Building a Tiny House? Where Does the Waste Go?

Cute little homes on wheels are all the rage today, but while the DIY TV shows talk about low cost and mobility, they seldom hint at how wastewater is handled by the homeowners.
Building a Tiny House? Where Does the Waste Go?
This example of a tiny house was on display at the Midwest Renewable Energy Fair in Wisconsin. These homes are gaining in popularity, but builders need to consider the appropriate onsite system for them. (Photo by Jim Kneiszel)

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Recently, Jim’s daughter and son-in-law were looking for a house to purchase to start their new life together. As they went through the process, they watched all the programs about house hunting on the cable TV channels. This is something Jim and his wife would probably never watch on their own, but they took a look so they could contribute to the conversation.

One of the programs was about the latest lifestyle rage — tiny houses. These are homes of 500 square feet or less, often as small as 200 square feet. It is amazing how a builder can pack all the modern conveniences into such a small space! In one of the episodes, the homebuyer indicated they had a hard time persuading the local county planning and zoning department to let him put the house on a secluded vacant lot. No further explanation was given, but it prompted Jim’s daughter to ask: What about the septic system?

Exploring the topic a little further reveals there are a number of issues with how these small houses do or do not fit into local zoning requirements, such as minimum sizes for permanent residences, the need for foundations, etc. These were probably the items of most concern to the buyer on the cable show. Most state codes and local septic ordinances are tied to numbers of bedrooms or numbers of water-using devices to estimate daily sewage flows, which are used to determine the size of the septic tank and the size of the soil treatment area.


Another complicating factor is that many of the tiny houses shown utilize composting toilets. We’re sure this is challenging to local administrators and county boards to determine how to deal with these requests, and to you as an installer to determine what to tell the person requesting you to design and install a system for them.  

Typically, your local government has guidelines to help determine some of the requirements for any onsite system. Minnesota and Wisconsin have many thousands of seasonal dwellings on lakeshores or hunting properties. So local units of government have had to deal with unusual situations on a regular basis.

To that end, we looked back at how we dealt with flows for these situations with a septic advisory committee. The tiny houses seen on cable TV would fit under one of two categories when estimating sewage flows for design purposes: For a small house with a regular flushing toilet and two other water-using devices such as a dishwasher, clothes washer, shower, etc., the lowest estimated daily flow would be 180 gallons. If a composting toilet was used, the estimated daily flow for what is then a graywater system would be 60 percent of that value or 108 gallons per day.


Septic tank size would be determined by the minimums necessary. So in the first case, the size would require a 1,000-gallon tank. For the second example, a 750-gallon tank would be required. Many local ordinances require higher minimums based on the likelihood that those “seasonal” residences would become fully occupied. In the case of the tiny houses seen on TV, the owners anticipated this would be their residence. By that standard, the minimums would then become 1,000 gallons for both systems. We would opt for a 1,500-gallon tank in anticipation the situation will likely change in the future, but that is just our designer background coming into play.

Sizing and location of the soil treatment area would be done the same as for a typical system. All setbacks — horizontal and vertical — would remain the same. The soil-sizing factor would be determined through soil analysis. You may encounter some minimum trench or soil area requirements. Once again, our designer background would say a little bigger may be better.

If the system is considered graywater-only, it needs to be clear that no toilet wastes are allowed. A number of areas do not allow the use of composting toilets because of previous bad experiences due to odors or vectors (insects). As always, as an installer it is important to know these requirements and be prepared to answer the owners’ questions.

If your local zoning administration has not dealt with the issue of tiny houses, these suggestions can be a starting point for discussion to determine system requirements. We would be interested to hear if tiny house onsite systems have been an issue for you or have been addressed in your area. If so, what was the outcome and were the new owners of these trendy little houses satisfied?


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