Can One Individual Wastewater System Serve Multiple Dwellings in Hawaiʻi?
Hawaiʻi law now allows one individual wastewater system (IWS) to serve up to five bedrooms without limiting the system to a particular number of dwellings, dwelling units, or county-defined accessory units. The units may be attached or detached. The five-bedroom total still matters, and the law does not allow a new request to connect an additional dwelling unit to an existing cesspool.
What did Act 224 change?
Act 224 was signed on June 25, 2025 and took effect upon approval. The law states that an IWS may serve up to five bedrooms regardless of the number of:
- Dwellings or dwelling units
- Accessory units as defined by the counties
The law applies whether those units are attached or detached.
Before this change, the administrative rule described in the act's findings allowed an IWS to serve no more than five bedrooms in one or two dwelling units. Act 224 removed the dwelling-count restriction while keeping the five-bedroom ceiling.
The Hawaiʻi Department of Health Wastewater Branch (DOH) currently links Act 224 as the amendment that changes the number of dwellings allowed for an IWS.
What does the five-bedroom limit mean for a property?
Count the bedrooms across every dwelling or accessory unit that would use the proposed IWS. For example, a three-bedroom main house and a two-bedroom detached unit total five bedrooms. A four-bedroom main house and a two-bedroom unit total six and would exceed the limit stated in Act 224 for one IWS.
Do not rely only on room labels in a listing or floor plan. In its October 28, 2025 bedroom definition guidance, DOH explains that a room that could reasonably be used for sleeping is presumed to be a bedroom when it meets the physical criteria in Hawaiʻi Administrative Rules section 11-62-03. The property owner or representative may rebut that presumption with a credible representation, such as design plans from an engineer or architect or an owner's statement that the room will not be used as a bedroom.
A licensed civil engineer should confirm the wastewater design basis before the owner relies on a bedroom count for planning or construction.
What does the law not approve?
Act 224 answers a narrow wastewater question. It does not by itself approve:
- A particular building or property use
- The location, size, or type of the IWS
- A plumbing route between separate structures
- A project that cannot meet site, treatment, disposal, or agency requirements
- New dwelling connections to an existing cesspool
Act 224 expressly says the new section does not apply to a new request to connect additional dwelling units to an existing cesspool. If a property has an existing cesspool, adding another dwelling to it is not authorized by this law.
Why the site still controls the design
A shared IWS may be legally possible under the five-bedroom rule and still be impractical on a particular lot. The engineer may need to evaluate:
- Total wastewater flow and bedroom count
- Existing and proposed structures
- Available treatment and disposal area
- Setbacks, wells, water features, slopes, soil or rock conditions
- Access for construction and long-term maintenance
- The route and elevation of building sewers from each structure
- Future additions or uses that could change the design basis
DOH's IWS homeowner guidance directs owners to begin with a Hawaiʻi licensed civil engineer. The engineer evaluates the property, performs or coordinates the needed survey and testing, determines the wastewater requirements, and submits plans to the Wastewater Branch for review. Construction follows plan approval, and final use requires the applicable inspection and closeout process.
For more detail about what the engineer may review, see What Happens During an Engineer Site Visit. To organize the property first, use Documents to Gather Before an Engineer Reviews Your Property.
Questions to answer before planning one shared IWS
- How many existing and proposed bedrooms are there across all structures?
- Which rooms might DOH treat as bedrooms under the current definition?
- Are the units attached or detached, and where would each building sewer run?
- Is the current system a cesspool, an approved IWS, a sewer connection, or unknown?
- Is the proposal for a completely new IWS or an added connection to an existing system?
- Is there enough suitable area for treatment, disposal, setbacks, construction, and maintenance access?
- Could a future addition push the total above five bedrooms?
Practical next step
Gather the tax map key, site plan or survey, floor plans, bedroom count, information for every dwelling, and any existing wastewater records. Then ask a Hawaiʻi licensed civil engineer to evaluate whether one IWS is appropriate for the property.
If you need help organizing the property information and reaching an appropriate licensed professional, send the property details. Hawaiʻi Cesspool Help does not issue permits or determine whether a proposed system will be approved.
Official sources
- Act 224, Session Laws of Hawaiʻi 2025
- Hawaiʻi Department of Health Wastewater Branch
- Department of Health Bedroom Definition Guidance, October 28, 2025
- Department of Health individual wastewater systems homeowner guidance
Requirements and agency guidance can change. Confirm the current design and approval path with the Hawaiʻi Department of Health and a Hawaiʻi licensed civil engineer before relying on this information for a project.