Septic Design for Remodels and ADUs
Adding an accessory dwelling unit (ADU) or expanding your home creates one of the most common reasons for septic system changes. An ADU adds bedrooms and bathrooms, which increases the total wastewater flow your property must handle. Washington State requires septic systems serving ADUs to be sized using the combined bedroom count of both the main house and the ADU, not just the original structure.
Most counties in Washington use the number of bedrooms to calculate the minimum design flow, generally around 120 gallons per bedroom per day. This means a one‑bedroom ADU can effectively double the load on an older system built for a smaller home. Before construction begins, the system must be evaluated to determine whether it can be upgraded or if a new design is needed for the ADU.
Many existing systems no longer meet current standards, even without an ADU. If your septic system falls short in treatment level, tank size, or drain field capacity, local health departments can require a full design update during the remodel or ADU process. This is why septic design for remodels and ADUs often includes a full system assessment, not just a simple expansion.
How ADUs Affect Septic Capacity
When you add an ADU, several factors change at once. The design must address both the new load and how that load integrates with the existing system layout and any future maintenance needs.
- The total number of bedrooms determines the design flow, not just the original house.
- Existing tanks and drain fields may need to be resized or reconfigured.
- Separation distances from wells, property lines, and water features must still be met.
- Reserve area requirements must be maintained for the combined system.
- Permitting often requires a current system inspection or evaluation.
- Tank access and risers must remain clear and usable for the entire life of the system.
For attached ADUs, the system is usually tied into the main residence’s septic design unless space or soil conditions make a separate layout more practical. In these cases, the design shows how the original system and any new components will work together while meeting all health‑department rules. A detailed site and soil evaluation helps confirm whether the existing drain field area can support the added load or if a new field is needed.
Planning Expansions and Remodels Right
Any bedroom or bathroom addition that increases the number of sleeping rooms counts toward septic design calculations. Even a small remodel that converts a den into a bedroom or adds an extra bathroom can trigger septic review. Counties in King, Pierce, and Thurston typically require a septic design update when a project increases the total design flow.
Planning septic design early keeps your ADU or remodel project on schedule. Once the design is complete, it becomes part of your building‑permit package. The design must show how the system will handle the higher load, where new components will be placed, and how the reserve area will be maintained. Because the design is created to code from the start, it also supports smoother inspections when the project is finished.
For projects that also include a new construction component or a small commercial addition on the same property, we tailor the design to the combined use so the system performs reliably for years to come.
Update Your Septic for Your ADU or Remodel
If you are planning a remodel or an ADU, proper septic design is a key step before breaking ground. All Septic Designs LLC evaluates existing systems and designs upgrades or new systems for properties across King, Pierce, and Thurston Counties. Explore our full range of services on the Services page, or contact us today to discuss your ADU or remodel project and get a clear septic design solution.
