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Do I need a soils report for my ADU in LA? When LADBS asks, and what it costs

On a flat lot in the City of LA part of the Valley, most ADUs are permitted without a soils report: when none is submitted, LADBS has the footings built to its expansive-soil rules instead 1. A report comes into play mainly on hillside lots, where a grading pre-inspection decides whether one is needed 2, or when the design uses a post-tensioned slab 3. When one is needed, a Glendale geotechnical firm puts a modified report at about $1,500 and a full report at $7,000 to $9,000 45.

Edited by Ryan Rambo, construction managerFacts checked 2026-10-10

Does every ADU in Los Angeles need a soils report?

No. LADBS does not ask for a soils report on every ADU. Its expansive-soils bulletin, as of February 2026, says that when no soils report is submitted to the Grading Division, the footings are designed to a set of fixed requirements instead 1. That is the path most one-story backyard houses on flat Valley lots take.

The fixed requirements assume the soil could swell and shrink. New footings must be at least 24 inches deep at exterior walls and 18 inches at interior footings, and reinforced with four ½-inch bars, two at the top and two at the bottom 6.

The same bulletin applies those rules when a report is submitted but did not evaluate expansion, or found that the soil is expansive 1. So a report that tests the soil and finds it is not expansive lets your engineer design to the report instead. When a report is used, the ADU correction sheet says it must be approved by the Grading Section, and the approval letter becomes part of the plans 7.

When will LADBS ask me for a soils report?

There are four common situations on a house lot. None of them depends on whether the ADU is detached, attached or a conversion.

  1. The lot is in a Hillside Area. A project in the Hillside Area typically gets a Grading Pre-Inspection, which determines whether a soils and geology report prepared by a licensed engineer is required 2. The ADU correction sheet tells applicants to provide the pre-inspection report and comply with its conditions 8.
  2. The project needs grading. LADBS says permits are required for grading or earthwork in the Hillside Grading Area, for removal and re-compaction, and for back-fill, and that a soils and/or geology report is usually required before those permits issue 9. On hillside grading work, exploration by a licensed soils engineer and a licensed engineering geologist is required unless the Department waives it 10.
  3. The foundation is a post-tensioned slab or mat. LADBS requires a soils report for slab-on-ground foundations such as a post-tensioned mat or raft 3.
  4. The inspector sees something. Building to the fixed footing rules does not stop the building or grading inspector from requiring a soils report at any time, based on site conditions 11.

On a hillside lot there is also a way out of a grading permit. The Department may waive it for excavation under a building when the site is relatively level, or when the digging is only for footings or grade beams no deeper than 5 feet 12.

My lot is in a liquefaction zone. Does my ADU need a geology report?

Usually not. As of February 2026, LADBS exempts buildings accessory to a home, including ADUs under three stories, from the seismic hazard zone investigations for liquefaction, earthquake-induced landslides and fault rupture 13. The bulletin explains that under the California Residential Code these investigations are not required for wood- or metal-framed one- and two-family dwellings 14.

The exemption has limits. The building cannot be part of a development of four or more dwellings 15, and the bulletin counts each ADU as a unit. It also says geologists and engineers are expected to investigate where seismic hazards may exist, even outside a mapped zone 16.

This exemption covers the seismic hazard study only. It does not remove the hillside or expansive-soil rules above.

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How do I know if my lot is a hillside lot, and how common is that in the Valley?

Look up your address in ZIMAS, City Planning's web map that gives zoning information for properties in Los Angeles 17. The Hillside Area is a zoning term: it is any area shown on the city's Hillside Area Map 18. If your lot is also in a fire zone, see ADU rules in the Granada Hills fire zone.

On most of the Valley floor the answer is no. We counted the original ADU building permits LADBS issued in 2025 in the 35 City of LA ZIP codes in the Valley. Of 2,041, 117, about 6%, were flagged as in the Hillside Ordinance Area 19. The share depends on where the hills are 19:

On a flat lot, the soils question usually comes down to the expansive-soil footing rules. On a hillside lot, plan for the grading pre-inspection and for the chance that it asks for a report.

What is the difference between a modified and a full soils report?

The two terms come from geotechnical firms, not from the LADBS ADU correction sheet, which only says that any soil, foundation or geology report must be approved by the Grading Section 7. A Glendale geotechnical firm describes them this way:

The same firm says Los Angeles accepts modified reports for ADUs, including garage conversions 22. That is the firm's statement, not a city rule. On a hillside lot, the grading pre-inspection decides what the city needs 2, and hillside grading calls for both a soils engineer and an engineering geologist unless the Department waives it 10.

How much does a soils report cost for an ADU?

We found one Los Angeles-area geotechnical firm that publishes its prices. Applied Earth Sciences in Glendale puts a modified report at $1,500 on average, done in one to two weeks 4. It puts a full report at $7,000 to $9,000, taking three weeks to two months depending on complexity 5. These are one firm's averages, not a city fee. Get a written quote for your own lot.

Not having a report has a cost too. Without one, the footings follow the city's expansive-soil rules 1: new footings 24 inches deep at exterior walls and 18 inches at interior footings, with four ½-inch bars 6. Whether a report would pay for itself through a lighter foundation is a question for your structural engineer.

For the other lot-driven lines in an ADU budget, see ADU soft costs in Los Angeles. Whether your lot is flat, hillside or somewhere in between is the kind of thing a lot check settles.

Does a garage conversion need a soils report?

Usually not, because the existing foundation can often stay. LADBS's ADU bulletin for conversions, as of August 2026, lets existing foundation components remain if they are in good condition and met the conventional construction rules when the building was built or at any time since 23. Any new footings, for example under an added room, must meet the expansive-soil depth and reinforcement rules 6.

Before you count on the garage, check that it was permitted with the garage permit lookup. For how long plan check takes once the soils question is settled, see ADU plan check timelines at LADBS.

Quick answers

Do I need a soils report for an ADU on a flat lot in Reseda?

Usually not. None of the 168 original ADU permits LADBS issued in Reseda 91335 in 2025 was in the Hillside Ordinance Area 19, and without a report the footings follow the expansive-soil rules 1.

Who approves a soils report for an ADU in LA?

The LADBS Grading Section, and its approval letter becomes part of the plans 7.

Can LADBS ask for a soils report after I start building?

Yes. The building or grading inspector can require one at any time based on site conditions 11.

Does a post-tensioned slab need a soils report?

Yes. LADBS requires a soils report for a post-tensioned mat or raft foundation 3.

Sources

  1. Information Bulletin P/BC 2026-116: Foundation Design for Expansive Soils (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  2. DIR-2017-4229-SPP-1A, appeal staff report (describes the Grading Pre-Inspection), Los Angeles City Planning. Facts checked 2026-10-10
  3. Information Bulletin P/BC 2026-116: Foundation Design for Expansive Soils (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  4. Do ADU / Garage Conversions Need A Geotechnical Soil Report?, Applied Earth Sciences (Glendale geotechnical firm). Facts checked 2026-10-10
  5. Do ADU / Garage Conversions Need A Geotechnical Soil Report?, Applied Earth Sciences (Glendale geotechnical firm). Facts checked 2026-10-10
  6. Information Bulletin P/BC 2026-150: Accessory and Junior Accessory Dwelling Units (revised 08-13-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  7. Plan Check Correction List: Accessory Dwelling Units (PC/STR/Corr.Lst.20A, rev. 10/01/2025), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  8. Plan Check Correction List: Accessory Dwelling Units (PC/STR/Corr.Lst.20A, rev. 10/01/2025), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  9. Grading Division, Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  10. Information Bulletin P/BC 2026-068: Rules and Regulations for Hillside Exploratory Work (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  11. Information Bulletin P/BC 2026-116: Foundation Design for Expansive Soils (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  12. DIR-2017-4229-SPP-1A, appeal staff report (describes the Grading Pre-Inspection), Los Angeles City Planning. Facts checked 2026-10-10
  13. Information Bulletin P/BC 2026-044: Exemptions from Liquefaction, Earthquake Induced Landslides, and Fault-Rupture Hazard Zone Investigations (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  14. Information Bulletin P/BC 2026-044: Exemptions from Liquefaction, Earthquake Induced Landslides, and Fault-Rupture Hazard Zone Investigations (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  15. Information Bulletin P/BC 2026-044: Exemptions from Liquefaction, Earthquake Induced Landslides, and Fault-Rupture Hazard Zone Investigations (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  16. Information Bulletin P/BC 2026-044: Exemptions from Liquefaction, Earthquake Induced Landslides, and Fault-Rupture Hazard Zone Investigations (revised 02-12-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10
  17. Zoning Search (ZIMAS), Los Angeles City Planning. Facts checked 2026-10-10
  18. Zoning glossary: Hillside Area, Los Angeles City Planning. Facts checked 2026-10-10
  19. Building and Safety - Building Permits Issued from 2020 to Present (N), dataset pi9x-tg5x (Greenlight query, Valley ADU permits issued in 2025 by Hillside Ordinance Area flag), City of Los Angeles Open Data / LADBS. Facts checked 2026-10-10
  20. Do ADU / Garage Conversions Need A Geotechnical Soil Report?, Applied Earth Sciences (Glendale geotechnical firm). Facts checked 2026-10-10
  21. Do ADU / Garage Conversions Need A Geotechnical Soil Report?, Applied Earth Sciences (Glendale geotechnical firm). Facts checked 2026-10-10
  22. Do ADU / Garage Conversions Need A Geotechnical Soil Report?, Applied Earth Sciences (Glendale geotechnical firm). Facts checked 2026-10-10
  23. Information Bulletin P/BC 2026-150: Accessory and Junior Accessory Dwelling Units (revised 08-13-2026), Los Angeles Department of Building and Safety (LADBS). Facts checked 2026-10-10

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Greenlight is not a contractor, architect or engineer, and this is not legal advice. Rules change; every fact here shows the date we last checked it.