# Can Your Private Well Support an ADU? A Four-Gate Evidence Check

Source: https://brictale.com/water/wells/can-my-private-well-support-an-adu
Published: 2026-09-06
Audience: Homeowner
Published by Brictale, a consumer home-intelligence publication. https://brictale.com

## Short answer

A private well can support an ADU only when the local authority accepts the source and the combined homes have documented potable water, adequate quantity, recovery, pressure, and storage. There is no national pass/fail number. Use the local test duration and threshold, then mark each gate proved, missing, or needs review before pricing the design.

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An existing private well may be able to serve a new accessory dwelling unit (ADU), but a familiar well log or a strong flow at one faucet is not enough to answer the question. You need to show that the proposed two-home water system clears four different gates: the local approval rule, potable-water evidence, measured quantity and recovery, and pressure/storage under the combined demand.

Use this worksheet before paying for a final ADU water design. Its job is to identify evidence, not to invent a national well-yield threshold. A result of **proved** means you have a dated record that matches the authority's requirement. **Missing** means the question cannot yet be decided. **Needs review** means the evidence exists but a health department, licensed well professional, engineer, or other authority must interpret it.

{{visual:four-gates-adu-well-map}}

## Safety boundary: observe and collect records only

This is a non-diagnostic planning worksheet. Homeowner work is limited to safe observation from outside the equipment area, reading an existing gauge without touching controls, photographing labels, collecting existing records, and arranging laboratory or professional testing. Do not open energized electrical controls, test live wiring, manipulate pressurized pump or pressure equipment, or change a valve, pressure-switch setting, precharge, control, or treatment setting. Do not open, enter, or work inside a well, well pit, tank, or other confined space. Do not remove a well cap, pull a pump, open a pressure vessel, or work on wet or damaged equipment. Send those actions to a qualified well contractor, water-system professional, engineer, or licensed electrician as applicable. Stop and ask for professional help whenever a component is energized, pressurized, leaking, wet, damaged, or unclear.

## 1. Start with the local approval gate

The first question is not “How many gallons per minute does my well make?” It is “What does the authority that will review this ADU require me to submit?” Private-well and ADU rules are local. A county may require a well-site review, an accepted water-supply application, a well log, a particular pump-test duration, laboratory analyses, a covenant, or a professional submittal. Another jurisdiction may use different documents or thresholds.

For example, Snohomish County, Washington says an ADU building permit cannot be issued until the health department verifies adequate and potable water. For an existing private well, its page lists an approved well-site inspection, an accepted individual-water-supply review, and a covenant for a two-connection well. It also says the review must document at least 400 gallons per day per residential connection, or 800 gallons total, and requires a well log/report and laboratory results. Those are Snohomish County requirements, not a national rule. Read the [Snohomish County Health Department building-permit requirements](https://www.snohd.org/709/Drinking-Water-Requirements-for-Building) for that local process.

King County, Washington gives a different example. Its guidance says an individual private well serving a single-family residence and up to one ADU on the same property must apply through a certified on-site sewage designer or water-system engineer. The county reviews site, quality, quantity, pump-test, and supporting documentation before the local services department decides whether the source is adequate and legal. See the [King County private-well review guidance](https://kingcounty.gov/so-so/dept/dph/health-safety/environmental-health/healthy-water-air-soil/drinking-water/permitting-process/creating-replacing-water-system).

Record the following before you compare your well with any generic internet advice:

| Local-review input | What to record | Status |
| --- | --- | --- |
| Reviewing authority | County, city, state, tribal, or other office; contact and page URL | Proved / missing |
| ADU arrangement | One parcel, two dwelling units, shared or separate connections | Proved / needs review |
| Required source documents | Well log, site review, water-supply form, covenant, septic approval | Proved / missing |
| Required quantity test | Test duration, method, season, flow or daily-yield threshold | Proved / missing |
| Required quality test | Analytes, laboratory certification, sample timing, retest rules | Proved / missing |
| Professional sign-off | Designer, engineer, well contractor, or water professional | Proved / needs review |

Do not fill the threshold column with a number copied from another county. If the authority has not supplied a duration or threshold, mark it “not yet confirmed” and ask the permitting office what it accepts for an existing well serving an ADU. A completed local-rule record is the first gate; it does not prove that the well passes.

{{visual:local-rule-evidence-stack}}

## 2. Gate two: prove the water is potable for the proposed use

Quantity and potability are separate questions. A well can recover well and still need treatment or fail a local drinking-water review. Start with the authority's analyte list and sampling instructions. Then compare the report date, collection method, laboratory credentials, and results with those instructions.

EPA recommends annual testing of a private well for total coliform bacteria, nitrates, total dissolved solids, and pH. It also recommends testing promptly after flooding, nearby land disturbance or new construction, repair or replacement of part of the well system, or a noticeable change in odor, color, taste, or other water quality. EPA says to use laboratories certified for drinking-water testing. Use the [EPA private-well testing guidance](https://www.epa.gov/privatewells/protect-your-homes-water) as a maintenance baseline, then follow the local ADU review's list if it is stricter or different.

Some local reviews set specific limits. King County's individual-well guidance lists two satisfactory bacteriological analyses, a nitrate maximum contaminant level of 10 mg/L (10 ppm), and an arsenic maximum contaminant level of 0.01 mg/L (10 ppb). It says samples should be collected and submitted to certified laboratories by a water professional. Those values describe that county's listed review path; they are not permission to substitute a home test strip or to treat a different jurisdiction's rule as identical.

For the worksheet, capture each result rather than writing only “water tested.” Include the sample date, well or tap sampled, laboratory name and certification, analyte, result and units, reporting limit if shown, and the local acceptance status. If a treatment system is installed, record whether the authority requires a post-treatment sample and who must design or verify the treatment. Treatment may address a quality result, but it does not increase well yield or prove the source can serve two homes.

Use this evidence checklist:

- [ ] The report is recent enough for the local review and any event-triggered retest rule.
- [ ] The sample was collected from the location and by the method the authority accepts.
- [ ] The laboratory is certified for drinking-water testing.
- [ ] Every locally required analyte is listed with a result and units.
- [ ] Any exceedance has a written response from the health department or qualified water professional.
- [ ] Treatment, if proposed, has a design, maintenance plan, and post-treatment verification path.

Stop the ADU water decision at this gate if the water has not been tested after a relevant change, if the report is not traceable to a certified lab, or if a result exceeds the applicable health or permit requirement. Do not solve an uncertain result by assuming that boiling, a filter cartridge, or a larger tank makes the water acceptable. Ask the local health authority what action and retesting it requires.

## 3. Gate three: measure quantity, drawdown, and recovery

“Yield” is not the same as the amount of water you happened to use yesterday. The Alaska Department of Environmental Conservation says a well-yield test, also called a pump or production test, provides drawdown, aquifer recovery, and a sustainable production rate for judging the well's long-term capacity. Its private-water-system guidance also says representative residential peak demand is concentrated in morning and evening periods, so total daily volume alone does not prove that a system can deliver water when both homes use it. Read the [Alaska DEC private-water-system guidance](https://dec.alaska.gov/media/8481/private-water-systems-bmp.pdf) for this distinction. Its examples are planning guidance, not your authority's ADU threshold.

A pump-test record is useful only when you know what was measured and whether it matches the local rule. Record the static water level before pumping, the pumping rate, the water level during pumping, the lowest observed level, the pump intake or shutoff protection level, the test duration, and the water level during recovery after pumping stops. Also record the date, season, weather or drought context if known, test method, equipment, and person who performed or witnessed it.

The local test can be more specific than general extension guidance. King County lists a two-hour pump test for the individual-well review and says that a result of 5 gallons per minute or less triggers a four-hour test plus proof that the well produces at least 400 gallons per day. It also warns that low-production areas may require testing during seasonally low groundwater periods. Treat those details as a King County example of how a rule can branch; copy your own authority's duration, method, and threshold into the worksheet instead.

{{visual:pump-test-recovery-profile}}

Use the record to answer four questions:

1. What rate was actually sustained for the authority's required duration?
2. Did the water level remain above the pump's safe operating limit?
3. After pumping stopped, did the water level recover, and over what measured time?
4. Does the test represent the dry or low-groundwater condition that the authority cares about?

Do not turn a single faucet-flow reading into a pump-test result. Do not infer recovery from the fact that the pump eventually shut off. If the old record has no test duration, no water-level data, no method, or no connection to the current pump and well, mark quantity as missing or needs review. A controlled test may be appropriate, but the test procedure and discharge plan should come from the reviewing authority and a qualified well professional.

## 4. Gate four: check two-home peak demand and storage

Even when the source has an acceptable measured yield, the distribution system must deliver water when both residences use it. List the proposed occupancy, bedrooms, bathrooms, fixtures, appliances, irrigation, outdoor taps, and any use that could overlap. Pay special attention to morning and evening periods, simultaneous showers, laundry, toilets refilling, treatment backwash, and irrigation. Separate existing-home demand from new ADU demand so that an apparently comfortable current system is not treated as proof of future capacity.

As Brictale planning guidance, first model whether nonessential or overlapping uses can be shifted or reduced; that is not an Alaska DEC claim or a permit strategy. Alaska DEC's actual alternatives are conditional: if the well can meet peak demand, the pump is sized for peak demand; if the well can meet total demand but not peak demand, equalizing storage is needed; and if it cannot meet total demand, a supplemental water source may be needed. DEC describes equalizing storage as an atmospheric storage tank with a booster pump, with the well pump refilling the tank as its level falls. Those are system-planning options, not a permit approval. Its guidance says a pressure tank should match the well pump and uses about 25 percent effective storage as a general sizing rule of thumb. That is not a universal drawdown value: the actual reserve depends on the tank, pressure-switch settings, precharge, pump, and system design. A larger pressure tank may add some reserve but may not solve a low-yield problem. See the [Alaska DEC pressure-tank and storage guidance](https://dec.alaska.gov/media/8481/private-water-systems-bmp.pdf).

Equalizing storage is a different arrangement: the well pump fills an atmospheric storage tank as its level falls, and a booster pump supplies the peak demand. Alaska DEC describes this as an option when the well can meet total demand but not the peak rate. Storage, controls, low-water protection, freezing protection, treatment, and pump sizing are system-design matters. Do not select a tank size, adjust a pressure switch, or rewire a pump from a generic chart; have the authority-required professional design and review it.

{{visual:two-home-pressure-storage-cutaway}}

The basic system record should include:

| Capacity question | Evidence to collect | What it proves |
| --- | --- | --- |
| Existing source flow | Accepted pump-test rate and duration | What the well sustained under that test |
| Recovery | Water-level readings after pumping | How the source replenished after demand |
| Pressure | Static and operating pressure, gauge location, cut-in/cut-out settings | Whether distribution pressure is documented |
| Usable tank reserve | Tank model, rated volume, pressure settings, measured or manufacturer drawdown | The reserve available before the pump cycles |
| Peak demand | Fixture list, flow measurements, overlap assumptions, irrigation and treatment loads | What the two homes could request at once |
| Added storage | Tank type, usable volume, controls, refill rate, low-water protection | Whether storage can bridge the documented gap |

Nebraska Extension describes a typical private-well distribution system as a pump, motor, pressure storage tank, and control device such as a pressure switch. That separation matters: a well's source production and the house's pressure equipment are different parts of the system. Use the [University of Nebraska–Lincoln private-well system description](https://extensionpubs.unl.edu/publication/g2153/private-drinking-water-wells) when assembling the equipment record.

## 5. Fill the four-gate homeowner worksheet

Make one row for every decision input and attach its source. The source can be a local rule page, an official form, a well log, a pump-test sheet, a certified laboratory report, a pressure-gauge measurement, a tank label, or a professional design note. A web search result, an unverified seller claim, or memory of a neighbor's permit is not a substitute for the record the authority will accept.

| Gate | Input | Your value | Source/date | Status and next action |
| --- | --- | --- | --- | --- |
| Approval | Reviewing authority and current ADU page |  |  |  |
| Approval | Required test duration, method, and threshold |  |  |  |
| Approval | Required forms, covenant, and professional sign-off |  |  |  |
| Potable water | Required analytes and certified lab |  |  |  |
| Potable water | Results, units, and retest or treatment condition |  |  |  |
| Quantity | Static level, pumping rate, duration, lowest level |  |  |  |
| Recovery | Recovery readings and low-season relevance |  |  |  |
| Distribution | Pressure readings and switch settings |  |  |  |
| Distribution | Tank model, nominal volume, usable drawdown |  |  |  |
| Demand | Existing occupancy, ADU occupancy, fixtures, overlap |  |  |  |
| Storage | Added usable volume, refill rate, controls, protection |  |  |  |

For every blank, write a specific request. “Need pump test” is better than a blank, but “ask the county whether an existing-well ADU requires a four-hour dry-season test and who may perform it” is better still. For a water-quality blank, name the analyte and sample instruction. For a distribution blank, identify whether you need a pressure reading, a tank model, a flow measurement, or an engineer's review.

Keep the four gates separate when you summarize the result. A local office may accept the paperwork while the pump test remains incomplete. A certified laboratory report may prove potability while the two-home peak demand remains unmodeled. A strong test result may show quantity while pressure storage is undocumented. Do not average the gates into a single confidence score.

## 6. Interpret the worksheet without a made-up pass/fail number

Use this decision language:

- **Documented path:** the local authority, quality evidence, quantity/recovery record, and distribution/demand review are all present, and the authority or qualified designer confirms that the proposed system meets the applicable rule.
- **Evidence-ready but not approved:** the records are complete enough to submit, but the authority has not yet accepted them. This is the normal status before formal review.
- **Needs a controlled test or design review:** one or more inputs are missing, the test does not match the local procedure, recovery is unclear, simultaneous demand is not accounted for, or pressure/storage equipment is undocumented.
- **Do not price a final water plan yet:** a required local gate is unknown, the source is not shown to be potable, a test shows an unresolved quantity problem, or a proposed treatment/storage idea has no professional design path.

Only the applicable authority can make the permit decision. This worksheet can show that you have a transparent evidence trail and can prevent an expensive design conversation from starting with the wrong assumption. It cannot approve a connection, waive a covenant, certify a laboratory result, or establish a safe pump setting.

## 7. Decide what to request next

If all four gates are proved and the records match the local process, send the package to the reviewing authority or the qualified designer the authority requires. Ask for written confirmation that the proposed ADU arrangement, test records, water-quality results, and any covenant or treatment plan are acceptable before ordering equipment.

If approval is unclear, request the current ADU/private-well checklist and ask which office owns each decision. If potability is missing, use a certified drinking-water laboratory and follow the local sampling instructions. If quantity or recovery is missing, ask for the accepted pump-test protocol, duration, season, discharge method, and reporting format. If pressure, storage, or peak demand is missing, have a qualified well or water-system professional evaluate the existing equipment and proposed two-home demand.

{{visual:evidence-status-comparison}}

If the well fails an applicable requirement, pause the ADU design. The next path might be treatment, demand reduction, added storage, a new well, public water, or another option permitted by the jurisdiction, but the right choice depends on the failed gate and the authority's rules. EPA advises contacting the public health department when a private-well sample exceeds a health standard, and it notes that some conditions may require a new source or new well. Read [EPA's guidance on test results and treatment](https://www.epa.gov/privatewells/protect-your-homes-water) before relying on a treatment claim.

The useful answer to “Can my private well support an ADU?” is therefore a documented path, not a generic number: **local rule identified, potable water evidenced, quantity and recovery measured, and pressure/storage checked against the combined two-home demand.** When one gate is missing, the honest result is “next test or design review,” not “yes” and not “no.”

## Evidence

- [Drinking Water Requirements for Building Permits](https://www.snohd.org/709/Drinking-Water-Requirements-for-Building) — Snohomish County, Washington; applies to that county's ADU building-permit review and is not a national rule.
- [Drinking Water Requirements for Building Permits](https://www.snohd.org/709/Drinking-Water-Requirements-for-Building) — Snohomish County, Washington existing private wells; the numeric yield requirement must not be generalized to another jurisdiction.
- [Creating or replacing a water system](https://kingcounty.gov/so-so/dept/dph/health-safety/environmental-health/healthy-water-air-soil/drinking-water/permitting-process/creating-replacing-water-system) — King County, Washington; local review criteria for an individual private well serving a single-family residence and up to one ADU on the same property.
- [Creating or replacing a water system](https://kingcounty.gov/so-so/dept/dph/health-safety/environmental-health/healthy-water-air-soil/drinking-water/permitting-process/creating-replacing-water-system) — King County, Washington local review page; values and sampling process are not a universal ADU rule.
- [Best Management Practices for Private Water Systems](https://dec.alaska.gov/media/8481/private-water-systems-bmp.pdf) — Alaska Department of Environmental Conservation guidance for private water-system planning; its demand examples and professional-test recommendation are not an ADU permitting threshold or a national rule.
- [Best Management Practices for Private Water Systems](https://dec.alaska.gov/media/8481/private-water-systems-bmp.pdf) — Alaska Department of Environmental Conservation private-water-system sizing guidance; the 25 percent figure is a general rule of thumb in that document, not a universal equipment value or ADU approval criterion.
- [Best Management Practices for Private Water Systems](https://dec.alaska.gov/media/8481/private-water-systems-bmp.pdf) — Alaska Department of Environmental Conservation private-water-system planning guidance; storage, controls, treatment, freezing protection, pump sizing, and code approvals require project-specific professional design.
- [Protect Your Home's Water](https://www.epa.gov/privatewells/protect-your-homes-water) — EPA consumer guidance for private wells in the United States; local permitting programs can require a different analyte list, sampling method, or timing.
- [Private Drinking Water Wells](https://extensionpubs.unl.edu/publication/g2153/private-drinking-water-wells) — University of Nebraska-Lincoln Extension general private-well system description; it is not a design approval or ADU capacity determination.