How Big Can an ADU Be in California? ADU Size Rules

California law allows most accessory dwelling units up to 1,200 square feet, and many cities reduce or waive certain impact fees for ADUs under 750 square feet. Size rules vary by type, so the right answer depends on whether your ADU is detached, attached, or a conversion.

California's housing crunch has made accessory dwelling units (ADUs) an increasingly popular solution for homeowners seeking more space or rental income, but before building one, it's essential to understand the state's size regulations.

How big can an ADU be in California? The answer depends on factors such as property zoning, local ordinances, and state laws. Knowing these limits upfront helps you design an ADU that maximizes livable space while staying fully compliant with regulations.

Here's what we'll cover in this guide:

• ADU size limits by type: detached, attached, garage conversions, and JADUs.
• Statewide standards vs. local rules in Los Angeles and nearby cities.
• Height limits, setbacks, floor area rules and how they impact usable square footage.
• Good sizes for an ADU based on use, layout, and budget planning.
• LADU's approach and how it streamlines design, permits, and construction arrangements.

You'll soon know the maximum ADU size you can build on your lot, the minimums that apply, how height and setbacks affect your design, and the fastest path to approvals in LA through LADU's architect-led process.

At LADU, we are ADU experts in LA committed to providing residents with the most streamlined approach for building an ADU.

With beautiful pre-planned and custom designs, ADU building permit assistance, and a team of licensed construction partners, we'll ensure the entire process is stress-free and straightforward, from the initial consultation to the final walkthrough.

Speak with our ADU experts today to begin planning the ADU of your dreams.

California ADU Size Limits by State Law

Detached ADU Sizes: 800 to 1,200 Square Feet

Detached ADUs offer the most flexibility on many lots. Under California state law, cities must allow at least:

• 850 square feet for a one‑bedroom detached ADU.
• 1,000 square feet for two or more bedrooms.

Many cities still allow a detached accessory dwelling unit up to 1,200 square feet when local code supports it.

What is the maximum size for an ADU in California? In practice, the maximum ADU size in California for a detached unit sits between 800 and 1,200 square feet on most parcels, with the bedroom‑based caps above serving as floor allowances that cities cannot reduce.

Height, setbacks, and other site conditions then shape the final square footage you can fit.

Attached ADU Size in California: 50% of Main Home, Up to 1,200 Square Feet

The ADU size limit in California for an attached ADU ties to the size of the primary home. An attached ADU can be no larger than 50% of the main home's square footage and may not exceed 1,200 square feet.

The statewide bedroom‑based allowances still apply, which helps on smaller homes.

Examples:

• A 1,200-square-foot house could add an attached ADU up to 600 square feet.
• A 1,700-square-foot house could add an attached ADU up to 850 square feet.
• A 2,400-square-foot house could add an attached ADU up to 1,200 square feet.

LADU's team confirms the right path during zoning and code checks so your plan meets state laws and local size regulations without surprises.

ADU Size Requirements for Conversions of Existing Structures

Garage and accessory structure conversion limits are based on the space you already have.

If you convert a legal, existing structure and keep the footprint, the project often sits outside the 1,200 square-foot cap. Many cities allow a small expansion, commonly up to about 150 square feet, for entries or code clearances.

Conversions are a powerful way to capture square footage in tight setback conditions and still meet ADU regulations.

For statewide references, visit California Department of Housing and Community Development (HCD).

Minimum and Maximum ADU Size: California Rules

Minimum ADU Size California: Habitability Basics

Cities often set a minimum ADU size between 150 and 220 square feet. Even at that scale, the unit must function as an independent dwelling with:

• A kitchen with a cooking appliance and food prep area.
• A bathroom with a toilet, sink, and shower or tub.
• A sleeping area.
• A separate exterior entry.

These ADU size requirements reflect safety and livability standards. LADU can design compact plans that meet the minimum size while still feeling spacious.

Junior ADU Size: 500 Square-Foot Cap

A Junior ADU (JADU) sits entirely inside the main home or within an attached garage. The maximum size is 500 square feet.

A JADU includes an efficiency kitchen, and many jurisdictions allow or require a shared bathroom with the primary residence. JADUs can pair with a full ADU on many single‑family lots, creating flexible options for family or long‑term rental.

What Is a Good Size for an ADU?

The right size ADU meets your goals, budget, and lot size. Here are common ADU sizes that balance livability and cost:

• Studio: 350 to 500 square feet. Works for a guest space or a rental with a clear sleeping nook and compact kitchen. Storage walls, tall cabinets, and combined washer‑dryer units keep the plan efficient.
• One‑bedroom: 500 to 650 square feet. Adds privacy and better furniture placement. A U‑shaped or L‑shaped kitchen under a window brings light without wasting wall space.
• Two‑bedroom: 750 to 1,000 square feet. Ideal for family, caregivers, or higher rental income. Hallways must stay short.

These ranges align with common ADU minimum size rules and the ADU size restrictions that come from height and setbacks. During concept design, we model layouts at multiple square footage targets so you can compare cost, rental income potential, and comfort.

Height Limits, Setbacks, and Lot Coverage That Affect Max ADU Size California

ADU Height Limit

ADU height restrictions dictate roof form, second stories, and lofts. Local code controls the final number, but common patterns under ADU law and local ordinances include:

• Detached ADU: often allowed up to 16 feet in height.
• Near major transit or on some multifamily lots: up to 18 feet may be allowed.
• Attached ADU: frequently tied to the main house height, commonly permitted up to about 25 feet.

Setbacks and Placement

State laws require cities to allow side and rear setbacks as little as 4 feet for many ADUs. Front setbacks, driveway access, and corner lot rules vary by jurisdiction. These lines define where the structure can sit and help protect light, privacy, and fire access.

On narrow lots, a long and shallow layout often fits better than a box design. On wider lots, a square footprint can reduce exterior wall area and cost.

FAR, Lot Coverage, and Fee Relief for 750 Square Feet

Floor area ratio (FAR) and lot coverage can cap ADU size on some parcels. State law requires cities to allow at least 750 square feet regardless of FAR and lot coverage. Many cities also reduce or waive some impact fees for ADUs up to 750 square feet.

This rule often shapes smart sizing strategy:

• Targeting 749 to 750 square feet can cut fees in many places.
• Compact storage and built‑ins make a sub‑750 plan feel larger.
• If long‑term family use is the goal, growing past 750 square feet may be worth the added cost.

Local ADU Size Rule Variations in Los Angeles and Nearby Cities

ADU Size Los Angeles: City Baseline

The City of Los Angeles generally aligns with California ADU size limit standards for total ADU size, height, and 4‑foot side and rear setbacks.

LADU's team handles common plan‑check focus areas, including fire department clearances, utility tie‑ins, hillside review, and corrections on privacy windows, grading notes, and Title 24 energy forms.

Our code‑first approach reduces back‑and‑forth hassles and helps your plans move faster.

Examples of Stricter ADU Size Caps

Some nearby jurisdictions set tighter maximums or different caps for certain lot types. For example, a 950 square foot limit appears in some city codes outside LA, such as Walnut Creek. Differences like these are why local checks matter before you set your target square footage.

How LADU Navigates City-by-City Code

Our architects work across Los Angeles County on flat lots, small lots, hillside parcels, and multifamily properties. We map the ADU size limit California rules to your address, then layer on utility and grading constraints so your submittal matches local expectations.

ADU Types: How ADU Size Choices Shape Design

Detached ADU: Privacy and Maximum Flexibility

Detached ADUs offer the most control over privacy and orientation. Size choices interact with setbacks, window placement, and utility routes. A 400- to 500-square-foot plan can frame a private patio between the main house and the ADU.

Moving up to 800 to 1,000 square feet allows two bedrooms and a larger great room while still fitting within common height and yard limits. LADU designs service runs for water, sewer, and power early so your preferred square footage stays intact during plan check.

Attached ADU: Footprint Efficiency

An attached ADU shares a wall with the main home. The 50% cap and the 1,200 square-foot ceiling guide size. Floor plans must account for sound separation and a dedicated exterior entry.

Careful circulation planning avoids cutting up the main house and protects the ADU's usable space. This route often reduces excavation and can shorten utility runs.

Garage or Accessory Structure Conversions

Conversions reuse the existing footprint and framing, which can simplify zoning. Many garages need structural upgrades, new slabs, and insulation to meet code. Small expansions are often allowed for entries or code clearances.

Conversions are a strong value play when setbacks or lot coverage are tight.

Planning ADU Bedrooms and Layouts by Square Footage

Studios and One-Bedrooms

For ADUs 350 to 650 square feet, layout is everything. Key moves include:

• One wet wall: Align the kitchen and bathroom to simplify plumbing and save space.
• Storage wall: Full‑height closets or built‑ins along one side reduce clutter.
• Daylight: A large window near the living and dining zone expands the feel of a compact plan.

These strategies meet ADU minimum size rules and keep cost per square foot predictable.

Two Bedrooms and Family-Forward Layouts

At 750 to 1,000 square feet, two bedroom units fit well with a single bathroom or a 1.5‑bath layout. Keep hallways short, tuck laundry near the bathroom, and group mechanicals in a shared closet.

The ADU size restrictions you face on setbacks and height guide room dimensions, window types, and where to place doors for smooth circulation.

Accessibility and Building Systems

Clearances, door widths, and bathroom layouts must support safe movement for all users. Mechanical closets need enough space for equipment, filters, and service access. Laundry areas placed near the kitchen or bathroom shorten vents and save square footage.

Energy code planning, including insulation and window selections, protects comfort and operating costs.

ADU Permitting Updates That Impact Size and Speed

Pre-Approved ADU Plans

In 2025, cities are now required to offer pre‑approved ADU plan schemes. These can shorten review times for common layouts and popular ADU sizes. Building an ADU has never been easier.

LADU evaluates where pre‑approved plans fit and when a custom or pre‑designed scheme will serve you better. We match your lot, setbacks, and target square feet to the right path so design speed does not sacrifice fit or long‑term value.

Legalizing Existing ADUs and AB 2533

AB 2533 opens new paths in 2025 for permitting unpermitted ADUs that meet health and safety standards. In some cases, fees are reduced and size changes are possible to bring a space into compliance.

LADU documents the existing square footage, proposes targeted fixes, and guides submittals so the unpermitted ADU can be legalized without a full rebuild.

Submittal Strategy in Los Angeles

Clear, complete submittals speed plan check. Expect to provide:

• A current site survey or base map with property lines, setbacks, and utilities.
• Floor plans, elevations, and a simple roof and drainage plan.
• Structural notes or calculations where needed, explained in plain language.
• Energy compliance forms for heating, cooling, insulation, and windows.
• A utility worksheet for water, sewer, gas, and electrical connections.
• Photos of the site and any structure to be converted.

Costs, Financing, and Pricing Tied to Square Footage

How Square Footage Drives Cost

Total square footage is a major cost driver. Larger footprints add additional costs for foundation, framing, exterior finishes, and systems.

Height matters too. A one‑story 75-square-foot unit may cost less per square foot than a two‑story 1,000-square-foot plan with stairs and extra structure.

Site conditions such as grading, utility distances, and hillside work add complexity. LADU balances these factors so you can pick an ADU size that fits your budget and schedule.

Under-750 Strategies and Fee Savings

Many homeowners target 749 to 750 square feet to capture fee relief available in many cities. Tradeoffs include smaller bedrooms, fewer closets, or a single bathroom. Smart moves at this size include:

• Combine the laundry with a linen cabinet.
• Use pocket or barn doors to save swing space.
• Choose a galley or L‑shaped kitchen to keep circulation clear.

These choices keep quality high without pushing square feet over a fee threshold.

LADU Pricing Model and Financing Options

LADU offers flat‑fee design packages, transparent pricing for custom design, and competitive construction pricing arranged through our network of licensed partners.

Financing options and a free consultation help you plan with confidence. Learn more about ADU construction and how we align scope with your budget from day one.

How LADU Makes ADU Sizing Decisions Simple in LA

Free Site Assessment and Feasibility

Start with a free site assessment. We confirm ADU size limits, setbacks, height, and utility tie‑ins on your lot. You receive a clear feasibility snapshot so you know what you can build before you invest in full drawings.

Architect-Led Design for Faster Approvals

LADU is an architecture‑led team focused on LA codes and review habits. We model multiple size options, run early code checks, and prepare submittals that respond to local reviewers. This approach cuts corrections and keeps momentum.

One Team. One Contract. One Goal.

Design + Permitting + Build = LADU. Avoid gaps between ADU design, obtaining ADU permits, and hiring a contractor.

Our integrated delivery protects your budget and timeline from the first sketch to final inspection. After permitting is complete, we arrange construction with licensed ADU partners so you can build your ADU without stress.

California ADU Size: Key Takeaways

So, how big can an ADU be in California?

Many detached and attached units can reach up to 1,200 square feet, with common caps of 850 square feet for a one‑bedroom and 1,000 square feet for two or more. Conversions often match the existing footprint. Junior ADUs max out at 500 square feet.

Local size regulations can shift the final number, and site factors matter. A height restriction of 16 feet is common for many detached ADUs but 18 feet is permissible in some cases.

Standard 4‑foot side and rear setbacks apply in most cases, and many cities offer fee relief at or under 750 square feet, which can be a smart sizing strategy. The ideal ADU revolves around livability, rental goals, lot size, and budget.

LADU's architect‑led team translates ADU size limits into strong layouts and faster approvals. See if your property qualifies today. Schedule a free site assessment today to get started.

Have questions about the California ADU size limit or a case‑specific submittal in Los Angeles? Call (213) 855-3334 for clear guidance on plan check and permits.

LADU: ADUs Designed Better. Built Smarter.