6 Types of ADUs to Consider: A Complete Guide for Homeowners

Looking to add extra living space to your property? Accessory Dwelling Units (ADUs) offer six distinct options to suit different needs and budgets. These include detached new construction (backyard cottages), home additions, garage conversions, basement conversions, second-story conversions, and junior ADUs within your main house.

Each type has unique features and requirements, making them suitable for different purposes – whether you’re planning to create a rental unit, accommodate family members, or add value to your property. Understanding these options is crucial for making an informed decision about which ADU type best fits your situation.

Type 1: Detached New Construction ADUs (Backyard Cottages)

Detached new construction ADUs, also known as backyard cottages or standalone structures, represent a popular choice for homeowners seeking to add a separate living unit to their single family residential lot. These structures function as independent living spaces, perfect for creating rental income opportunities or housing elderly parents. As a completely separate structure from your primary residence, these units offer maximum privacy and flexibility in design.

Design Options and Size Considerations

When planning a detached ADU, size typically ranges from a compact studio apartment layout to more spacious designs up to 1,200 square feet, depending on local ordinances. Design considerations must account for your property’s unique characteristics and local regulations. These units can be customized with various layouts:

• Single-story cottages with open floor plans
• Two-story laneway houses with separate bedrooms
• Compact granny flats with small environmental footprint
• Functional living spaces that maximize every square foot

Foundation and Utility Requirements

Building a detached ADU requires careful planning for utility connections and foundation work. Key requirements include:

• Proper foundation installation meeting building codes
• New utility connections for:
  • Water supply
  • Electrical service
  • Sewer lines
  • Gas lines (if needed)
• Separate meters for utilities
• Private entrance installation
• Adequate drainage systems

Creating these connections often involves significant excavation and coordination with local authorities to ensure compliance with regulations.

Estimated Costs and Timeline

The investment in a detached new construction ADU varies based on size, location, and design choices. Here’s a typical cost breakdown:

Timeline typically spans 4-8 months from permit approval to completion.

Type 2: Home Addition ADUs

Home addition ADUs represent an efficient way to create additional living space by expanding your existing home structure. These attached units share walls with the primary residence, making them more energy-efficient and cost-effective regarding utility connections. This option is particularly popular for creating in-law suites or accommodating adult children.

Common Placement Options

The most effective placement options for home addition ADUs include:

• Side yard expansions
• Rear home additions
• Wrap-around additions
• Converting existing spaces into extended living areas

Each placement option must consider setback requirements, yard space availability, and impact on the existing home’s structure.

Structural Requirements

Adding an ADU to your existing home demands careful attention to structural integrity:

• Foundation reinforcement or expansion
• Load-bearing wall considerations
• Roof tie-in requirements
• Proper insulation between shared walls
• Compliance with current building codes
• Seismic requirements (especially important in areas like San Francisco)

Connection to Main House Utilities

One advantage of attached ADUs is the ability to tie into existing utility systems, though upgrades may be necessary:

• Electrical panel capacity assessment
• Water heater sizing evaluation
• HVAC system modifications
• Shared or separate meter options
• Integration with existing plumbing
• Potential need for system upgrades

These units typically require less investment in utility infrastructure compared to detached ADUs, making them an attractive option for affordable housing solutions. The shared walls and existing structure integration often result in less energy consumption and lower overall construction costs.

Type 3: Garage Conversion ADUs

Garage conversion ADUs represent one of the most cost-effective ways to create additional living space from existing garage space. This type of conversion has become increasingly popular in residential areas as adus provide homeowners with versatile options for long term rentals or accommodating young adults. Converting an existing garage into a secondary housing unit can significantly increase property value while offering affordable housing options.

Converting Attached vs Detached Garages

The approach to conversion varies depending on whether you’re working with an attached or detached garage:

Attached Garage Benefits:

• Easier utility connections
• Lower conversion costs
• Direct access to main house
• Simpler permit process
• More efficient heating/cooling

Detached Garage Benefits:

• Greater privacy
• More design flexibility
• Independent living space
• Better suited for short term rentals
• Separate entrance options

Required Modifications

Converting garage space into livable space requires several essential modifications:

Structural Changes:

• Raising the floor level
• Installing proper insulation
• Adding windows for natural light
• Upgrading walls to living space standards
• Installing proper ventilation

Utility Updates:

• Electrical system upgrade
• Plumbing installation
• HVAC integration
• Internet and cable connections
• Water heater addition

Parking Considerations

When converting a garage, parking requirements become a crucial factor:

• Check local ordinances for minimum parking requirements
• Consider creating alternative parking spaces
• Evaluate street parking availability
• Plan for covered parking solutions if needed
• Account for impact on property accessibility

Type 4: Basement Conversion ADUs

Converting a basement into an accessory unit offers an excellent opportunity to add a basement apartment while maximizing existing space in your single family home. This type of adu development is particularly attractive for creating internal adus that can serve as in-law units or generate rental income through long term rentals.

Safety Requirements

Basement conversions must meet strict safety standards:

Essential Safety Features:

• Egress windows in bedrooms
• Fire-rated walls and ceilings
• Smoke and carbon monoxide detectors
• Multiple escape routes
• Proper ventilation systems

Building Code Requirements:

• Minimum ceiling height (typically 7 feet)
• Adequate natural light
• Fire-resistant materials
• Electrical safety measures
• Emergency lighting

Moisture Control Solutions

Controlling moisture is crucial for basement conversion ADUs:

Prevention Methods:

• Professional waterproofing
• Proper drainage systems
• Dehumidification equipment
• Vapor barriers
• Mold-resistant materials

Treatment Areas:

• Foundation walls
• Floor systems
• Window wells
• Entry points
• Utility penetrations

Access and Entry Options

Creating appropriate access points is vital for basement ADUs:

Entry Configurations:

• Separate external entrance
• Interior stairway access
• Walkout basement option
• Egress window wells
• Private entrance pathways

Design Considerations:

• ADA accessibility requirements
• Privacy for both units
• Emergency escape routes
• Natural light optimization
• Security measures

Cost Comparison Table:

These conversion projects typically require a building permit and must comply with local building codes to ensure safety and habitability standards are met.

Type 5: Second-Story ADUs

Second-story ADUs represent a smart solution for homeowners looking to maximize their property value without sacrificing ground-level space. This type of adu development works particularly well in residential areas where lot sizes are limited. Converting upper levels into livable space can create excellent opportunities for long term rentals or provide comfortable accommodations for young adults while maintaining the existing footprint of your single family home.

Attic vs Existing Space Conversions

Converting attic space into an ADU typically requires more extensive modifications compared to existing second-floor space. Attic conversions often need substantial structural changes to create adequate headroom and meet building codes. However, existing second-floor space may already have proper ceiling heights and basic infrastructure in place. The choice between these options largely depends on your home’s current configuration and local building regulations.

Cost Comparison:

Stairway and Access Requirements

Access considerations are crucial for second-story ADUs as they directly impact safety and convenience. Internal staircases must meet specific code requirements for width, rise, and run measurements. External staircases may be necessary for separate entry options, particularly when the unit is intended for short term rentals or as a guest house. The stairway design must accommodate emergency egress requirements while maintaining the aesthetic appeal of your home.

Primary Requirements:

• Minimum stair width of 36 inches
• Maximum riser height of 7.75 inches
• Minimum tread depth of 10 inches
• Proper lighting throughout stairwell
• Secure handrails on both sides

Structural Support Needs

Creating a second-story ADU requires careful evaluation of the existing structure’s ability to support additional weight and living space. Load-bearing walls, floor joists, and foundation elements may need reinforcement to ensure safety and stability. Professional engineering assessment is essential to determine necessary modifications and ensure compliance with local building codes.

Type 6: Junior ADUs (Inside Main House)

Junior ADUs offer an affordable housing option by converting a portion of your existing home into a separate living unit. These internal adus are typically smaller than other ADU types and often share some facilities with the main house. This approach to adu development can significantly increase property utility while maintaining the original character of your single family home.

Space Planning Requirements

Effective space planning is critical for junior ADUs to function as independent living quarters while maximizing available square footage. The layout must accommodate essential living functions within a compact area, typically ranging from 150 to 500 square feet. The design should create clear boundaries between the main house and the accessory unit while maintaining efficient flow and functionality.

Minimum Space Allocations:

Shared vs Private Facilities

Junior ADUs often utilize a hybrid approach to facility sharing, which can help reduce construction costs and maximize space efficiency. Common arrangements include shared laundry facilities or outdoor spaces while maintaining private sleeping and living areas. The decision between shared and private facilities should balance privacy needs with practical considerations and local regulations.

Typically Shared Facilities:

• Laundry rooms
• Outdoor spaces
• Main entrance
• Utility systems
• Storage areas

Private Facilities Often Include:

• Sleeping quarters
• Living space
• Kitchenette
• Personal storage
• Secondary entrance

Entry Options

Creating appropriate entry points for junior ADUs requires careful consideration of both practical and regulatory requirements. While some jurisdictions require separate external access, others allow internal connections between the main house and the ADU. The entry design should provide adequate privacy while meeting safety requirements and maintaining the home’s architectural integrity.

Common entry configurations include modified side entrances, repurposed secondary doors, or dedicated internal access points. The chosen solution should accommodate both daily use and emergency egress requirements while considering the comfort of all residents.

Choosing the Right ADU Type for Your Property

Selecting the most suitable ADU type requires careful consideration of various factors that will impact both the construction process and long-term satisfaction with your investment. This decision should align with your property’s characteristics, financial resources, and intended purpose. Taking time to evaluate these aspects will help ensure your ADU project meets your needs while maximizing property value.

Space Available on Your Lot

The available space on your property largely determines which ADU types are feasible. A large backyard might accommodate a detached ADU, while limited outdoor space might direct you toward garage conversion or basement options. Consider setback requirements, existing structures, and yard preservation when evaluating space. Measure potential areas carefully and account for construction access needs. Remember that some municipalities require dedicated parking spaces in addition to the ADU footprint.

Budget Considerations

ADU costs vary significantly based on type and scope. Here’s a general cost comparison:

Factor in ongoing costs such as maintenance, utilities, and potential property tax increases. Consider financing options like home equity loans or construction loans early in the planning process.

Privacy Needs

Privacy requirements often influence ADU design and placement decisions. A detached unit offers maximum privacy for both main house residents and ADU occupants. Conversions and attached units require thoughtful design elements like sound insulation, separate entrances, and strategic window placement. Consider how different ADU types might impact daily routines and household dynamics.

Intended Use (Rental vs Family)

Your planned use for the ADU should guide many design decisions. Long-term rentals might prioritize separate entrances and independent utilities, while family accommodations might benefit from more integrated designs. Consider future flexibility – an ADU that works for aging parents today might serve as a rental unit later. Think about features that would appeal to your intended occupants while maintaining resale value.

Getting Started with Your ADU Project

Beginning an ADU project requires careful planning and coordination with various professionals and regulatory bodies. Understanding the initial steps and requirements will help ensure a smoother development process and better final results.

Checking Local Rules

Start by thoroughly researching local zoning laws and building requirements. Many municipalities have specific regulations regarding:

Development Standards:

• Maximum ADU size and height
• Minimum setback requirements
• Parking requirements
• Design compatibility with main residence
• Utility connection standards

The approval process typically involves multiple departments and may require neighborhood notifications. Working with a local planning professional can help navigate these requirements effectively.

Finding the Right Contractors

Selecting qualified professionals is crucial for project success. Look for contractors with specific ADU experience and valid licenses. Request detailed proposals that include:

Schedule consultations with multiple contractors to compare approaches and ensure competitive pricing.

Understanding the Building Process

The ADU construction process typically follows these phases:

Phase 1: Planning and Design (2-4 months)

• Initial concept development
• Architectural drawings
• Engineering plans
• Permit applications

Phase 2: Approvals (1-3 months)

• Plan review
• Permit issuance
• Utility coordination
• Construction scheduling

Phase 3: Construction (3-8 months)

• Site preparation
• Foundation work
• Structural construction
• Interior finishing
• Final inspections

Understanding this timeline helps set realistic expectations and allows proper planning for temporary arrangements during construction. Regular communication with your contractor about progress and potential challenges helps maintain project momentum and address issues promptly.