How will Maui County’s new water-demand rules affect property owners, sellers, and future building plans?

Beginning January 1, 2026, Maui County will use a daily water-demand model based on Equivalent Single Dwellings (ESDs) to project water use and size meters, replacing the long-standing fixture-based Hunter Curve Method for new applications.

Why Maui County Is Changing How Water Demand Is Measured

Water planning has long been one of Maui’s most sensitive and complex infrastructure challenges. Limited supply, aging systems, and the need for responsible long-term management have driven the County of Maui to reevaluate how water demand is calculated for new development and expanded use.

In response, the Maui County Council adopted Ordinance 5759, directing the Department of Water Supply (DWS) to move away from the fixture-based Hunter Curve Method. That older approach estimated demand based on the number of plumbing fixtures in a building, such as toilets, sinks, and showers. While widely used, it often failed to reflect how water is actually consumed in real households.

Starting January 1, 2026, DWS will instead rely on a daily water-use model that estimates demand based on occupancy patterns and typical household activities. This shift is intended to improve accuracy, consistency, and long-range planning without increasing Maui’s overall water supply.

The new method applies to building permit applications and applications for new or additional water use submitted on or after January 1, 2026.

Understanding the Equivalent Single Dwelling (ESD) Standard

At the center of the updated methodology is the Equivalent Single Dwelling (ESD), a standardized unit that represents daily water demand rather than fixture count.

Under the new rules:

• One ESD equals 300 gallons of water per day

• Each water service receives an additional 0.5 ESD to account for outdoor water use, such as irrigation
  
  
  

DWS developed this standard by analyzing empirical local meter data, per-capita water usage, and national daily-use averages. Typical residential activities—laundry, bathing, dishwashing, cooking, and general consumption—were evaluated across different home configurations.

Daily water demand is estimated based on the number of bedrooms and bathrooms in a structure. Any room that is, or could reasonably be, used for sleeping is counted as a bedroom. The 300-gallon-per-day benchmark was established using a four-bedroom, two-bathroom home as a reference model.

This approach allows DWS to translate different property types and layouts into a consistent, measurable unit for permitting and infrastructure planning.

What the New Rules Mean for Single-Family Homes

For single-family residences, the ESD methodology is paired with standardized meter capacity limits designed to simplify review and approval.

Under the updated standards:

• 5/8-inch meter: up to 2.5 ESDs

• 3/4-inch meter: up to 3.5 ESDs

• 1-inch meter: up to 6 ESDs

These thresholds connect a home’s size and layout directly to meter sizing under the daily-use model. While the ESD method may identify unused capacity under the 300-gallon-per-day standard, it does not increase Maui’s total available water.

For homeowners considering renovations, additions, or future resale, understanding how bedrooms, bathrooms, and potential sleeping spaces are counted can be an important planning consideration.

How Multi-Family and Non-Residential Properties Are Evaluated

Multi-family projects use the same bedroom-and-bathroom methodology to estimate indoor water demand, with outdoor water use calculated separately. However, because water-use patterns differ from single-family homes, meter sizing relies on modeling rather than fixed fixture counts.

DWS uses the International Association of Plumbing and Mechanical Officials (IAPMO) Water Demand Calculator (WDC) to model multi-family demand. Based on project modeling, standard meter capacities have been established:

• 1-inch meter: up to 16 ESDs

• 1.5-inch meter: up to 60 ESDs

• 2-inch meter: up to 120 ESDs

• 3-inch meter: up to 240 ESDs

Developers may rely on these standard tables or submit custom WDC calculations.

For non-residential properties, water demand must be calculated by a licensed professional. Applications require both indoor use estimates and detailed landscape irrigation calculations, including plant types, irrigated area, and anticipated usage. Tools such as the EPA WaterSense Water Budget Calculator may be used to assist with outdoor estimates. All data is ultimately converted into ESDs for permitting and meter sizing.

Timing, Transition Rules, and Existing Water Entitlements

Applications submitted on or before December 31, 2025, will continue to be evaluated under the current fixture-based Hunter Curve Method. Applications submitted on or after January 1, 2026, will use the new ESD-based daily demand methodology.

Existing multi-family and non-residential customers will retain their current ESD entitlements based on either their approved use or the FY2026 source fee equivalent of $2,316 per ESD, whichever is greater. DWS has indicated it will not reduce existing entitlements or require reconciliation where prior fees exceed calculated entitlements.

Updated guidance materials, including the ESD Methodology and a Materials Submission Checklist, will be provided through the DWS Engineering Division.

Frequently Asked Questions

Does the new ESD method create more water capacity for Maui? ▼

No. The methodology changes how water demand is measured and managed, but it does not increase Maui's water supply.

Who should pay attention to these changes now? ▼

Homeowners, sellers, buyers, developers, and property owners planning renovations, new construction, or future water-use applications should be aware of the new standards.

When do the new rules take effect? ▼

The ESD-based water-demand methodology applies to applications submitted on or after January 1, 2026.