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Understanding ADU Laws: A Comprehensive Guide

ByRoof and Realm
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Accessory Dwelling Units (ADUs), commonly referred to as granny flats, in-law suites, or backyard cottages, have become a cornerstone of modern housing solutions. As states and cities across the U.S. face rising housing demands, ADUs provide an affordable, sustainable, and versatile way to add living space. However, navigating ADU laws can be complex, as regulations vary widely depending on location. Here’s an in-depth look at ADU laws, their implications, and what you need to know before having one built.

What Are ADUs?

An ADU is a secondary housing unit on a single-family or multi-family residential lot. These units can be detached (stand-alone structure), attached (connected to the main house), or internal (converted spaces like basements or garages). ADUs are designed to provide independent living facilities, including a kitchen, bathroom, and sleeping area.

Why ADU Laws Matter

ADU laws dictate where and how these units can be built. They balance housing development with local concerns such as zoning, density, and infrastructure. Understanding these laws ensures compliance and helps streamline the construction process.

Key Components of ADU Laws

1. Zoning Regulations (SB9)

  • Residential Zoning: Most ADUs are permitted in single-family and multi-family zones. Some cities are expanding allowances to more zones to increase housing supply.
  •  Lot Size: Many jurisdictions require a minimum lot size for ADU construction, though some have eliminated this restriction to encourage development.

2. Size Limits

California state law and local ordinances establish size and design standards for accessory dwelling units (ADUs). Here are the key regulations:

• Minimum Size: An ADU must have a minimum living area of 150 sq ft.
• Maximum Size:

  • 800 sq ft: The maximum size allowed on most residential properties.
  • 850 sq ft: For a one-bedroom ADU.
  • 1,000 sq ft: For ADUs with two or more bedrooms.
  • 1,200 sq ft: The absolute maximum, subject to local rules.
  • 50% of the main residence’s floor area: For attached ADUs.

• Height Limits:

  • Single-story ADUs are generally capped at 16 ft, but local ordinances may allow greater heights for two-story ADUs.

• Setbacks:

  • Side and rear setbacks must be at least 4 ft.

• Building Code Requirements:

  • Bedrooms and living rooms: At least 70 sq ft.
  • Kitchen: At least 50 sq ft.
  • Main bathroom: At least 30 sq ft.

• Converted Structures:

  • No size restrictions apply to ADUs created by converting existing structures like garages or barns.

While local agencies can adopt additional restrictions, they cannot prohibit the creation of an ADU up to 800 sq ft.

3. Parking Requirements

  •  Many ADU laws mandate additional parking spaces for the unit. However, in areas near public transit, cities often waive parking requirements to promote accessibility and reduce costs.

4. Owner-Occupancy Rules

  • Some jurisdictions require the property owner to live in either the main house or the ADU. However, states like California have recently eliminated this requirement to encourage ADU development.

AB-1033: Key Provisions and Impacts

  • Separate Sale of ADUs:
    ADUs can now be sold as independent housing units, but only if local municipalities adopt ordinances enabling this option.
  • Local Government Authority:
    AB-1033 is permissive, not mandatory, allowing cities and counties to decide whether to implement these rules. As a result, adoption and application may differ across California.
  • Expanded Investment Opportunities:
    This law removes previous restrictions, enabling private developers and homeowners to respond to the growing demand for housing by selling ADUs independently. However, adherence to both local ordinances and state requirements is essential.

5. Building Codes and Permits (SB897)

  • ADUs must comply with local building codes for safety and habitability, including plumbing, electrical, and structural standards.
  • A permit is typically required, and jurisdictions may streamline the process to support ADU construction.
  • California Title 24 is the state’s building code that sets standards for the design, construction, and maintenance of buildings to ensure safety, energy efficiency, accessibility, and environmental sustainability. It includes regulations for structural safety, fire protection, plumbing, electrical systems, energy conservation, and green building practices. Title 24 is updated every three years to reflect new technologies and environmental goals, making it a critical guide for architects, builders, and property owners in California.

6. Utility Connections

  •  ADUs may need separate utility connections, which can add to construction costs. Some laws allow shared connections to reduce expenses.

7. Environmental and Aesthetic Considerations

  •  Certain areas enforce design standards to ensure ADUs complement the neighborhood’s character.
  •  Environmental impact assessments may be required, especially in regions with specific ecological concerns.

State-Specific Highlights

California

California is a national leader in ADU-friendly legislation. Key features include:

  • No minimum lot size requirements.
  • Streamlined permit approvals within 60 days.
  • Parking exemptions for ADUs near public transit.

Oregon

Oregon allows ADUs in all residential zones and has removed owner-occupancy requirements, making it easier for homeowners to rent out both units.

Washington

Washington recently passed statewide legislation encouraging ADUs to address housing shortages, with relaxed parking and size regulations.

Benefits of ADU Laws

  • Increased Housing Supply: ADUs help address housing shortages, particularly in high-demand urban areas.
  • Affordability: They provide more affordable housing options for renters and extra income for homeowners.
  • Sustainability: ADUs promote efficient land use and reduce urban sprawl.
  • Flexibility: Homeowners can use ADUs for family members, rental income, or personal workspaces.

Challenges and Criticism

  • Cost of Construction: Despite supportive laws, building an ADU can be expensive, often ranging from $100,000 to $400,000.
  • Community Pushback: Some residents oppose ADU development, citing concerns about density and neighborhood character.
  • Complex Regulations: Navigating varying local requirements can be daunting without professional guidance.

Tips for Navigating ADU Laws

  • 1. Research Local Regulations: Start by reviewing your city or county’s ADU policies.
  • 2. Consult Professionals: Work with architects, contractors, or legal experts familiar with ADU laws.
  • 3. Plan for Costs: Consider construction expenses, permit fees, and potential utility upgrades.
  • 4. Engage with Neighbors: Open communication can ease concerns and foster support for your project.

ADU laws are evolving to meet modern housing challenges, offering homeowners opportunities to enhance property value and contribute to affordable housing solutions. By understanding and adhering to local regulations, you can navigate the process with confidence and create a functional, compliant, and sustainable living space.