ADU Structural Requirements in California: Complete Engineering Guide 2026

*Updated: February 2026*

California has emerged as the national leader in ADU construction, with over 25,000 ADU permits issued statewide in 2025. This building boom is driven by a series of state laws -- AB 68, SB 13, AB 881, AB 671, and the 2024-2025 legislative updates -- that streamlined ADU permitting and removed many of the barriers that previously prevented homeowners from building accessory dwelling units. However, the structural engineering requirements for ADUs remain rigorous, ensuring that every unit provides safe, durable housing that withstands California's seismic and environmental hazards.

At AAA Engineering & Design, our licensed Professional Engineers have designed over 500 residential projects across Southern California, including ADUs in Los Angeles, Orange County, San Diego, Riverside, and San Bernardino counties. This guide covers every structural requirement California homeowners and builders need to understand before starting an ADU project.

---

ADU structural requirements are the engineering standards and building code provisions that govern the design of an accessory dwelling unit's load-bearing systems. These requirements ensure that every ADU in California safely supports its intended loads, resists earthquake and wind forces, and performs adequately throughout its expected service life.

The primary code documents governing California ADU structural requirements include:

Understanding these requirements before starting your project prevents costly redesigns, permit delays, and construction change orders. Visit our structural engineering services page for professional assistance with ADU structural design.

---

Every California ADU requires a foundation system designed to the standards of CBC Chapter 18 and the geotechnical conditions at the specific building site. The foundation requirements vary based on:

**Soil bearing capacity:** California soils range from highly expansive clays to sandy alluvium to solid rock. The foundation design must match the soil type, with larger footings required for lower-bearing-capacity soils.

**Seismic Design Category:** All of Southern California falls within Seismic Design Categories D, D+, or E. Foundations in these categories require positive connections to the superstructure, including anchor bolts, hold-down bolts, and foundation straps.

**Frost depth:** While California's mild climate eliminates deep frost depth requirements in most areas, footings must extend to a minimum depth of 12 inches below adjacent grade per CBC Section 1809.4.

**Spread Footings with Stem Walls:** The most common California ADU foundation consists of continuous concrete footings with stem walls rising to the floor level. Typical footing widths range from 12 to 24 inches depending on soil bearing capacity, with 18 inches being standard for most Southern California sites.

**Post-Tension Slab-on-Grade:** Popular in areas with expansive soils, post-tension slabs use steel tendons tensioned after concrete curing to resist soil movement. Many California jurisdictions in Riverside County and portions of Los Angeles County specify post-tension slabs for ADUs on expansive soil sites.

**Raised Floor with Cripple Walls:** Some California ADU designs use raised floors with short cripple walls between the foundation and the first floor. These cripple walls require bracing per CBC prescriptive requirements or engineered shear wall design.

**Pier and Grade Beam:** Hillside ADU sites in California frequently require pier foundations with grade beams spanning between piers. This foundation type is common in the hillside communities of Los Angeles, San Diego, and the San Francisco Bay Area.

California ADU foundations require steel reinforcement meeting the following minimums:

| Foundation Element | Minimum Reinforcement | Size | Spacing | |-------------------|----------------------|------|---------| | Continuous Footing | 2 bars longitudinal | #4 minimum | Continuous | | Stem Wall | 1 bar horizontal | #4 minimum | 48" o.c. max | | Stem Wall | Vertical dowels | #4 minimum | 48" o.c. max | | Slab-on-Grade | Welded wire reinforcement | 6x6-W2.9xW2.9 | - | | Grade Beam | Per engineering design | #5 minimum | Per design |

For detailed information about foundation engineering solutions, visit our dedicated service page.

---

California's seismic hazard is the primary driver of structural engineering complexity for ADUs. Every ADU project requires seismic analysis using the following parameters from ASCE 7-22:

**Spectral Response Accelerations (Ss and S1):** These site-specific values are determined by the building's latitude and longitude using the ASCE 7 Hazard Tool. Values in Southern California typically range from 1.0g to 2.5g for Ss and 0.4g to 1.0g for S1.

**Site Class:** Determined by the soil conditions at the building site, ranging from Site Class A (hard rock) to Site Class F (liquefiable soils). Most California ADU sites default to Site Class D unless site-specific geotechnical data justifies a different classification.

**Seismic Design Category (SDC):** Determined by the combination of spectral accelerations and site class. Virtually all of Southern California falls within SDC D, D+, or E, requiring:

The structural engineer selects the appropriate lateral system based on ADU configuration and code requirements:

**Wood Structural Panel Shear Walls:** The most common lateral system for California ADUs. Plywood or OSB sheathing nailed to wood framing creates shear walls that resist earthquake forces. Nailing patterns, hold-down connections, and anchor bolt spacing are specified by the structural engineer based on calculated seismic demands.

**Steel Moment Frames:** Used when architectural requirements prevent adequate shear wall placement. Steel moment frames allow open floor plans with large window walls while providing the required lateral resistance. Common in California ADU designs with modern open-concept layouts.

**Braced Frames:** Occasionally used in California ADU garages or carport areas where moment frames are not economical and shear walls conflict with vehicle access requirements.

California seismic design requires positive connections at every point in the load path from roof to foundation:

Learn more about seismic retrofitting services for existing structures adjacent to new ADU construction.

---

The vast majority of California ADUs use wood-frame construction governed by CBC Chapter 23 and referenced standards including the National Design Specification (NDS) for Wood Construction.

Modern California ADU construction frequently incorporates engineered wood products that provide superior performance:

California's seismic environment demands robust connections throughout the wood-frame system:

| Connection Type | Minimum Requirement | Code Reference | |----------------|-------------------|----------------| | Stud-to-plate | 2-16d nails or equivalent clip | CBC Table 2304.10.1 | | Joist-to-plate | 3-8d toenails or clip | CBC Table 2304.10.1 | | Rafter-to-plate | Hurricane clip per design | ASCE 7-22 | | Header-to-trimmer | Per engineering design | Per structural plans | | Hold-down-to-foundation | Per engineering design | ASCE 7-22 | | Shear wall nailing | 3" to 6" o.c. per schedule | Per structural plans |

---

California law establishes the following ADU size parameters that affect structural requirements:

**Detached ADUs:** Up to 1,200 square feet on any lot with an existing or proposed single-family or multifamily dwelling. Two-story detached ADUs are permitted statewide as of January 2025.

**Attached ADUs:** Up to 1,000 square feet or 50% of the existing dwelling area, whichever is greater. Attached ADUs require structural analysis of the existing building to verify adequate capacity for the added loads.

**Junior ADUs:** Up to 500 square feet within the existing single-family dwelling footprint. JADUs involve structural modifications to existing walls, floors, and potentially the roof structure.

**Multifamily ADUs:** At least one detached ADU plus up to 25% of existing units as converted ADUs. Structural requirements depend on the existing building's structural system and the scope of modifications.

California's allowance of two-story ADUs creates additional structural requirements:

For comprehensive residential structural engineering services, explore our residential service offerings.

---

California's Title 24 Energy Code affects structural design through:

**2x6 wall framing:** Many California climate zones require 2x6 exterior wall framing to achieve energy compliance, which changes structural design parameters including wall height-to-thickness ratios and lateral bracing requirements.

**Raised heel trusses:** Energy code insulation requirements at roof-to-wall intersections often require raised heel trusses, affecting roof-to-wall connection design.

**Window-to-wall ratios:** Fenestration requirements influence shear wall placement and lateral system design.

**Solar readiness:** California's solar mandate for new construction, including ADUs, requires roof structural capacity for future solar panel installations adding 3-5 psf to roof dead loads.

---

While individual jurisdictions vary, California ADU structural plans generally include:

**Los Angeles:** LADBS requires structural plans on 24"x36" sheets with specific title block information, digital plan submittal through the ePlanLA system, and compliance with the Los Angeles Building Code amendments.

**Orange County cities:** Each city maintains independent building departments with varying plan check requirements, though structural standards are consistent with CBC 2025 statewide provisions.

**San Diego:** DSD has streamlined ADU permitting with reduced fees and expedited review, but structural plan completeness requirements remain comprehensive.

**Riverside and San Bernardino Counties:** Higher wind speed zones in desert areas and expansive soil conditions create additional structural requirements beyond standard coastal California designs.

---

| ADU Type | Size Range | Engineering Cost | Typical Timeline | |----------|-----------|-----------------|-----------------| | JADU (Interior Conversion) | 150-500 sq ft | $2,500-$5,000 | 1-3 weeks | | Garage Conversion | 300-600 sq ft | $3,000-$5,500 | 2-3 weeks | | Detached (Single-Story) | 400-800 sq ft | $4,000-$8,000 | 3-4 weeks | | Detached (Large, Single-Story) | 800-1,200 sq ft | $6,000-$10,000 | 3-5 weeks | | Attached ADU | 400-1,000 sq ft | $5,000-$9,000 | 3-5 weeks | | Two-Story Detached | 600-1,200 sq ft | $8,000-$13,000 | 4-6 weeks | | Hillside (Any Type) | Any | $8,000-$15,000 | 5-8 weeks |

**Soil conditions:** Expansive soils, hillside sites, and areas with high water tables require more complex foundation designs. California properties with geotechnical hazards add $2,000-$5,000 to structural engineering costs.

**Existing structure analysis:** Attached ADUs and garage conversions require investigation and analysis of the existing building. Properties without original structural plans require additional field investigation adding $1,000-$3,000.

**Architectural complexity:** Open floor plans, large window walls, vaulted ceilings, and cantilevers increase structural engineering scope and cost.

**Jurisdiction requirements:** Some California jurisdictions require additional structural analyses, such as progressive collapse resistance or fire-resistance-rated assembly design, adding scope to the engineering engagement.

---

---

Our licensed Professional Engineers work across all California jurisdictions, understanding the nuances of each building department's requirements. We have completed ADU structural engineering in over 40 California cities.

With over 500 residential structural engineering projects completed, we bring unmatched experience to every California ADU engagement. Our designs are efficient, code-compliant, and contractor-friendly.

Our firm has provided structural engineering services in California for over two decades. We have worked through multiple code cycles and understand how California building code requirements have evolved.

Our structural plans pass plan check on the first submission over 94% of the time across all California jurisdictions. This saves clients weeks of delay and eliminates revision costs.

Every California ADU project receives a comprehensive structural engineering package including PE-stamped calculations, structural plans, connection details, and special inspection requirements.

Explore our ADU engineering services for detailed information about our ADU structural design capabilities.

---

Yes. Every California ADU that creates habitable space requires structural plans stamped by a California-licensed Professional Engineer (PE) or licensed Structural Engineer (SE). This applies to detached ADUs, attached ADUs, garage conversions, and JADUs. The only exception is non-habitable accessory structures under 120 square feet, which do not require engineering.

The California Building Code (CBC) 2025, effective January 1, 2025, governs all ADU construction in 2026. The CBC 2025 is based on the 2024 International Building Code with California-specific amendments. Seismic design follows ASCE 7-22, and wood-frame design follows the 2024 National Design Specification. Local jurisdictions cannot impose structural requirements that exceed the CBC for ADU projects.

No. Every California ADU requires a code-compliant foundation system. The minimum foundation for a California ADU consists of concrete footings extending at least 12 inches below adjacent grade with anchor bolts connecting the wood framing to the foundation. Prefabricated ADUs placed on permanent sites also require permanent foundation systems per CBC requirements.

Most of California falls within Seismic Design Category (SDC) D, with portions of Southern California in SDC D+ or E. The specific SDC for your property depends on the site's latitude and longitude, the soil conditions, and the building's risk category. SDC D and higher require engineered lateral force-resisting systems, hold-down connections, and special inspection programs.

Yes. California allows two-story detached ADUs up to 1,200 square feet statewide as of January 2025. Two-story ADUs require additional structural engineering for the increased gravity loads, higher seismic forces, and inter-story drift control. Foundation requirements are also greater for two-story structures. The structural engineering cost for a two-story ADU is typically 40-60% higher than a comparable single-story unit.

Expansive soils are common throughout California, particularly in inland areas. CBC Section 1808.6 requires special foundation designs for expansive soils, including deeper footings, moisture barriers, and potentially post-tension slab systems. The structural engineer uses geotechnical report recommendations to design foundations that accommodate soil expansion and contraction cycles without structural damage.

California ADU construction requires multiple structural inspections including foundation (before concrete pour), underfloor framing (for raised floors), wall framing, shear wall nailing, hold-down installation, roof framing, and final structural inspection. Special inspections by independent testing agencies are required for certain elements including concrete placement, high-strength bolt installation, and welding on steel connections.

Many California jurisdictions require geotechnical reports for ADU projects, particularly on hillside lots, in areas with known expansive soils, or within liquefaction zones. Even where not required, a geotechnical report provides the soil bearing capacity, site class, and foundation recommendations the structural engineer needs for accurate and efficient design. The cost of a geotechnical report ($2,000-$5,000) is offset by the savings from optimized foundation design.

---

---