Not for a conventional system. Maricopa County says a conventional septic system may be designed by the homeowner or anyone the owner authorizes, if it meets the state rules and county guidelines. Alternative systems such as mounds, drip or aerobic units must be designed and sealed by an Arizona-registered Professional Engineer.
Conventional systems: the owner can design them
The county's FAQ asks the question outright: "Do I have to hire an engineer to design my system and draft my plans?" Its answer: "No. Only an alternative system requires an engineer to properly design, select and lay out the onsite system." Conventional systems "may be designed by the homeowner or any other party authorized by the owner provided all requirements listed in the Arizona Aquifer Protection Permit Rule and the Maricopa County Onsite Wastewater guidelines are observed and satisfied."
So the owner, a designer or an installer the owner authorizes can draw the plans. What they draw still has to fit the state's rule for this system. The 4.02 General Permit in A.A.C. R18-9-E302 covers a system "consisting of a septic tank dispensing wastewater to an approved means of disposal" with a design flow under 3,000 gallons per day, and it says "Only gravity flow of wastewater from the septic tank to the disposal works is authorized by this general permit." The standard design is meant for sites where the site investigation finds no limitations. If the soil, slope or groundwater on the lot is a problem, a standard design may not fit, and that is when an alternative design comes into play.
Alternative systems: a sealed Professional Engineer design
The county defines an alternative system as "any type of septic system other than a septic tank using gravity to convey effluent for disposal into trenches, beds, chambers, or pits." Its typical examples are aerobic system, cap system, sand lined trench, intermittent sand filter, Wisconsin mound, composting toilet, peat filter, textile filter, pressure distribution, subsurface drip irrigation, and Ruck system.
For all of those, the county's wording is: "Alternative OSWTFs must be designed by and stamped with the seal of a Professional Engineer (P.E.), registered to work in the state of Arizona and with expertise in the on-site wastewater field." (OSWTF is the county's abbreviation for an onsite wastewater treatment facility.) Which components can go into an alternative system comes from an approved list kept by the Arizona Department of Environmental Quality, and the county notes it "may have additional requirements or restrictions."
The engineer stays involved after the plans are approved. For alternative systems, the state rule requires a "Certificate of Completion signed by the current engineer or designer of record assuring that installation of the facility conforms to the design approved under the Construction Authorization." It adds that an inspector may not authorize backfill before that engineer or designer "has verified proper installation of the system."
Not needing an engineer is not the same as skipping review
Every new system goes through the county. Its homeowner guide lists the application types: Phase 1 applications for properties that need a site and soils inspection for the design, Phase 2 Notice of Intent to Discharge applications for proposed new conventional or alternative systems, alteration applications, minor review applications for additions and remodels, and abandonment applications. The county reviews the design whoever draws it, and the system cannot discharge until it is approved (see what a septic Discharge Authorization is in Maricopa County).
Two points trip up owner-designers:
- Changes after approval cost time. The county FAQ says "Any proposed changes to the septic system that occur after permit issuance will require an additional plan review," that "Additional fees will apply," and that undocumented changes noticed during the construction inspection lead to delays and potentially more fees.
- The house sets the demand. The guide says any increase in plumbing fixtures or bedroom equivalents "can increase the system demand and require system review by MCESD." A design drawn for a two-bedroom casita doesn't quietly grow into a three.
Which way should you go?
- Level lot, workable soils, standard gravity layout: an owner-drawn conventional design is allowed. If you are not confident about sizing and soil testing, hiring a designer is still your call. The county doesn't require one.
- Poor soils, high groundwater, tight setbacks, or a system type that isn't gravity: budget for an Arizona P.E. with onsite wastewater experience before you submit anything.
- Not sure which category you fall into: start with the county's Phase 1 application, which exists for properties that need a site and soils inspection for the design.
The house-side plumbing that feeds the tank is a separate piece of work. HQ is licensed and insured, with free estimates and upfront pricing, for residential plumbing. To see what drives the install price, read septic system cost. If you are comparing a septic system to a connection, septic vs sewer lays out the trade-offs. This page is general information, so confirm current requirements with Maricopa County Environmental Services before you buy equipment.