ADU Sizing - From the Smallest to Typical Dimensions

An accessory dwelling unit can be as compact as a well-designed studio apartment or as spacious as a modest home. The range is wide — from 150 square feet at the absolute minimum to 1,200 square feet or more at the upper end — and the right size for your project depends on a mix of local regulations, lot constraints, intended use, and budget.

Understanding how ADU sizing works is one of the most important early steps in any project. Size drives cost, it determines what bedroom and bathroom configurations are possible, it affects permitting complexity and impact fees, and it shapes whether your unit will function as a comfortable long-term residence or a more focused space for guests or rental income.

This guide breaks down ADU sizing from the smallest allowable dimensions to the most common configurations, with an eye toward what actually fits — and functions — at each size range.

What counts as an ADU, and what every unit needs

Before getting into square footage, it helps to understand the baseline requirements that define an ADU. Regardless of size, an ADU must function as a complete, independent living space. That means every unit needs to include a kitchen (with a sink, cooking appliance, and refrigeration), a bathroom (with at minimum a toilet, sink, and shower), a sleeping area, and its own separate entrance.

These requirements hold whether your ADU is 200 square feet or 1,200. The practical implication is that even the smallest ADU carries a baseline level of complexity in terms of plumbing, electrical, and ventilation — and that has a direct impact on both design and cost.

A junior ADU, or JADU, is a related but distinct category. JADUs are limited to 500 square feet, must be created within an existing home or attached structure (like a garage), and can share a bathroom with the primary residence. They require only an efficiency kitchen rather than a full one. JADUs are currently a California-specific classification, though other states are developing similar frameworks.

The regulatory landscape for ADU sizing

ADU size limits vary significantly depending on where you live, and the regulatory picture has been shifting rapidly. Here's what you need to know about how size regulations work across the country.

California: the most established framework

California has the most detailed statewide ADU regulations in the country, and they set the standard that many other states reference. The key size parameters include:

• Minimum size. State law sets a floor of 150 square feet for all ADU types, though individual cities may set higher minimums (San Francisco, for example, requires 220 square feet).
• Maximum size for detached ADUs. The statewide ceiling is 1,200 square feet, and roughly 391 cities and counties use this standard. Some jurisdictions set lower caps — several San Diego-area cities cap at 1,000 square feet, for instance.
• Maximum size for attached ADUs. These cannot exceed 50% of the primary home's living area, though the state guarantees that homeowners can build at least an 850-square-foot one-bedroom or a 1,000-square-foot two-bedroom unit regardless.
• The 800-square-foot guarantee. Even if a property is maxed out on floor area ratio or lot coverage, California law requires that an 800-square-foot ADU still be permitted.
• The 750-square-foot threshold. ADUs under 750 square feet are exempt from impact fees statewide, which can save thousands of dollars in permitting costs.
• Junior ADUs. Capped at 500 square feet, JADUs must be built within existing space and may share a bathroom with the primary dwelling.

Other states with ADU-friendly frameworks

• Washington. State law requires urban municipalities to allow at least two ADUs per residential lot. Local governments cannot require ADUs to be smaller than 1,000 gross square feet.
• Oregon. Cities with populations over 2,500 must allow at least one ADU per single-family lot. Maximum sizes are set locally but commonly fall in the 800- to 900-square-foot range or 75% to 85% of the primary home's size. Portland caps at 800 square feet.
• Maine and Connecticut. Both states require most residential lots to allow ADUs, though specific size regulations are set at the local level.
• Colorado. New ADU legislation took effect in 2025 for many metro areas, with size standards still being established by individual municipalities.
• New York City. Under rules enacted in late 2024, ADUs on one- and two-family lots cannot exceed 800 square feet and cannot cover more than 33% of the required rear yard. Detached ADUs must be within 100 feet of the street curb.
• Texas and Florida. Neither state has comprehensive statewide ADU legislation. ADU allowances are set city by city — Austin and Houston permit them, Miami and St. Petersburg are opening up — and size regulations vary accordingly.

The common thread: most jurisdictions that permit ADUs allow units between 800 and 1,200 square feet, with minimums typically ranging from 150 to 300 square feet.

ADU sizing by range: what fits and what works

Under 400 square feet: the compact studio

At the smallest end of the spectrum, ADUs in the 200- to 400-square-foot range function as efficiency studios. A typical footprint at this size might be roughly 16 by 20 feet or 14 by 25 feet. Design at this scale is almost entirely open-concept — the living, sleeping, and kitchen areas share a single room, with only the bathroom walled off.

What fits at this size:

• Kitchen. A galley-style or single-wall kitchenette with a compact refrigerator, two-burner cooktop, sink, and a few feet of counter space. Full-size appliances are possible but will dominate the layout.
• Bathroom. A three-quarter bath (toilet, sink, and shower stall) is standard. A full bathtub is possible but will consume a disproportionate amount of floor area.
• Sleeping and living area. These share the same space. Murphy beds, loft sleeping areas, and convertible furniture help maximize utility.
• Storage. Limited to built-ins, closet nooks, and vertical shelving. There's typically no room for a dedicated closet.

This size range works well for short-term rentals, home offices, or as a guest house where people won’t stay long-term. It's also the most cost-efficient entry point — though be aware that cost per square foot tends to be higher for very small ADUs because the fixed expenses of plumbing, electrical, and foundation work are spread across less area.

400 to 600 square feet: the functional one-bedroom

This is the range where an ADU starts to feel like a true independent home rather than a studio. A 500-square-foot unit might measure 20 by 25 feet or 16 by 31 feet, and that additional space opens up meaningful layout possibilities.

What fits at this size:

• Kitchen. A fully equipped kitchen with standard-size appliances, enough counter space for real meal prep, and room for a small dining nook or breakfast bar.
• Bathroom. A full bathroom with a tub-shower combination is comfortable at this range. Some layouts can accommodate a stacked washer-dryer unit in or adjacent to the bathroom.
• Bedroom. A separate, enclosed bedroom is the key upgrade from the studio range. At 500 square feet, you can fit a queen bed, nightstands, and a closet while still maintaining a distinct living area.
• Living area. Enough room for a sofa, coffee table, and media setup, though the space will feel cozy rather than expansive.

This is the sweet spot for many homeowners. A one-bedroom ADU in this range works for long-term rental tenants or as an in-law suite.

In California, keeping the unit under 500 square feet can also trigger exemptions from certain requirements like soils reports in some jurisdictions.

600 to 850 square feet: the spacious one-bedroom or compact two-bedroom

At this mid-range, layouts become noticeably more flexible. You can design a generous one-bedroom with a separate dining area, a home office nook, and in-unit laundry — or you can fit two smaller bedrooms with a shared bathroom.

What fits at this size:

• Kitchen. A full kitchen with room for an island or peninsula, pantry storage, and a full-size refrigerator.
• Bathroom. One full bathroom plus the option for a half bath if the layout prioritizes a two-bedroom configuration.
• Bedrooms. A one-bedroom layout at 750 to 850 square feet can include a primary suite with a walk-in closet. A two-bedroom layout at this size will have more modestly sized rooms but still meet building code minimums (bedrooms must be at least 70 square feet with a minimum 7-foot horizontal dimension in any direction).
• Living area. Enough square footage for a comfortable living room, a designated dining space, and potentially a small workspace.
• Laundry. In-unit washer and dryer become easy to accommodate at this range, typically in a closet configuration.

This range is popular for multigenerational living, where the occupant needs a full home experience at a smaller scale. Two-bedroom configurations at this size are also strong performers for rental income, as they appeal to a broader tenant pool.

850 to 1,200 square feet: the full-size ADU

At the upper end of what most jurisdictions allow, ADUs in this range feel comparable to a modest apartment or small home. Two bedrooms with one or two bathrooms are standard, and some layouts accommodate three bedrooms at the 1,200-square-foot maximum.

What fits at this size:

• Kitchen. A full-size kitchen with ample cabinetry, counter space, and room for a dining table nearby.
• Bathrooms. Two full bathrooms are comfortable, allowing for a primary suite with an en-suite bath and a second bathroom accessible from the common area.
• Bedrooms. Two generous bedrooms with closets, or three bedrooms at tighter proportions. Each bedroom at this size can comfortably fit a queen or king bed.
• Living and dining. Separate, defined spaces for living and dining — or a large open-concept great room.
• Additional features. Enough room for a dedicated laundry area, a linen closet, a coat closet by the entry, and potentially a small mudroom or storage room.

ADUs at this scale are genuine homes. They work for small families, couples who need distinct spaces (home office, guest room), and rental units where you're targeting tenants who want a full living experience. From an investment standpoint, larger ADUs cost more in total but deliver a lower cost per square foot — the per-square-foot cost at 1,200 square feet is typically meaningfully less than at 400 square feet because fixed construction costs are distributed over more area.

How size affects the cost of your ADU

ADU construction costs range widely depending on location, type, and finish level, but size is one of the primary drivers. Nationally, construction costs typically fall between $150 and $300 per square foot, though they can reach $400 to $600 per square foot in high-cost markets like Los Angeles, San Francisco, and New York.

Here's how total project costs generally scale by size:

• Under 400 sq ft. $60,000 to $150,000 total. Higher cost per square foot due to fixed expenses.
• 400 to 600 sq ft. $100,000 to $200,000 total. This is where economies of scale begin to improve.
• 600 to 850 sq ft. $150,000 to $275,000 total. A balanced range for cost and livability.
• 850 to 1,200 sq ft. $200,000 to $400,000+ total. Lowest cost per square foot but highest total investment.

These ranges cover construction costs and do not include land (since ADUs are built on existing lots), but they do encompass design, permitting, site preparation, and utility connections. Garage conversions tend to come in at the lower end of each range since they leverage an existing structure, while detached new construction runs higher.

Two cost thresholds are worth keeping in mind in California: ADUs under 750 square feet are exempt from impact fees, and ADUs under 500 square feet may qualify for additional permitting exemptions in some jurisdictions. These savings can amount to several thousand dollars and are worth factoring into your sizing decision.

Of course, no conversation about ADU cost would be complete without acknowledging the impact of choosing a prefab or custom solution. Read more about this with Prefab ADUs - What is the True Cost Savings.

Choosing the right size for your goals

The best ADU size is the one that aligns with how the unit will actually be used. A few guiding questions:

• Who will live there? A single occupant or couple can be comfortable in 400 to 600 square feet. A small family or roommates will likely need 750 square feet or more.
• Is it for rental income? One-bedroom units in the 500- to 750-square-foot range are the most common rental ADUs because they balance construction cost with broad tenant appeal. Two-bedroom units attract more tenants but cost more to build.
• Will it serve aging parents or multigenerational needs? Prioritize accessibility — wider doorways, single-story layouts, a walk-in shower rather than a tub — which may require slightly more square footage to execute well.
• Are you planning to use it yourself someday? If there's any chance you'll downsize into the ADU and rent out the primary home, size it for the long term.
• What does your lot actually allow? Setbacks, lot coverage limits, and the position of existing structures will constrain your options. A 1,200-square-foot ADU is only an option if your lot can physically accommodate it within required setbacks — typically 4 feet from side and rear property lines for detached units.

Working with Block Renovation on your ADU project

ADU projects involve a unique combination of challenges: zoning research, permitting, architectural design, utility coordination, and construction — all on a property that already has an occupied home. The contractor you choose needs to understand not just how to build the structure, but how to navigate the regulatory process and coordinate the project around your daily life.

Block Renovation connects homeowners with thoroughly vetted, licensed contractors who have experience with ADU projects. Every contractor in Block's network passes through a rigorous vetting process that includes license verification, background checks, and workmanship reviews. You'll receive competitive proposals with detailed scopes, compare them side by side with help from a dedicated project planner, and have expert support from planning through final walkthrough.

Whether you're building a compact studio for rental income or a full-size two-bedroom for family, getting the size right from the start sets the foundation for everything that follows.

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