Types of ADUs: A Comprehensive Guide

What is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary housing unit – also known as an accessory structure, granny flat, or backyard cottage – on the same lot as your primary dwelling. It’s a fully equipped, independent living unit with its own kitchen, bathroom, sleeping area, and exterior entrance. Whether built as a free-standing tiny house (a detached structure) or carved from existing space (a basement ADU or garage apartment), ADUs expand your square footage, create extra living space, and add a reliable stream of income.

Transforming unused space into an ADU is a smart way to generate rental income while increasing your property’s overall value.

Key Characteristics of ADUs

• Types: ADUs can be detached structures (standalone cottages or carriage house lofts), attached units (garage apartments, rear-yard additions), or converted ADUs (basement conversion, attic conversion).
• Purpose: Serve as housing for family members, generate rental income (long- or short-term), or provide affordable housing options for 1–2 person households.
• Ownership: ADUs remain tied to the primary property and share utilities with the primary structure; they cannot be sold separately.
• Regulations: Must comply with local zoning laws, building permit application submittal, development regulations, and setback standards (rear setback, minimum lot size), all overseen by the Planning Department and department of building.

The table below provides a detailed comparison of ADU types, highlighting key factors such as cost, privacy, rental potential, and design flexibility to help you choose the best option for your needs.

Detached ADUs

A detached ADU is a free-standing dwelling unit built on the same lot as the primary single-family dwelling, physically separate from the main house. As a versatile accessory unit, it delivers fully independent livable space—kitchen, bath, living area, and separate entrance—while minimizing your environmental footprint. Ideal for property owners who value privacy and flexible living spaces, detached ADUs often follow Pre-Approved ADU Plans and streamlined building permit processes under local Accessory Dwelling Unit Programs, helping control ADU cost per sqft and comply with building codes and size requirements.

Detached ADUs offer maximum privacy and flexibility, ideal for rental income or multigenerational living.

Examples of Detached ADUs

• Backyard Cottage: A cozy residential unit—built from scratch or via building construction—that offers a full-blown living space for guests or long-term tenants. Easily paired with Pre-Approved Plans to simplify permit submittal and manage average cost.
• Studio Apartment: A compact secondary unit with an open floor plan, delivering efficient habitable living space (often ~400 sqft average) ideal for singles, couples, or college students.
• Home Office or Workspace: A dedicated separate living unit perfect for remote work or a home-based business, with built-in storage space and minimal distractions from the primary structure.
• Multigenerational Housing: A private suite for extended family—aging parents or adult children— balancing independence with proximity, often meeting owner-occupancy requirements and setback standards (e.g., rear yard setback).
• Vacation Rental Unit: A short-term rental ADU (e.g., Airbnb) that adds a reliable stream of income and maximizes ROI on your residential property, all while adhering to regional county regulations and building permit review fees.

Scenarios Where Detached ADUs Shine

• Rental Income: As legal dwelling units, detached cottages attract long-term rentals and short-term stays, delivering reliable additional income for homeowners in high-demand areas.
• Housing Family Members: Ideal for multigenerational living, these private living spaces let relatives stay close while enjoying their own residential living space.
• Guest Accommodations: A free-standing dwelling unit offers a dedicated space for out-of-town guests without disrupting your primary house routine.
• Future-Proofing Homes: With flexible layouts, these secondary units can transition into retirement homes or live-in caregiver suites as needs evolve.
• Creative Spaces: Perfect for art studios, music rooms, or workshops—converting nonliving spaces into inspiring dedicated spaces separated from the main home.
• Quarantine or Emergency Housing: Provides safe, independent lodging during health crises or renovations, thanks to compliant ADU construction under local building permit processes.

Design Considerations

• Embrace open floor plans and multi-functional furniture to maximize efficiency in your residential dwelling unit.
• Incorporate eco-friendly elements—solar panels, green roofs—to reduce your environmental footprint.
• Use landscaping, fences, or hedges to enhance privacy and meet minimum building separation requirements.

Pros

• Increased Privacy: These units offer more autonomy than attached ADUs—perfect for renters or family members seeking independence.
• Design Flexibility: Freedom in architectural design and interior finishes, unbound by your primary structure.
• Property Value Boost: Adding a fully independent unit significantly raises marketability and resale value.

Cons

• Higher Construction Costs: Construction ADUs from the ground up requires a larger investment than conversions.
• Longer Timeline: More complex planning, permitting, and build time compared to internal ADUs or garage conversions.

Attached ADUs

An attached ADU is a secondary housing option that shares at least one wall with the primary single-family dwelling. This effective option leverages the home’s existing structure to add additional housing options—from accessory apartments to ADUs Internal conversions—while keeping ADU cost and construction ADUs streamlined under local Accessory Dwelling Unit Programs (e.g., San Francisco’s amnesty program, Santa Cruz County pre-approved plans).

Attached ADUs blend with your main home’s architecture while adding valuable living space for family or renters.

Examples of Attached ADUs

• Garage Conversion: Repurpose a preexisting structure into a full residential dwelling unit.
• Basement Apartment: An internal conversion option that creates a self-contained suite ideal for 1–2 person households.
• Side or Rear Addition: A popular home renovation project that meets size maximums and setback rules.
• Over-the-Garage Unit: A Carriage house conversion or loft for extended family, friends, or moderate-income households.
• Integrated Home Office or Studio: A Studio ApartmentA–style workspace with separate entrance for professionals or college students.

Scenarios Where Attached ADUs Are Useful

• Multigenerational Living Housing aging parents or adult children while maintaining proximity and independence and providing a safe and accessible living space for elderly family members who need occasional care – expanding your property’s range of residences.
• Rental Income Renting out the unit to long-term tenants for supplemental income or using it as a short-term rental (e.g., Airbnb) in areas where local regulations permit.
• Caregiver Accommodation Housing live-in caregivers, such as nannies or healthcare providers, close to the main household.
• Guest Accommodations Offering a private space for visiting friends or family without disrupting the primary residence.
• Affordable Housing Solutions Providing affordable housing options for family members or tenants in areas with high housing demand.
• Home Office or Workspace Utilizing the attached ADU as a dedicated office or creative studio, separated from the distractions of the main home.
• Aging in Place Preparing for future needs by creating a smaller, accessible living space for homeowners to downsize while staying on their property.

Design Considerations

• Building ADUs: Follow a clear building permit process and submit a complete building permit application with your Property Information Map.
• Design Principles: Integrate seamless architectural design and design concept to match your home’s aesthetics.
• Size Requirements: Adhere to Maximum size limits, rear yard setback, and provide additional parking space if required.
• Partnerships for Housing Planning: Coordinate with regional entities and the Planning Department to ensure compliance.

Pros

• Cost-Effective Construction These units have lower costs than detached ADUs because they share walls, utilities, and sometimes existing infrastructure (e.g., plumbing and electrical systems).
• Space Efficiency Ideal for smaller lots where building a standalone structure isn’t feasible.
• Ease of Supervision Allows homeowners to oversee tenants or family members more quickly due to proximity.
• Blends with Main Home Can be designed to seamlessly integrate with the architecture of the primary residence, maintaining the overall aesthetic.

Cons

• Privacy Concerns Proximity to the main home may reduce privacy for occupants.
• Limited Design Flexibility Design must align with the structure of the existing home, which can limit customization options.
• Shared Utilities Attached ADUs often share utility systems with the main home, which might require upgrades or careful planning.

Garage Conversion ADUs

A garage conversion ADU repurposes a preexisting structure into a fully independent residential dwelling unit, complete with its own kitchen, bathroom, and sleeping area. As a popular option among single-family dwelling owners, it leverages existing utility connections and maximizes otherwise underused nonliving spaces—making it one of the most cost-effective forms of ADU construction.

Garage conversions are cost-effective solutions that repurpose existing spaces without expanding your property footprint.

Examples of Garage Conversion ADUs

• Studio Apartment A single-car garage (approximately 220 sq. ft.) converted into a compact studio apartment with an open layout. Features include a kitchenette, bathroom, and living/sleeping area.
• One-Bedroom Unit A two-car garage (approximately 400-500 sq. ft.) transformed into a one-bedroom unit with a separate bedroom, living room, kitchen, and bathroom.
• Home Office or Workspace A garage converted into a dedicated office or creative studio with workstations, storage, and a small kitchenette for convenience.
• Guest Suite A garage redesigned as a guest suite with a private entrance, bedroom, and bathroom for visiting family or friends.
• Income-Generating Rental A fully equipped rental unit designed for long-term tenants or short-term vacation rentals like Airbnb.

Scenarios Where Garage Conversion ADUs Are Useful

• Rental Income Homeowners in high-demand areas convert their garages into rental units to generate passive income. For example, a detached garage converted into a 550 sq. ft. ADU can significantly increase property value and provide consistent rental revenue.
• Affordable Housing Garage conversions offer affordable housing options for tenants or family members while helping combat housing shortages in California.
• Multigenerational Living Families use garage conversions to create private spaces for aging parents or adult children who need independence but want to remain close to the main household.
• Home-Based Business Entrepreneurs convert garages into offices or studios for startups, saving on commercial rental costs while maintaining separation from the main house.
• Guest Accommodations: A converted garage is a comfortable guest house for visitors without disrupting the primary residence’s routine.
• Maximizing Property Potential In areas with limited land or small lots, converting an existing garage avoids taking up additional yard space while adding functional living areas.

Design Considerations

• Maximizing Natural Light If setback regulations restrict windows, use glass sliders or French doors to bring in light.
• Efficient Layouts Incorporate open-plan designs with innovative storage solutions to maximize limited space.
• Eco-Friendly Features Add solar panels, energy-efficient appliances, and water-conserving landscaping to enhance sustainability.
• Blending with the Main House For aesthetic cohesion, match the architectural style of the ADU to the primary residence.
• Outdoor Enhancements Create pathways or small porches to provide privacy and curb appeal.

Pros

• Cost-Effective Compared to building new structures, garage conversions are more affordable since the foundation and walls already exist.
• Faster Construction Timelines Garage conversions can be completed more quickly than detached ADUs or home additions.
• Increased Property Value Garage conversions increase property value while offering flexibility in use, such as a rental unit, guest house, or workspace.

Cons

• Limited Space The size of the garage may limit the scope of the ADU, restricting design and functionality.
• Parking Adjustments Converting a garage may require alternative parking solutions, which could pose challenges in areas with limited street parking.
• Structural Constraints Existing garage layouts may require creative solutions to ensure the space is functional and comfortable.

Junior ADUs (JADUs)

A Junior Accessory Dwelling Unit (JADU) is a compact, self-contained living space (up to 500 sqft) created within an existing home. Ideal for 1–2 person households, JADUs offer an effective option for homeowners seeking affordable additional housing options without significant new construction—perfect as an option for college students or a house for friends.

A JADU is a great choice for cost-conscious homeowners looking to add value by maximizing underutilized interior spaces.

Examples of JADUs

• Converted Bedroom A spare bedroom is transformed into a JADU with a kitchenette, private entrance, and shared or private bathroom.
• Garage Conversion (Partial) A portion of an attached garage is converted into a JADU while retaining some garage space for storage or parking.
• Attic or Basement Conversion An unused attic or basement is repurposed into a compact living unit with its own entrance and efficient kitchen.
• Side Addition with Minimal Expansion A small expansion (up to 150 sq. ft.) is added to the home to create a separate entrance and ingress/egress for the JADU.
• Integrated Home Office A JADU is designed as a dual-purpose space that functions as both a home office and occasional guest accommodation.

Scenarios Where JADUs Are Useful

• Multigenerational Living Housing elderly parents or adult children in a semi-independent space while maintaining proximity to the main household. For example, a homeowner converts an unused bedroom into a JADU for an aging parent who needs occasional assistance but values privacy.
• Rental Income Renting out the JADU to generate supplemental income while keeping development costs low. For instance, a homeowner creates a JADU to rent to a student or single tenant.
• House-Hacking Strategy Living in the JADU while renting out the main house to maximize rental income. For example, a homeowner might move into the JADU and rent out their larger primary residence to cover mortgage payments.
• Guest Accommodations Providing a private space for visiting family members or friends without disrupting the main household. For example, a family converts part of their home into a JADU for frequent visits from out-of-town relatives.
• Home Office or Studio Using the JADU as a secluded workspace, studio, or creative area with minimal distractions. For example, a professional converts an unused bedroom into a JADU-style office with its entrance for meeting clients.
• Affordable Housing Solution Offering affordable housing options in high-demand areas by creating smaller, cost-efficient units. For example, homeowners provide low-cost rental housing by converting part of their home into a JADU.
• Combining ADUs and JADUs To maximize housing density, add both an ADU and a JADU to the same property. For instance, a homeowner builds a detached ADU in their backyard while converting part of their home into a JADU, creating three separate living spaces on one lot.

Design Considerations

• Maximizing Efficiency Use compact layouts and multi-functional furniture to maximize limited space.
• Soundproofing Ensure proper soundproofing between the JADU and the main home for privacy.
• Utility Integration JADUs often share utilities with the main house, reducing costs but requiring careful planning.
• Separate Access Include a private entrance to enhance the independence of the JADU.

Pros

• Affordability Lower construction costs than traditional ADUs due to shared utilities and minimal structural changes.
• Efficiency It makes use of underutilized spaces like bedrooms, garages, or attics without significantly expanding the property footprint.
• Regulatory Advantages Exempt from impact fees in many jurisdictions and often subject to simpler permitting processes.
• Flexibility It can be quickly reintegrated into the main home if it is no longer needed as an independent unit.

Cons

• Limited Size Restricted to a maximum of 500 square feet, which may not meet all space requirements.
• Shared Utilities Often relies on existing home utilities, which could limit functionality or require adjustments.
• Privacy Constraints Proximity to the main living area might reduce privacy compared to detached ADUs.

Internal ADUs

An Internal ADU (or room addition) transforms areas like basements, attics, or underused ground-floor rooms into legal, separate living units. Among the quickest and most budget-friendly building ADUs methods, this approach adds valuable livable space without expanding your property’s footprint – ideal for moderate-income households or properties on Multi-Family Lots.

Opt for an internal ADU to create affordable living options while minimizing construction impact and maximizing your existing space.

Examples of Internal ADUs

• Basement Conversion: Transform an unused basement into a self-contained rental unit with its own entrance separate from the main house, full kitchen, bathroom, and direct access to utilities.
• Attic Studio: Repurpose an attic into a cozy studio apartment (ideal for 1–2 person households) with skylights, compact kitchen appliances, and integrated storage.
• Ground Floor Renovation: Convert part of the main living area into a multigenerational living space—complete with kitchenette and private entry—while maintaining the flow of your existing home.

Scenarios Where Internal ADUs Are Useful

• Multigenerational Living: Offers semi-independent quarters for aging parents or young adults, keeping extended family close yet private.
• Rental Income: A cost-effective way for property owners to generate reliable income for property owners by leasing to long-term tenants or 1–2 person households.
• Guest Accommodations: Provides dedicated living quarters for visiting friends and family without encroaching on your primary residence’s routine.
• Affordable Housing Solutions: Maximizes existing square footage to meet demand under local zoning regulations, making it an accessible option in high-cost markets.

Design Considerations

• Smart Layouts: Use open-concept design to enhance perceived space and functionality.
• Lighting: Prioritize natural light where possible and add recessed fixtures to brighten basements or enclosed areas.
• Soundproofing: Incorporate insulation and acoustic barriers to ensure privacy between units.
• Accessibility: Design entrances, doorways, and fixtures to meet safety standards and accommodate residents of all abilities.

Pros

• Cost-Effective Minimal construction is required, as the existing structure is utilized.
• Quick to Complete Faster timelines compared to building detached or attached ADUs.
• Maximized Space Usage Makes efficient use of underutilized areas like basements and attics.

Cons

• Limited Privacy Proximity to the main living space may reduce independence.
• Shared Utilities Often shares plumbing, electrical, and HVAC systems with the main house, which may require careful planning.

Why Consider Building an ADU?

Adding an ADU to your single-family home or primary single- or multifamily dwelling unlocks a variety of forms—from Detached Accessory Dwelling Units to Basement ConversionA internal units—delivering affordable, independent living space and a free-standing dwelling unit option without sacrificing your main residence. Whether you’re a seasoned investor or new to ADU construction, the ABCs of ADUs make it easier than ever to expand your property’s potential.

Increased Property Value

ADUs are a valuable type of home improvement that ADUs increase overall marketability. Legal residential dwelling units boost curb appeal and appeal to a wider buyer pool, helping your property stand out among peers during a house review or appraisal.

Rental Income

Create a reliable stream of income for property owners by renting your ADU to long-term tenants, vacationers, or even 1-2 person households. This additional housing option provides consistent revenue with minimal management, making it a top choice for a project among property owners.

Additional Living Space

Perfect for multigenerational families, existing multi-family dwelling deliver privacy and flexibility—ideal as guest suites, home offices, hobby studios, or house for friends. From portions of conversion ADUs to full-scale detached cottages, you gain functional square footage without sacrificing yard space.

San Diego’s Supportive Regulations

San Diego leads the way in Types of ADUs allowed, offering streamlined building permit processes, amnesty programs, and zoning incentives that simplify compliance with requirements for ADUs added – making it the go-to region for hassle-free ADU development.

Proper planning is key to a successful ADU project—align your design with future needs to maximize ROI and functionality.

How to Choose the Right ADU for Your Property

Selecting the ideal ADU means balancing practical factors with your personal goals and making informed decisions. By assessing your lot’s dimensions, local rules, and budget, you’ll pick a form of housing that enhances your home’s value and utility.

Property Size and Layout

Review your site’s footprint and topography to determine if you can fit a new, free-standing dwelling unit or need an internal conversion. Consider existing driveways and setbacks—ADUs on lots may also affect additional parking space and service access for utility connections.

Zoning Laws

Local regulations for existing or proposed single-family and multi-family buildings govern your ADU’s maximum size, height limits, and rear yard setback. Research whether your municipality allows a wide range of ADU types—from detached cottages on Multi-Family Lots to garage conversions—to ensure compliance.

Intended Use

Define your ADU’s purpose: a rental for 1–2 person households, a private guest suite with a separate entrance, or a home office. Your choice will shape layout, finishes, and features—ultimately influencing income for property owners and long-term ROI.

Budget

Estimate costs per square foot across ADU types. New construction of a detached unit typically carries higher up-front expenses, while conversions of existing structures (basement, garage) offer a more budget-friendly type of housing option. Align your financial plan with the form that meets both your needs and your bottom line.

The right ADU depends on your goals—whether it’s rental income, family use, or property value. Assess lot size, budget, and long-term plans to make the best choice.