The order that saves money
- Hire an OWTS professional. They choose the system design for your ADU and site, run percolation tests and soil borings, and file the layout application. DEHQ lists OWTS professionals, septic contractors and pumpers on its septic page.
- Find your existing as-built. DEHQ's online document library holds as-built drawings of existing systems.
- Get layout approval. DEHQ field-inspects; the site plan must show both dwellings served by a compliant system and a 100% reserve area for a future replacement field.
- Then the building permit. DEHQ advises waiting for layout approval before applying, because the ADU may not be approvable or the required system "may be much more expensive than you initially expected".
- OWTS installation permit once the layout's terms are met (building and grading plan review, pump designs, well water tests, grading inspection). The layout approval expires after one year.
Tank sizes when the ADU shares a tank
| Main dwelling | ADU | Minimum tank |
|---|---|---|
| 1 bedroom | 1 bedroom | 1,000 gallons |
| 2 bedrooms | 1 bedroom | 1,200 gallons |
| 2 bedrooms | 2 bedrooms | 1,500 gallons |
| 3 bedrooms | 1 bedroom | 1,500 gallons |
| 3 bedrooms | 2 bedrooms | 1,500 gallons |
| 4 bedrooms | 1 bedroom | 1,500 gallons |
From DEHQ's FAQ. Normally each dwelling needs its own tank; these reduced minimums exist to let owners reuse an existing tank. The disposal field is sized separately from soil permeability, soil depth, bedroom count and the area left after setbacks, and DEHQ says an ADU "in almost every case" would exceed the existing field.
What it means when you hire
- Two contracts, in sequence. The OWTS professional and septic contractor come first; the ADU builder second. A builder who wants to submit plans before DEHQ approval is asking you to risk redesign fees.
- Tell both what else you plan. DEHQ says the OWTS professional weighs desired future development such as pools, decks, sheds and solar, along with driveways, grading and easements, when choosing a design; the 100% reserve area also has to be delineated on the plan.
- Check the license. County ADU builders mostly match an active CSLB license (91.5% in our file); look yours up with our San Diego County contractor license lookup.
- Correspondence with DEHQ goes to lwqduty.deh@sdcounty.ca.gov.
What 334 County ADU permits show
| Measure (October 2023 to September 2026) | ADU | Addition |
|---|---|---|
| Permits | 334 | 158 |
| Firms | 213 | 134 |
| Usual filed value | $128,106 | $78,705 |
| Usual days from issue to completion | 351 | 434 |
| Firms matched to an active CSLB license | 91.5% | 96.3% |
ADU permits that reach the County's building file usually finish: 85.1% of those a year or more old were complete, about 351 days after issue. The septic stage happens before that clock, so add it to any schedule a builder gives you. The middle half of ADU builders usually file $110,493 to $160,022; size-by-size values and the County's fee rules are in ADU cost in San Diego County.
Compare San Diego County ADU builders →
Related
Plumbing and sewer permits: San Diego County plumbing permit. Finding a permit: San Diego County building permit search. ADU basics: do I need a permit for an ADU.
What this does not tell you
Our permits come from the County's building records; DEHQ septic layouts and installation permits are a separate file we did not review, so we cannot say how many ADUs needed a new system. The DEHQ FAQ is dated February 2021; the Local Agency Management Program on DEHQ's septic page sets the current rules. This covers unincorporated San Diego County only, not cities such as Escondido or San Diego.
How we know this
Septic steps, tank sizes and approval terms are from the County of San Diego DEHQ's ADU FAQ for properties served by onsite wastewater treatment systems (revised 1 February 2021), read 23 October 2026. Permit counts are County of San Diego building permits in the unincorporated area classified as ADUs and additions, October 2023 to September 2026; completion uses permits a year or more old and license matches use the CSLB file.
Numbers come from San Diego County Planning & Development Services building permits on homes in the unincorporated county (the County's open-data permit, contractor and status-history files), data through October 20, 2026. Nobody paid to be on this page and nobody can.
Questions
- Can I build an ADU on a septic system in San Diego County?
Often, but feasibility is decided by the County's Department of Environmental Health and Quality through an OWTS Layout Application prepared by an OWTS professional. DEHQ inspects the site and, if requirements are met, issues terms of approval. The site plan must show both dwellings served by a compliant system plus a 100% reserve area.
- Can an ADU share my existing septic tank in San Diego County?
Rarely as is. DEHQ says an ADU is a second dwelling and in almost every case would exceed the existing tank and disposal field. A shared tank is possible if it is big enough; for a 3-bedroom house and a 1- or 2-bedroom ADU the minimum is 1,500 gallons.
- Do I need a perc test for an ADU in San Diego County?
Yes. Every layout application must include percolation test data from the locations of all proposed disposal and reserve areas. DEHQ accepts older tests only from a previous approved layout or a recorded Health Department Certification for a system of similar design in the same spot.
- How long is a DEHQ septic layout approval good for?
One year (12 months). The OWTS installation permit is issued once the layout's terms are met, which can include DEHQ review of building and grading plans, pump designs, well water tests and a grading inspection.
- How much does an ADU cost in unincorporated San Diego County?
On the permit, the usual County ADU was filed at $128,106 (334 permits, October 2023 to September 2026). That is construction value and leaves out the septic design, perc testing and installation.
Build your own list
Every firm with that kind of permit on record. Pick what you value. The list moves.
Related guides
- San Francisco Section 311 neighborhood notice: which additions trigger the 30-day notice, and how it changes your schedule
- ADU cost in San Diego County: what the permits were filed at, by size
- ADU vs home addition: what the permit file shows
- Austin HOME initiative: three units on one lot, 1,800 sq ft small lots, and what it means when you hire a builder