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Accessory Dwelling Units (ADU) are a second (usually smaller) dwelling unit on the same lot as a detached single family home; they may be attached or detached.
Many existing septic systems, and some properties served by septic systems, are not capable of handling the increase in sewage flow from an ADU. To be considered an ADU under septic rules, and therefore be sized at 300 gallons per day (GPD) the unit may not have more than 2 bedrooms. By comparison, most single family dwellings (up to 4 bedrooms) are sized at 450 GPD.
Regardless of your circumstances, the first step is to contact Planning and Zoning to ensure an ADU is an allowed use at your property and learn about the zoning standards that apply.
Building a single family dwelling and an ADU
- A site evaluation approval that is adequate for the dwelling and the ADU is required. Find information on our records and information on how to apply for a site evaluation.
- Apply for a septic permit. Two septic permits are required if the dwelling and ADU will be on separate systems. Learn how to apply for a septic permit
For previously developed properties, you may have the option to connect to the existing system, or install a new system.
Building an ADU with a separate septic system
- Site evaluation – There must be adequate area for the initial system to serve the ADU, and enough area for a future replacement system to serve both the existing dwelling and the ADU. Contact us to see if there is an adequate site evaluation approval on record, otherwise, apply for a site evaluation.
- Apply for a septic permit
Connecting an ADU to an existing system
In most cases, applications to connect an ADU to an existing system sized at 450 GPD serving a 4 bedroom single family dwelling, will require that the existing system be upgraded or replaced. If the existing system serves a dwelling with 1-3 bedrooms you may be able to connect to the existing system without modifications, however, this must be determined by Septic Program staff.
To determine if your existing septic system is suitable for an ADU, apply for a septic authorization notice through Development Direct. If it is determined that your existing septic system is not adequately sized, one of the following will be required:
- A new site evaluation to determine what type and size of system you will need, if we do not have adequate soil evaluation records for your lot.
- Alterations to your existing system may be permitted if we already have sufficient soil records or if a new site evaluation is approved.
Additionally, if the system currently includes an alternative treatment technology (ATT), the ATT must be approved by DEQ for 750 GPD. The vast majority of ATTs installed for 1-4 bedroom homes are not approved for this amount of flow and would require an upgrade or replacement.
If the septic requirements for an ADU are not possible for logistical, sizing, or financial reasons you may consider some potential alternatives:
- Guest House or RV – This option equates to a significantly lower GPD sizing requirement which can mean an easier path to approval.
- If the second dwelling is intended for family members that require assistance and care, a Temporary Dwelling for Care may be allowed. This does require review and permitting every few years depending on your property's zoning.
To determine if any of these options are allowed uses on your property, first contact Planning and Zoning at zoninginfo@clackamas.us or at 503-742-4500.
