If you’re building a house anywhere in Hawaii that isn’t on a sewer line, the wastewater system is one of the first things to plan, not one of the last. The number of bedrooms you want, where the house sits on the lot, and even whether you can add an ohana unit later all depend on what the septic system can handle.
This guide is for owner-builders, people buying raw land, and anyone planning a new home outside the sewer system. It covers what the state requires and the order things need to happen in.
Cesspools are off the table
Since March 2016, Hawaii has banned new cesspools statewide. Every new home not connected to a sewer needs an approved individual wastewater system (IWS), usually a septic tank with a leach field, or an aerobic treatment unit. It doesn’t matter how rural the lot is or what the neighbors have.
The order of operations
Getting the sequence right saves months. Here’s how it typically works:
- Get a site evaluation early, ideally before you finalize house plans. A licensed engineer checks the soil and runs a percolation test. The results decide where the system can go and how big it has to be, which affects where the house can go.
- Design the house and the wastewater system together. Bedroom count and footprint drive the system size and layout.
- Submit the IWS plans to the Department of Health. The engineer files them with DOH’s Wastewater Branch.
- Apply for the county building permit. For homes on an IWS, DOH reviews the wastewater system before it signs off on the building permit. Hawaii County’s guidance recommends getting DOH approval before you apply to keep the permit from stalling.
- Build, inspect, and get DOH’s approval to use before anyone moves in.
One timing trap: DOH’s approval lapses if the county doesn’t issue your building permit within a year. If your project stalls, you may have to resubmit.
Our step-by-step conversion guide explains the DOH application, inspection and approval steps in more detail. They work the same way for new construction.
How the system is sized
State rules size a home’s system by bedrooms, not by square footage or bathrooms.
- Design flow is 200 gallons per day per bedroom.
- Minimum septic tank size is 1,000 gallons for up to four bedrooms and 1,250 gallons for five.
- Leach field size depends on how fast your soil absorbs water. Per bedroom, it ranges from about 70 square feet in very fast-draining soil to 330 square feet in slow soil. If the soil drains slower than 60 minutes per inch, a standard leach field isn’t allowed at all.
Watch out: what counts as a bedroom
Under Hawaii’s wastewater rules, any room 70 square feet or larger is presumed to be a bedroom. That den, office or “bonus room” on your plans may count. Because each bedroom adds to the required system size, this can change your design and your cost. Talk it through with your engineer before your plans are final.
Limits that shape what you can build
A few rules set hard limits, and they matter a lot if you’re thinking about a second dwelling.
- Five bedrooms per system. One IWS can serve up to five bedrooms total, whether they’re in one house or split between a main house and an ohana.
- 1,000 gallons per day per system.
- 10,000 square feet of land per system. Lots recorded before August 30, 1991 are an exception.
So if you want a four-bedroom house and a two-bedroom ohana on one system, you’re over the limit. Planning for the ohana from the start, even if you build it later, avoids an expensive surprise.
Setbacks
Every part of the system has to sit a minimum distance from certain features. A few of the main ones under state rules:
- Buildings: at least 5 feet for the tank, leach field or seepage pit.
- Property lines: 5 feet for the tank and leach field, 9 feet for a seepage pit.
- Streams, ponds, lakes and the ocean: at least 50 feet, measured from the certified shoreline for oceanfront lots.
- Large trees: 5 feet for the tank, 10 feet for the leach field or pit.
- Drinking water sources that serve a public water system: 500 feet for the tank and 1,000 feet for the leach field or pit.
On a small or oddly shaped lot, setbacks can leave very little room. If you have a private well or catchment system, ask your engineer how the system will be placed in relation to it. Oceanfront applications may also need a certified shoreline survey.
Building on lava rock
Large parts of Hawaii Island, including much of Puna, Kona and Ka’u, sit on lava with little or no soil. That creates two problems. Digging is slow and expensive, and fractured lava lets wastewater move too quickly to be treated.
State rules deal with this directly. On fractured lava, a standard leach field isn’t enough. The design has to use a soil replacement system, where poor material is dug out and replaced with suitable fill, with at least three feet of separation to groundwater. Raised mound systems are another option, reviewed case by case. Excavation often means hydraulic hammers or rippers rather than a shovel and bucket.
This is a big reason lava lot systems cost more. It’s also why an early site evaluation matters so much on the Big Island.
When an aerobic treatment unit makes sense
There’s no blanket rule requiring an ATU on any particular lot. Your engineer chooses based on the site. ATUs come up most often where groundwater is shallow, where setbacks are tight, or where a variance is needed. Near the coast in areas with less than three feet of separation to groundwater, state rules point toward an aerobic unit discharging to a mound or drip system.
If you get an ATU, it has to be a certified model, and you’ll need an active service contract with inspections at least every six months for as long as you own the home.
Owner-builders: what you can and can’t do yourself
Plenty of people build their own homes in Hawaii, but the wastewater system is one part you can’t do yourself. A Hawaii-registered engineer has to design it, and a contractor with one of the license types DOH accepts (A, C-9, C-37, C-37a or C-43) has to install it. Plan for both in your budget and schedule.
What it costs
New systems cost roughly the same as conversions, minus closing an old cesspool. Most land between $20,000 and $50,000, with lava rock, tight lots and advanced treatment pushing toward the top or beyond. Our cost guide goes into the details.
If you’ve got a lot picked out or plans in progress, we can help you figure out what kind of system fits before you lock in the design. Request a free estimate.
Frequently Asked Questions
Can I build a new home with a cesspool in Hawaii?
No. New cesspools have been banned statewide since March 2016. New homes not on a sewer need an approved septic system or treatment unit.
Do I need septic approval before my building permit?
For homes on an individual wastewater system, the Department of Health reviews the system before it signs off on the county building permit. Getting DOH approval first usually keeps the building permit moving.
How big a septic tank do I need for a new house in Hawaii?
At least 1,000 gallons for up to four bedrooms and 1,250 gallons for five. Your engineer sizes the whole system, including the leach field, based on bedroom count and how your soil drains.
Can I put an ohana unit on the same septic system?
Possibly, as long as the main house and ohana together have no more than five bedrooms and the system is designed for it. Above that, you’ll need a separate system and enough land for it.
Does an office count as a bedroom for septic sizing?
It might. Hawaii’s wastewater rules presume that any room 70 square feet or larger is a bedroom. Check with your engineer before your plans are final.
Can I install the septic system myself as an owner-builder?
No. The system must be designed by a Hawaii-registered engineer and installed by a contractor with a license type DOH accepts.
Why is septic more expensive on the Big Island?
Much of Hawaii Island is lava rock with little soil. Digging takes heavy equipment, and fractured lava requires a soil replacement system or another engineered design, which adds cost.
