. If you are the service provider, it is important the owners understand you need to know the location of each component and have access for scheduled maintenance visits. If you are the service provider, make sure easements are in place allowing access.


Each septic tank will be sized according to current regulations generally based on number of bedrooms and water-using devices (dishwasher, laundry, garbage disposals, etc.). If individual tanks are installed and the residences are the same size, this is not a problem. If there is a shared septic tank or the residences are different sizes, the use agreement becomes more complicated.  

Even with separate tanks, if one residence uses more water there will be issues to address for sizing required pump tanks and the soil treatment areas. I observed this for a property near where I live. A resort was separated into two properties to share a common aboveground mound system. Each residence had a separate septic tank installed with piping to a common pump tank supplying the mound. 

One of the residences has two bedrooms and the other has three bedrooms. There is a difference in estimated use between the residences of 150 gpd. This affected the size of the pump station and the mound in terms of the total estimated daily sewage flow. As part of the agreement, there was a distribution between the parties of the cost of construction based in the estimated difference in use. Since the total cost of the system was more than $30,000, the difference was not insignificant.

I would recommend as a service provider that it would be prudent to measure flow to the whole system through use of water meters in each residence if they are not required by regulation. Short of having water meters, there should be a cycle counter installed on the pump so the amount of effluent delivered to the mound can be monitored. All is good if use is within the estimates. But if use is higher, you can work with the owners to reduce their water use before they hydraulically overload the system.

There are other potential problems with shared systems that need to be addressed in the use agreements. What if one of the residences wants to expand, build an addition or add bedrooms? This type of situation should be handled through the permitting authority, but as a maintenance provider you need to be aware of any of these changes. It probably means increased maintenance and your agreement to provide it will change.


What problems have you encountered with shared septic systems? I am sure there are numerous other issues I haven’t dealt with here. What happens when one dwelling sells? What happens when there is a dispute? The list goes on!  


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