Can my septic system handle a casita or addition?

By bedrooms and fixtures. For a single-family home, Arizona's septic tank rule sets the tank's "design liquid capacity" and "daily design flow" from "the number of bedrooms and fixture count." Here's the start of that table:

Fixture count adds up fixture units. Under the same rule, a 1.6 gallon per flush toilet counts as 3, a bathtub or single shower stall as 2, a single bathroom sink as 1, a clothes washer as 2, and a kitchen sink with dishwasher as 2.

The design flow drives the size of the drain field as well as the tank. So when bedrooms or fixtures go up, the flow the system has to handle goes up with them.

### What does a casita do to the numbers?

Here's an example using the table. Say you have a three-bedroom home with 18 fixture units, which puts it at a 1,000 gallon tank and 450 gallons a day. You add a casita with one bedroom, a toilet, a shower, a sink and a kitchenette sink:

• Toilet (3) + shower (2) + bathroom sink (1) + kitchen sink (2) = 8 more fixture units, for 26 total
• One more bedroom, for 4 total

Counted as part of the home, that's the four-bedroom row: a 1,250 gallon minimum tank and 600 gallons a day. The existing 1,000 gallon tank no longer meets the table, and the drain field was sized for less flow.

Even a bathroom alone can tip the balance. Using the same table, that three-bedroom home at 18 fixture units adds a full bathroom with a toilet (3), a bathtub (2) and a sink (1). That's 24 fixture units, over the 21 limit for three bedrooms, so the row becomes a 1,250 gallon tank and 600 gallons a day. No new bedroom was added, but the fixture count moved the home up a row.

This is only an illustration. How a detached casita is counted is the county's call, and none of the county documents we found spells that out. For buildings other than a single-family home, the rule uses a separate flow table and makes the tank "2.1 times the daily design flow." Get the answer for your lot from the county review below, not from arithmetic.

A review before you build. Maricopa County's Environmental Services Department runs a Minor Plan Review, which it says is "also known as a Reconnect/Remodel Review." It uses that review "to determine if the existing septic system is of sufficient capacity to accommodate flow from a new or replacement structure on the parcel or for a modification to the existing structure or to ensure that any structures added to the property meet all setback requirements." Its list of situations that call for one includes "Adding a guest house, pool house or other permanent structure to the property" and "Remodeling of an existing home."

The county's homeowner guide puts it simply: "Any increase in the number of plumbing fixtures or bedroom equivalents can increase the system demand and require system review by MCESD." The county health code backs that up: "No person may install, materially alter, or add to any on-site wastewater treatment facility without first applying for and obtaining a written approval from the Department."

### Will the city hold up my building permit?

It may. The homeowner guide says "Many building jurisdictions will require MCESD septic review and/or approval prior to issuing building permit approval or Certificate of Occupancy." The county's guide for cities describes the "proactive approach" as withholding a building permit "until MCESD has issued a Construction Authorization Certificate or has approved the continued use of the current septic system." So plan for the septic review first, not after framing starts. The county's FAQ also notes that "Environmental Services can only release building permits for unincorporated areas," so inside a city, the city issues the building permit.

If you're not sure whether your home is on septic at all, see how to tell if you have septic or city sewer in Phoenix.

Then it gets upgraded. The county's review instructions say "If, after the Minor Plan Review, MCESD determines the existing septic system is inadequate for the proposed changes, an Alteration to replace either the tank or disposal field or a new NOID application to replace the tank and disposal field may be required." A NOID, or Notice of Intent to Discharge, is the county's application for a new system.

In practice, that can mean:

• A larger tank to match the new bedroom and fixture count
• More drain field to handle the higher daily flow
• A full replacement if the old system is also near the end of its life
• A sewer connection instead, if a city line is close. See septic to sewer conversion in Phoenix.

Setbacks matter too. The review checks that any structure added to the property meets setback requirements, so a casita planned too close to the tank or field can force a redesign. If one part of the system has already failed, the county's FAQ says "an Alteration permit to replace that component will be required."

A few steps save redesigns later:

1. 1Find your septic records. Tank size and the approved design flow tell you where you're starting from.
2. 2Locate the tank and field. Keep the casita footprint, driveway and any new trees off them.
3. 3Count fixtures early. A second bathroom or a full kitchen in the casita raises the count fast.
4. 4Get the tank pumped and checked. An inspection tells you whether the existing system is worth building around. See how long a septic system lasts.
5. 5Apply for the Minor Plan Review before you pull the building permit. Maricopa's septic program is reachable at 602-506-6666.

For what a new or bigger system involves, see what a septic system costs and the difference between a septic tank and a leach field.

HQ Plumbing & Air handles the plumbing for casitas and additions across the Phoenix area, and we're licensed and insured, with free estimates.