Do you need a well permit or adequate water supply determination to build in Maricopa County?

Building a single custom home in Maricopa County does not require a city-style well permit or a subdivision-level water supply determination. What you do need depends on your water source. If you drill your own well, you file a Notice of Intent to Drill with the Arizona Department of Water Resources (ADWR). A household well qualifies as an exempt well, meaning a pump rated 35 gallons per minute or less. A formal 100-year Assured Water Supply determination is a subdivision requirement. It is triggered when land is split into six or more lots inside an Active Management Area, which covers the Phoenix metro. One home on an already-legal lot does not have to clear that 100-year test. The county still permits and inspects your house, and your site plan must show a legal water source. But the heavy water-supply paperwork is aimed at developers carving up land, not at one family building one home.

Here is what each requirement is, when it applies, and how the rules differ inside and outside an Active Management Area.

The well filing for one home: Notice of Intent to Drill

For one home on its own lot, the water filing you owe is an ADWR Notice of Intent to Drill, not a county well permit. Arizona registers wells at the state level through ADWR. The agency puts the rule plainly: "Prior to drilling a new well, or deepening or modifying an existing well, a Notice of Intent to Drill must be filed with ADWR." You file the form, ADWR reviews it, and only then can a licensed driller go in the ground.

A home well almost always qualifies as an exempt well. Under A.R.S. 45-454, an exempt well has a pump rated at 35 gallons per minute or less used for domestic, non-irrigation use. That cap easily covers a single household, so nearly every rural home well in Maricopa County is exempt. Exempt does not mean unregistered. You still file the notice, still pay the fee, and still use a licensed driller. It only means lighter rules on how much the well can pump.

The fee is small. ADWR's filing fee is $150 in most cases. It drops to $100 when the well sits outside an Active Management Area or Irrigation Non-Expansion Area, serves domestic use only, and pumps 35 gpm or less. For the full step-by-step, see our guide on what an ADWR Notice of Intent to Drill is. The point here is simple. This filing, not a county "well permit," is the water step for a single home that drills its own well.

What an adequate water supply determination actually is

A water supply determination is ADWR's formal finding that an area has enough water for 100 years. It is built for subdivisions, not single homes. ADWR runs two parallel programs under the 1980 Groundwater Management Act. Both test for a 100-year supply. The difference is geography:

• Assured Water Supply applies inside an Active Management Area (AMA), the groundwater-stressed zones that include the Phoenix metro. Inside an AMA, a developer of a subdivision of six or more lots must show a 100-year assured supply to get plat approval and sell lots. ADWR documents this with a Certificate of Assured Water Supply. It can also come from a city or utility that holds a Designation of Assured Water Supply and serves the land.
• Adequate Water Supply applies outside the AMAs. There a subdivider seeks an adequacy determination, and ADWR notes that fewer criteria must be met than for an assured supply.

Both programs, in ADWR's words, "evaluate the availability of a 100-year water supply considering current and committed demand." The trigger is a subdivision: land split into six or more lots, offered for sale. That is the activity the 100-year rule polices. A single home on a lot that already exists is not a subdivision. So it does not need its own assured-supply certificate.

Why a single home usually skips the 100-year test

A single home skips the 100-year determination because the rule attaches to subdividing land, not to building a house. The 100-year requirement is meant to stop developers from selling lots into a community that will run dry. When you buy one existing, legal lot and build one home, you are not creating new lots. So the assured-supply review does not apply to you. You still must show the county a legal, working water source on your site plan. That can be a well, hauled water to a storage tank, or a utility connection. But you do not file an assured-supply certificate.

The wrinkle is lot splits. Arizona law lets an owner split a parcel into five or fewer lots without formal subdivision review. People often call this a lot split or "wildcat" split. Land divided this way historically skipped the 100-year water test. That is exactly how parts of Rio Verde Foothills grew without a locked-in supply. So the split history of your parcel matters. A lot from a clean, long-standing split is usually a simple build. A parcel from a recent split, especially one that dodged water review, can draw extra scrutiny from ADWR and the county before a permit issues.

In 2024, ADWR moved to tighten the gap. It worked to stop new construction tied to splits that never proved a water supply. This corner of the rules is still settling. Confirm your parcel's split history and water status with ADWR and Maricopa County before you buy, so a paperwork surprise does not stall your build.

What Maricopa County checks when you build

Maricopa County checks that your house meets code and that your site plan shows a legal water source and waste system. The county itself does not run the 100-year water test. Rio Verde Foothills and much of the rural county is unincorporated, so Maricopa County is your building department. It reviews your plans to the adopted building code, permits the work, and inspects it. As the county frames a permit, it is your assurance the plans and finished structure "will be inspected for minimum code safety."

Three water-related items show up in that county process:

• Your site plan must show the water source. The county wants to see your well, your storage tank, or your utility connection on the plan, along with the septic system, and they all have to fit within required setbacks on the lot.
• Septic ties to the well. For a domestic well on a parcel of 5 acres or less, the county environmental health office reviews the well-to-septic spacing before drilling, so the well, the septic, and the home have to be placed together.
• The well filing is separate from the building permit. Your ADWR Notice of Intent to Drill is a state step that runs on its own track, parallel to the county building and septic permits.

Sequence these so they do not collide. Many owners start the well and the perc test first, because both take weeks and both shape the home design. For the full rural checklist, see our guide on building on rural land in Rio Verde, and for the broader water options, how to get water to a home in Rio Verde Foothills. Because the adequacy rules and split policies are still changing, confirm the current water requirements for your specific parcel with ADWR and Maricopa County before you commit.

Building with Jematell Homes

We plan water and wastewater early so they never stall your permit. We would rather answer your questions before you build than after, so get in touch any time.

Sources

• Arizona Department of Water Resources: Assured and Adequate Water Supply (AAWS) Overview
• Arizona Department of Water Resources: Well Drilling in Arizona
• A.R.S. 45-454 (Exemption of small non-irrigation wells; definitions)
• Maricopa County Residential Construction permits