Can ADWR or Maricopa County deny a building permit over water in Arizona?

Water can stop a building project in Arizona, but whether ADWR or Maricopa County can deny a permit over water depends entirely on the scale of what you are building. For a subdivision of six or more lots inside an Active Management Area (AMA), which covers the Phoenix metro, the answer is a clear yes. A city, town, or county may approve the subdivision only if the developer first gets an assured water supply certificate from the Arizona Department of Water Resources (ADWR), proving a 100-year supply. No certificate, no approval. For one home on an existing, legal lot, the settled rule is narrower. That single-home build does not have to clear the 100-year subdivision test, though you still must show the county a legal water source. The contested, fast-changing part is the middle ground: parcels created by lot splits that never proved a water supply. That corner of the law is unsettled, the state has moved to tighten it, and the rule that applies can turn on your specific parcel's history. This page separates what is settled from what is still moving, so you know which rules you are actually under.

The settled rule: subdivisions of six or more lots

The settled rule is that a county cannot approve a six-or-more-lot subdivision inside an AMA without an ADWR assured water supply certificate. This is the firm framework, codified for decades. A.R.S. 45-576 requires that "a city, town or county may approve a subdivision plat only if the subdivider has obtained a certificate of assured water supply from the director" or has a written commitment of water service from a provider that holds a Designation of Assured Water Supply. ADWR runs this program under the 1980 Groundwater Management Act.

"Assured water supply" has a strict meaning in the statute: "sufficient groundwater, surface water or effluent of adequate quality will be continuously available to satisfy the water needs of the proposed use for at least one hundred years." So a developer must prove a 100-year supply before lots can be platted and sold. The trigger is a subdivision, which Arizona law treats as land split into six or more lots offered for sale. Outside the AMAs, a parallel adequate water supply program applies with fewer criteria, but the same idea holds: prove the water before you carve up the land.

This is where ADWR's water rules have real teeth. They can and do block a multi-lot development that cannot show a 100-year supply. For a developer, water is a gate that comes before the plat.

The settled rule for a single home

For one home on an existing legal lot, the settled rule does not require a 100-year assured water supply test. The 100-year requirement attaches to subdividing land, not to building a single house on a lot that already exists. When you build one home on one already-legal parcel, you are not creating new lots, so the assured-supply certificate does not apply to you. You do still have to show the county a legal, working water source on your site plan, whether that is a well, a connection to a water provider, or hauled water to a storage tank.

If you drill your own well, the water step is an ADWR filing, not a county water permit. ADWR registers wells at the state level, and "prior to drilling a new well, or deepening or modifying an existing well, a Notice of Intent to Drill must be filed with ADWR." A typical home well qualifies as an exempt well. Under A.R.S. 45-454, an exempt well is one with a pump "having a maximum capacity of not more than thirty-five gallons per minute" used for domestic, non-irrigation purposes. That cap easily covers a single household. For the well process, see our ADWR Notice of Intent to Drill page and the broader well and adequate water supply page for Maricopa County.

So a single home on a clean, long-standing legal lot is a routine build from a water standpoint. The heavy water-supply paperwork is aimed at developers, not one family.

What is contested and evolving

The contested ground is parcels created by lot splits that never proved a water supply, and the rules there are changing. Arizona law has long let an owner split a parcel into five or fewer lots without formal subdivision review. Because that stays under the six-lot subdivision trigger, land divided this way historically skipped the 100-year water test. People often call this a lot split or a "wildcat" split. It is exactly how some rural areas, including parts of the Rio Verde Foothills in unincorporated Maricopa County, grew without a locked-in water supply. For how those splits work, see our wildcat lot split page.

This is the part the Legislature and ADWR are actively reworking, so it is not settled. A.R.S. 45-576 itself shows the open question. The statute directs that "the director shall study and submit" to state leaders "a report on whether and how a person that seeks a building permit for six or more residences within an active management area" should have to "apply for and obtain a certificate of assured water supply." In plain terms, whether and how the assured-supply requirement should reach individual building permits, not just plats, was set up for study, not locked into final form. That is the heart of the contested area.

What this means for you:

• The framework for subdivisions is settled. Six or more lots in an AMA need a certificate. That is not in flux.
• The treatment of split parcels is moving. Parcels carved by splits that dodged water review can draw extra scrutiny, and the rules are being tightened, so what applied a few years ago may not apply now.
• The single-home rule on a clean lot is settled. One home on a long-standing legal lot does not face the 100-year test, though it must still show a legal water source.

We are stating only what the statutes and ADWR say. Where the rule for a split parcel is genuinely unsettled, the honest answer is that it depends on current ADWR policy and your parcel's history, which you must verify.

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, but the county itself does not run the 100-year water test. Much of the rural county, including Rio Verde Foothills, is unincorporated, so Maricopa County is your building department. The county frames the permit as "your reassurance that your building plans, as well as the structure you build, will be inspected for minimum code safety." Three water-related items show up in that process:

• Your site plan must show the water source. The county wants to see your well, storage tank, or provider connection on the plan, along with the septic system, all within required setbacks.
• The well filing runs on its own track. Your ADWR Notice of Intent to Drill is a state step, separate from the county building permit.
• Split history can trigger questions. A parcel from a recent split, especially one that skipped water review, can draw extra review from ADWR and the county before a permit issues.

The bottom-line answer is that ADWR water rules can block a six-or-more-lot subdivision in an AMA, a single home on a clean legal lot is not held to that 100-year test, and the rules for split parcels are contested and being tightened. Because this corner of Arizona water law is changing and the answer can turn on your exact parcel, confirm the current water rule and your parcel's split history with ADWR and Maricopa County for your specific lot before you buy or commit (as of June 2026). For the rural build checklist, see our building on rural land in Rio Verde page.

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
• A.R.S. 45-576 (Certificates of assured water supply; building permits; study; definition)
• A.R.S. 45-454 (Exemption of small non-irrigation wells; definitions)
• Maricopa County: Construction Permit Information
• Arizona Department of Water Resources: Well Drilling in Arizona