What's the Deal with Septic Systems?

Author: Renée J. Schomp, Director & Scott Johnson, ADU Expert, Napa Sonoma ADU Center

Here at the Napa Sonoma ADU Center we see a lot of ADU projects fail to move forward because of the challenges associated with septic systems. So if you are located on a property with a septic system and not able to access city water, this blog post is for you.

This blog post covers:

  • What is a septic system and how does it work?

  • How is septic capacity measured?

  • Who are septic consultants I can hire to help me

  • Is there a workaround that would allow me to avoid the cost and hassle?

  • Is there any financial assistance available?

Septic systems can make or break an ADU project – so if a property is on a septic system, a crucial first step is identifying:

  • Does the existing septic system have the capacity to add an ADU?

  • If not, does it need to be upgraded or does a whole new system need to be put in?

  • How much will an upgrade or new system cost? (Costs can range from $2,000 - $250,000)!

  • Adding this cost to the estimated cost of the ADU project in total, is the project reasonable for your budget?

Before we dive into the topic further, let’s take a step back and make sure we even understand: What is a septic system and how does it work?

There are multiple different types of septic systems (see more on the Permit Sonoma website). The Environmental Protection Agency has an excellent short primer on the subject, which you can read here in full. Here is a helpful excerpt:

“Septic systems are underground wastewater treatment structures, commonly used in rural areas without centralized sewer systems. They use a combination of nature and proven technology to treat wastewater from household plumbing produced by bathrooms, kitchen drains, and laundry. A typical septic system consists of a septic tank and a drainfield, or soil absorption field.”

The EPA webpage goes on to explain:

  • How a typical conventional septic system works

  • How to assess if you have a septic system

  • How to find your septic system

  • Failure symptoms: Mind the signs!

And you can even check out an animated, interactive model of how a household septic system works created by the Guadalupe-Blanco River Authority.

How is septic capacity measured? Septic capacity is measured by the number of bedrooms the septic system was built to support. So for example if you have a septic system with capacity to support three bedrooms, and you have a primary residence with two bedrooms in it, you can build an ADU that is either a studio or one-bedroom ADU.

 
 

Permit Sonoma has an extremely detailed webpage, including FAQs and insights into septic systems and ADUs, available right here.

 
 

But hang on – is there a workaround that would allow you to avoid the cost and hassle of spending money on your septic system in order to build an ADU?

Maybe! Depending on your project – there is sometimes an option to do a "bedroom swap," where you remove a bedroom from your primary residence so that you can add an ADU without upgrading your septic capacity. A bedroom swap works differently depending on the jurisdiction you are in. In short, it requires that you work with your local jurisdiction and professionals to structurally alter an existing bedroom in the primary residence such that it would be challenging for it to be used as a reasonable bedroom – for example, by removing a closet and also making other changes to convert it into a home office or some other room type. In this manner, you can “swap” a bedroom out of the primary residence and into your new ADU.

Permit Sonoma’s guidelines, for example, state in part: “[A room] may be exempted from being considered a bedroom if there is no closet and at least one of the following is present:

a) permanently built-in bookcases, desks and other feature that encumber the room in such a way that it cannot be used as a bedroom;

b) a minimum 4 foot opening, without doors, into another room; or

c) a half wall (4 foot maximum height) between the room and another room.”

Since every project is different and rules change regularly over time and across jurisdictions, make sure to speak directly with your local jurisdiction if you are considering a bedroom swap. This may not be an option depending on your particular location and project.

  • For Sonoma County contact Permit Sonoma Well & Septic at PermitSonoma-Well-Septic@sonoma-county.org or at (707) 565-2849. You can also get many more details on Well & Septic matters from Permit Sonoma at their webpage right here.

  • For Napa County contact the Napa County Environmental Health Division at Environmental@countyofnapa.org or at (707) 253-4417. You can also get more details on well and septic matters from Napa County’s Environmental Health Division at their webpage right here.

Who are septic consultants I can hire to help me?

This is all so expensive – is there any financial assistance available?

Beyond the ADU financing we have information on via our website, these resources are available for some to receive assistance specifically with septic matters:

  • USDA Rural Development Housing Repair Program (RH 504): Lower income and elderly homeowners in rural areas may qualify for a loan or grant via the USDA for home repairs and maintenance, including a septic upgrade – see more information on the USDA website. 

  • Site Cleanup Subaccount Program: This program is suitable for groups of landowners to apply collectively for funding. Your local Resource Conservation District may be able to help you apply for the funding.

Long story short – if you want to build an ADU and your existing house is on a septic system, contact your local septic agency to find out if you can add an ADU before scheduling your ADU Feasibility Consultation with our nonprofit to kickstart your build!

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