Published September 3, 2026 · Updated October 8, 2026
Start with the water bill. If your Board of Water Supply bill carries a City sewer charge, the house is on the county sewer. If it does not, the house has its own system in the ground — a cesspool or a septic tank — and the yard, the closing documents and the state’s records will tell you which. Or we open it and tell you; the assessment is free.
It matters more than it used to. Hawaiʻi’s Act 125 requires every cesspool in the state — roughly 88,000, more than 11,000 of them on Oʻahu, per the Department of Health — to be upgraded, converted or connected to a sewer by January 1, 2050. A septic tank is not a cesspool, and a sewer connection is the finish line. So “which one do I have” is the first question, and the answer also decides what you pay every month.
The water bill
On Oʻahu, sewer service is billed by the City and County’s Department of Environmental Services, and its charge rides on the Board of Water Supply bill. If there is a sewer line on that bill, you are connected. If there is not, you are not.
The charge has two parts. As of July 1, 2026, a single-family home on metered water pays a $48.27 monthly base charge plus $11.83 per 1,000 gallons of water used, with a 20% discount on the usage portion if you have no separate irrigation meter. The City’s own sample bills, with that discount applied, put a house using 4,000 gallons a month at $83.76, 6,000 gallons at $107.42, and 9,000 gallons at $131.08. Rates had not changed since 2016. In 2026 the City restructured the charge, lowering the base fee and raising the per-gallon rate, so bills fell for light users and rose for heavier ones. More increases are scheduled: 8.5% in July 2027 and 9% every July from 2028 through 2031.
That is what “on sewer” costs. Hold the number; we come back to it.
The yard
If there is no sewer charge, walk the property.
A cesspool is a pit. There is usually a single lid — often a round concrete cover, sometimes a square one, sometimes nothing visible at all because it was buried decades ago — and there is no drain field, because the pit does the disposing. Cesspools were the standard on Oʻahu for a long time and were only banned for new construction in 2016, so an older home outside the sewered neighborhoods is a cesspool until proven otherwise.
A septic tank has two lids, one over the inlet and one over the outlet, sometimes raised to grade on risers. Downstream of it there is a drain field: a stretch of lawn that stays a little greener or a little softer than the rest, sometimes with a small distribution box between the tank and the field. If you can find two lids and a strip of suspiciously healthy grass, it is septic.
An aerobic treatment unit looks like a septic system with electricity. There is a control panel on an exterior wall with an alarm light, and a blower you can hear running. If your house has one of those, you have an ATU, and it needs power and a service schedule.
A sewer connection shows up as a cleanout — a capped pipe near the property line or the house — and nothing else. No tank, no lids, no field.
Lids get buried under lawns, lanais and driveways, so not finding one proves nothing. Finding one is what an inspection is for.
The records
Three places keep the answer on paper.
Your closing documents. Hawaiʻi sellers must disclose a cesspool, so if you bought recently, the disclosure form or a past inspection report names the system.
The Department of Health. The Wastewater Branch keeps records of individual wastewater systems, and it is the agency that permitted whatever was installed legally.
The Hawaiʻi Cesspool Prioritization Tool. The University of Hawaiʻi and the Department of Health map every known cesspool parcel in the state and assign it a priority level. If your parcel is on that map, you have a cesspool. We built our Oʻahu cesspool data page from the same source, matched to county ownership records — 6,763 parcels on this island, 1,340 of them in the highest tier.
What each one means under the 2050 law
Cesspool. It has to go. The law says January 1, 2050; the state’s Cesspool Conversion Working Group recommended 2030 for Priority 1 areas and 2035 for Priority 2, and the Legislature has not adopted that schedule yet. Your priority level tells you how much of a margin to expect. The Hawaiʻi cesspool conversion guide covers the three ways out and what they cost.
Septic. Act 125 is a cesspool law. A permitted septic system is not a cesspool, so the 2050 requirement does not apply to it. What a septic system has instead is a clock of its own — the tank needs pumping on a schedule, and the drain field has a working life. Keep it pumped and it keeps working.
Sewer. You are done, and you pay for it monthly.
Sewer fee versus keeping a tank
This is the comparison nobody lays out, so here it is with the City’s numbers and ours.
On sewer, a single-family home using 4,000 to 9,000 gallons a month pays about $84 to $131 a month at the rates in force since July 1, 2026 — roughly $1,005 to $1,573 a year — and the rate rises 8.5% next July and 9% a year after that through 2031.
On a septic tank, the routine cost is the pump-out. Most tanks on Oʻahu need it every two to four years, and we charge $599 to $799 for a tank of 1,500 gallons or less with the lid at the surface and the truck within 75 feet. Spread across the interval, that is $150 to $400 a year, or about $12 to $33 a month. A tank also carries a drain field that will eventually need work, and that is the cost the monthly figure hides.
On a cesspool, pumping is $599 for a pit of up to 1,500 gallons with the same access, every one to three years depending on size and use — $200 to $600 a year. But every dollar spent keeping a cesspool going is spent on a system the law has already retired, and the conversion at the end of it is a $25,000 to $50,000 job.
The honest read: month to month, a working tank is far cheaper than a sewer bill. The sewer fee is the price of never owning the problem — no field to fail, no conversion to fund, no 2050. Which one is the better deal depends on how long you plan to hold the house and what your lot can support, and the second half of that is exactly what a site assessment answers.
What to do with the answer
If it is a cesspool, read the conversion guide, check your priority level, and get the free assessment before you decide anything — cesspool conversion is the job, and the number comes from your lot, not from a page. Small lots have a sizing rule to clear first.
If it is a septic tank, find out when it was last pumped. If nobody knows, get it pumped and measured once, and the interval stops being a guess.
If you cannot tell, call (808) 630-9966. A septic and cesspool inspection locates the system, opens it, and tells you what you have and what condition it is in — which is the one thing every buyer, seller and owner on this island should know before 2050 gets any closer.