Do old oil wells change what you can build in Placentia?

You’re considering a Placentia property, or planning a building, and an old oil well comes up. It can change where a proposed foundation goes, what investigation is required and which gas control measures may be required. An abandoned well does not automatically settle whether construction can proceed.

The Placentia Fire & Life Safety Department (PFLSD) addresses proposed construction and occupancy in its Combustible Soil Gas Hazard Mitigation Guideline. It is a department guideline, not a municipal code section. It requires investigation under a Registered Professional: a California Registered Professional Engineer, Registered Professional Geologist or other credentialed professional acceptable to PFLSD.

This is general information and not legal advice.

Which locations fall within the guideline?

The guideline applies to proposed buildings in specified areas around oil fields, wells, gas seepage zones and landfills, plus other locations PFLSD identifies as subject to gas migration.

Its scope includes any location inside a CalGEM oil or gas field administrative boundary or within 100 feet beyond it. Outside an oil field boundary, it includes locations 100 feet or less from an active or abandoned oil or gas well.

Other triggers reach farther: 300 feet or less from a gas seepage zone, and 1,000 feet or less from the refuse footprint of an operating or closed disposal site or landfill.

There is an exception for a “Hydrocarbon Free Oil/Gas Well” when complete surface-to-total-depth data has gone to CalGEM for review and certification, and that certification is provided to PFLSD. Ask the department which scope provision applies to your proposed building.

What does the 10-foot restriction mean?

The guideline says that, to the maximum extent feasible, a proposed slab or foundation shall not be constructed over or within 10 feet of an abandoned oil or gas well.

That is the Building Restriction Zone. The guideline defines an abandoned well as one plugged and abandoned to CalGEM standards. It still sets the 10-foot zone and venting requirements for abandoned wells.

If avoiding the zone is not feasible, construction may be allowed with mitigation. Requirements include a methane work plan prepared by a Registered Professional, a soil gas report within a 25-foot radius of any abandoned well in the zone, mandatory mitigation procedures and professional review. The well abandonment must have current CalGEM approval.

Proposed one- or two-family dwellings within the zone remain subject to further evaluation and/or mitigation. The department also states plainly that it advises against constructing any structure over any well.

How do you check for a well?

CalGEM’s Well Finder maps known wells using a property address or APN, but it does not capture every well.

A blank map does not establish that a property has no buried well. The CalGEM Construction Site Well Review FAQ explains that wells abandoned many years ago can be below the surface, with nothing visible above them. It encourages owners checking for unmapped wells to consult a qualified petroleum professional.

Listed detection methods include land survey techniques, magnetometer studies and ground penetrating radar studies. CalGEM also assists local permitting agencies by identifying known wells through Construction Site Well Review. A review may be triggered when the permitting agency believes wells could be on the property.

For your project, ask PFLSD and the Building division what well location information they require.

What must happen before a proposed building is approved?

A proposed building within the guideline’s specified areas requires a completed soil gas investigation and a report submitted to PFLSD for review and approval.

The investigation must be conducted under a Registered Professional. The department may require third-party review, and the applicant must pay the fees charged for that review. Finding a well on a map does not replace the required investigation and report.

Keep the report, proposed foundation location and well location together when asking what the department requires.

We were helping a Placentia owner consider an addition when an old oil-well record appeared during the property research. He believed the well no longer mattered because it had been abandoned years earlier. We discussed why the location, abandonment record and possible soil-gas requirements still needed to be checked before he finalized the building plans.

When do venting and other gas controls apply?

Distance from a well can trigger mitigation regardless of the measured gas concentration.

All abandoned oil or gas wells within 25 feet of a proposed building shall be vented. The guideline also covers certain wells within 300 feet when they sit under or within five feet of a paved road, lot or other continuous impermeable surface within 25 feet of the building. PFLSD, with CalGEM concurrence, may waive venting upon findings that it is not feasible, with additional mitigation.

Sub-slab passive venting is required beneath a proposed building within 25 feet of an abandoned well, and within 300 feet of an active gas seep zone. Its design must be stamped by a California Registered Professional Engineer. A gas membrane barrier is required for a building within 10 feet of an abandoned well and may be required in other listed locations.

Concentration adds separate triggers. At 5,000 ppm or greater at any location, mandatory mitigation applies to all buildings within 300 feet. Above 12,500 ppm, all buildings within 300 feet need a specific soil gas mitigation plan approved by a Registered Professional.

What paperwork and maintenance follow construction?

Final abandonment documentation, professional certification and ongoing owner maintenance remain part of the guideline’s requirements.

For wells being abandoned or re-abandoned, CalGEM’s requirements current when the mitigation plan is submitted apply. Final CalGEM abandonment approval must reach both PFLSD and the building department before occupancy is approved.

A Registered Professional Engineer must inspect all gas control measures and submit signed certification before building use or PFLSD final approval. The certification includes that testing found the building free from concentrations of the gases the control system was designed to mitigate.

Maintenance stays with the property owner. Owners receive written maintenance and testing instructions and must hire qualified personnel for specialized testing. Written testing documentation must be retained for at least five years and made available to PFLSD upon request.

PFLSD may require additional tests when there is reason to believe gas concentration is elevated. Ask for the maintenance instructions and retained testing records when reviewing an installed system.

What should buyers and sellers ask about their specific project?

Ask PFLSD and the Building division how the guideline applies to your actual proposal.

The guideline speaks to proposed buildings and their construction and occupancy. It does not say whether it applies to an existing house sale, a remodel or an ADU, so ask the department.

If you are preparing a sale, use the California seller disclosure guide for general disclosure paperwork questions; the Placentia guideline itself does not mention seller disclosure. When your purchase also involves HOA documents, use the HOA disclosure guide to organize that document review alongside your questions about the proposed construction.

If your project raises questions about street work, consult the Placentia street rehabilitation guide when preparing questions for the city. The soil gas guideline requires prior approval from the City/County Engineer or Public Works before mitigation systems can be installed in the public right of way.

Ask separately about building location relative to an active well. The guideline expressly leaves out the California Fire Code requirements governing that issue.

The short version

Old oil wells can change a proposed building’s location, investigation requirements and gas control design in Placentia.

Start with the well location and the proposed foundation. Ask PFLSD and the Building division which distance triggers apply, what report must be submitted and what approvals are required before occupancy. If gas controls are installed, obtain the written maintenance instructions and keep the required testing documentation.

As you plan your next step, you can also check your home’s value with the home value tool.