Types of ADUs: A Comprehensive Guide

What is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary housing unit located on the same property as a primary residence. It is fully equipped for independent living, typically including a kitchen, bathroom, sleeping area, and a separate entrance. ADUs are also known by various names, such as granny flats, in-law suites, or backyard cottages.

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 (standalone structures), attached (connected to the main home), or converted from existing spaces like garages, basements, or attics.
• Purpose They serve various purposes, including housing family members, generating rental income (long-term or short-term), or providing affordable housing options.
• Ownership ADUs cannot be sold separately from the main property and typically share utility connections with the primary home.
• Regulations ADUs must comply with local zoning laws and building codes, which vary by location.

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 standalone housing unit built on the same property as the primary residence but physically separate from it. These units are versatile and can serve various purposes based on the homeowner’s needs. Detached ADUs are ideal for those seeking maximum privacy and flexibility, whether for rental purposes, family housing, or creative use.

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

Examples of Detached ADUs

• Backyard Cottage A small, independent living space designed as a rental unit or guest accommodation. It typically includes a bedroom, bathroom, kitchen, and living area.
• Studio Apartment A compact unit with an open floor plan, ideal for single occupants or couples. This could be used for long-term rentals or as an artist’s studio.
• Home Office or Workspace A quiet, separate area designed for remote work or running a home-based business, free from the distractions of the main house.
• Multigenerational Housing A private living space for aging parents or adult children, offering independence while staying close to family.
• Vacation Rental Unit A detached ADU designed for short-term rentals, such as Airbnb, providing additional income for homeowners.

Scenarios Where Detached ADUs Shine

• Rental Income Detached ADUs are highly attractive to renters due to their privacy and independence. They can generate significant rental income, especially in high-demand areas.
• Housing Family Members Perfect for multigenerational living, they provide a private space for relatives while maintaining proximity to the main household.
• Guest Accommodations Ideal for hosting out-of-town guests without disrupting the main household’s routine.
• Future-Proofing Homes Detached ADUs can adapt to changing needs over time, such as becoming retirement homes for the owners or spaces for live-in caregivers.
• Creative Spaces They can be used as art studios, music rooms, or workshops, offering a dedicated area away from the primary residence.
• Quarantine or Emergency Housing Detached ADUs provide safe and independent living spaces in unforeseen circumstances, such as health crises or home renovations.

Design Considerations

• Detached ADUs often feature open floor plans and multi-functional furniture to maximize space efficiency.
• They can include eco-friendly features like solar panels or green roofs to enhance sustainability.
• Landscaping and privacy measures (e.g., fences or hedges) can create a distinct separation from the main house.

Pros

• Increased Privacy These units offer more autonomy than attached ADUs, making them ideal for renters or family members seeking independent living.
• Design Flexibility Greater freedom in architectural and interior design since it doesn’t need to match or integrate with the main house.
• Property Value Boost Adding a fully independent unit substantially increases the marketability and resale value of your property.

Cons

• Higher Construction Costs Building a standalone unit often involves significant investment compared to other ADU types.
• Longer Construction Timeline Due to its complexity, it requires more time for planning, permitting, and construction.

Attached ADUs

An attached ADU is a secondary housing unit that shares at least one wall with the main residence, offering flexibility in design and functionality. These units provide a cost-effective way to add independent living spaces while utilizing the primary home’s existing structure.

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

Examples of Attached ADUs

• Garage Conversion Transforming an existing garage into a fully functional living space with a bedroom, bathroom, and kitchen.
• Basement Apartment Converting an unused basement into a self-contained unit for rental income or family use.
• Side or Rear Addition Building an extension to the side or back of the house that serves as an independent living space with its own entrance.
• Over-the-Garage Unit Constructing a living space above an existing garage, ideal for properties with limited land area.
• Integrated Home Office or Studio Creating a professional workspace or studio within the home but separate from the main living areas.

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.
• 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

• Attached ADUs share walls with the main house, which simplifies utility connections and can reduce construction costs.
• Seamless integration with the architecture of the primary residence ensures aesthetic cohesion.
• Privacy features, such as soundproofing and separate entrances, can enhance functionality for both the main house and the ADU.

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 transforms an existing garage into a fully functional, independent living space. This type of ADU is a popular choice due to its cost-effectiveness, efficient use of space, and versatility. Homeowners can repurpose underutilized garage areas to create valuable additional living space without significant new 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 small, self-contained living space of up to 500 square feet created within the existing structure of a single-family home. JADUs are cost-effective and flexible housing solutions, ideal for homeowners who maximize their property’s utility without major structural changes. They are designed to provide independent living features while minimizing the need for additional construction.

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) refers to a secondary living space created by converting an existing section of a home, such as a basement, attic, or unused ground floor area, into an independent housing unit. Internal ADUs are among the most cost-effective and quickest options for homeowners who want to add functional living spaces without expanding their property footprint.

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 A homeowner transforms an unused basement into a rental unit featuring a separate entrance, living area, kitchen, and bathroom.
• Attic Studio An attic is repurposed into a cozy, independent studio apartment with skylights, compact kitchen appliances, and a sleeping area.
• Ground Floor Renovation Part of the leading home’s ground floor is converted into a multigenerational living space with a private entrance and kitchenette.

Scenarios Where Internal ADUs Are Useful

• Multigenerational Living Families convert attics or basements to create semi-independent spaces for aging parents or young adults, ensuring proximity while maintaining privacy.
• Rental Income Internal ADUs are ideal rental units, offering tenants affordability and homeowners a steady supplemental income.
• Guest Accommodations A dedicated, private area within the home ensures visiting guests have comfortable living quarters without impacting the homeowner’s daily routine.
• Affordable Housing Solutions In areas with limited housing availability, Internal ADUs provide cost-effective living options while maximizing the use of existing home structures.

Design Considerations

• Smart Layouts Incorporate open-concept designs to make compact spaces feel larger and more functional.
• Lighting Add ample lighting, such as recessed or natural light, especially in basements or enclosed spaces.
• Soundproofing Use insulation and soundproofing techniques to enhance privacy between the main house and the ADU.
• Accessibility Ensure entrances and amenities meet safety and accessibility standards, particularly for elderly or differently-abled occupants.

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?

Building an ADU comes with a wide range of benefits that cater to the needs of homeowners, families, and investors alike. Whether you aim to increase your property’s value, generate rental income, or create a functional living space, ADUs are a versatile and rewarding addition to any property.

Increased Property Value

ADUs significantly enhance the marketability and value of your property. They are a highly desirable feature for potential buyers, offering versatile and functional living spaces that cater to a variety of needs.

Rental Income

Homeowners can create a steady stream of rental revenue by building an ADU. ADUs are ideal for long-term tenants, vacation rentals, or even short-term leases, providing consistent income opportunities.

Additional Living Space

ADUs are perfect for multigenerational families, offering privacy and independence for elderly parents or adult children. They also serve as excellent guest accommodations, workspaces, or hobby areas.

San Diego’s Supportive Regulations

San Diego’s favorable ADU policies make construction more accessible than ever. Homeowners benefit from streamlined permitting processes, incentives, and zoning flexibility that encourage 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

Choosing the right ADU for your property involves a combination of practical considerations and personal goals. By evaluating key factors such as property size, local regulations, and budget, you can make an informed decision that aligns with your vision and maximizes the value of your investment.

Property Size and Layout

The size and configuration of your property play a critical role in determining which type of ADU is best suited for your needs. Ensure there is adequate space for construction and proper access to utilities.

Zoning Laws

Local zoning regulations dictate the types of ADUs permitted in your area. Research these rules to ensure your project complies with size limits, setbacks, and other requirements.

Intended Use

Clearly define the purpose of your ADU. Whether you’re building it for rental income, family use, or as a home office, the intended use will influence design, layout, and functionality.

Budget

Different ADU types come with varying costs. Detached units tend to be more expensive, while garage conversions and internal ADUs are more budget-friendly. Align your choice with your financial plan and long-term goals.

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.