REMODELING OR ADDING LIVING SPACES / ADUs

Adding bedrooms or separate living spaces for guests or renters means more people sending water and waste to your septic system. Local and state health codes require that you have septic capacity to match the added living capacity.

You may be able to add a bedroom in your home if the total number of bedrooms will not exceed the number your septic permit allows and there are no other building or permit limitations on your home.

Permitting an ADU is almost like building a second home on your property: You must show you have adequate water capacity from your well or an allocation from your water provider. Also, you must meet all requirements for a building permit. As for septic capacity, the minimum septic size for an ADU is for 2 bedrooms. If you have 2 bedrooms’ worth of unused capacity in your current septic system, you may be able to tie into the existing system. If not, you will either need to add a septic system dedicated to the ADU or alter the current septic system to increase capacity. (Our contacts at the county have told us that it can actually be less complicated to add a separate system than to alter an existing one.)

A septic designer will be able to help you determine the way forward.

My septic records indicate that my system is “non-conforming.” What does that mean?

It usually means that although the system may be performing just fine, there is something about your property that prevents you from installing a septic system that meets code. There are likely to be limits to making additions or alterations to the home and other structures on the property. Consult Seattle/King County Public Health’s information on house or building remodels.

Oops, I built a living space over the garage or converted my old potting shed into a cute guest cottage. Now what?

Adding living spaces and structures can be a fun and creative project. Unfortunately, when you cut corners on wastewater infrastructure, you run the risk of surface or groundwater contamination, premature failure of your septic system due to overuse, and/or roadblocks when seeking building permits or selling your home.

If you have added a living space or structure on your property that includes a kitchen and/or bathroom, you need a plan for water in and wastewater out of that living space, and it may be considered an ADU for building permitting purposes.

King County’s Sanitarian of the Day phone line (206-477-8177) might be a good place to start asking questions about your options, and you are allowed to remain anonymous.

As for building permits for ADUs and remodels, check out the webpage link in the section above.

Related Links:

Flowchart: Overview of Building Remodel Application on properties with Existing Septic Systems