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Accessory Dwelling Units (ADUs) in Marin County

Detached ADUs Remodels
 Marin County Contractor

Accessory dwelling units (ADUs), or granny flats, in-law suites, or backyard cottages, are self-contained living spaces on the same property as a single-family home. They are rapidly gaining popularity across the United States as more homeowners recognize their versatility and practicality.

ADUs offer a multitude of benefits:

  • Extra living space – An ADU creates more room for relatives, guests, aging parents, or anyone who needs a separate space. It can be used as a rental unit or office space as well.
  • Housing flexibility – An ADU provides flexible housing options, allowing multigenerational families to live together while maintaining privacy.
  • Property value increase – Adding an ADU can increase your property value. ADUs have desirable features that appeal to potential homebuyers.
  • Additional income – Renting out ADUs can provide homeowners with extra income, which can help offset housing costs.
  • Efficient use of space – ADUs make efficient use of space, especially in areas with limited land. Empty nesters can avoid downsizing by building an ADU.
  • Environmental benefits – ADUs promote sustainable development by using existing infrastructure more efficiently. This reduces urban sprawl.

In the following sections, we’ll explore ADUs in more depth – different types, costs, design considerations, regulations, and how to find the right builder to bring your ADU dreams to life.

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Types of ADUs

When considering adding an ADU to your property, one of the first decisions is whether you want a detached or attached unit. Here’s an overview of the differences:

Detached ADUs

A detached ADU, sometimes called a guest house or backyard cottage, is a completely separate structure from the main house. It has its own foundation, walls, and roof.

Benefits of detached ADUs:

  • More privacy – Detached ADUs feel like fully separate living spaces.
  • Flexible location – You can build a detached ADU wherever there is space on your lot, even at the back or side yards.
  • Custom design – Detached units can be designed in any architectural style.
  • No shared walls – Avoid noise transfer issues between the ADU and the main home.

Potential downsides:

  • Higher cost – Building a fully separate structure is more expensive than converting existing space.
  • Further from utilities – Electrical, plumbing, and other utilities may need to be extended.

Attached ADUs

Attached ADUs, also known as accessory dwelling units, are integrated into the existing home. They can be built by converting a portion of the main house or adding an addition.

Benefits of attached ADUs:

  • Lower cost – Making use of existing infrastructure reduces expenses.
  • Closer access – The ADU can be directly connected to the main living spaces.
  • Shared yard – The outdoor space can be easily shared.

Potential downsides:

  • Less privacy – Attached units may lack a separate entrance or feel less like separate spaces.
  • Limited locations – You can only build extensions where zoning allows.
  • Noise transfer – Shared walls and floors make sound transmission more likely.

The right option depends on your budget, space constraints, and privacy preferences. Our team of ADU experts can help you determine which type of unit best suits your property and needs.

ADU Design

When designing an ADU, there are several key factors to consider:

  • Compatibility with Main House – The ADU should complement the aesthetic of the existing home rather than clash with it. Using similar materials, colors, and architectural details will create cohesion.
  • Privacy – Careful placement and window positioning can maximize privacy between the main house and ADU. Strategic landscaping or fencing can also help delineate private outdoor spaces.
  • Access – Determine how the ADU will be accessed – through the main house, a separate entrance, or a detached structure. This impacts layout and flow.
  • Purpose – Will the ADU be used for rental income, multi-generational living, guests, or storage? The intended use will inform what spaces and amenities to include.
  • Zoning Regulations – Local zoning ordinances dictate ADU size limits, setbacks, height allowances, and more. Your design must adhere to these codes.
  • Site Constraints – Available space, trees, slope, orientation, easements, and utilities will influence ADU size, placement, and shape. Design within your property’s unique parameters.
  • Future Flexibility – Build flexiblely to potentially convert the ADU into a primary dwelling, office, or other use. An open layout supports multi-functionality.

By carefully weighing all of these factors, you can create a practical and aesthetic ADU design.

Permitting and Regulations

Constructing an ADU requires obtaining permits from your local city or county planning department. The permitting process ensures your ADU complies with zoning ordinances and building codes. Here’s an overview of the permits typically required:

  • Planning Permit: This permit allows your property to have an ADU and ensures it meets zoning requirements like setbacks, height limits, and maximum unit size. Planning staff will review your ADU plans and issue this permit first.
  • Building Permit: You’ll need a building permit to construct the ADU. This permit requires submitting detailed construction plans showing structural, electrical, plumbing, and other details. Building inspectors will inspect your project at various stages.
  • Encroachment Permit: If building the ADU requires working in the public right-of-way, like widening your driveway, you’ll need an encroachment permit from the city or county.
  • Fire Safety Permit: If your property is in a fire hazard zone, you may need a permit for fire-resistant construction techniques, access roads, vegetation clearing, and water storage tanks.

The permitting process ensures your ADU complies with local codes and regulations. Your contractor should advise you on permits and submit the planning and building permit applications. The permit process typically takes 1-3 months depending on the jurisdiction.

Site Evaluation

Assessing your property’s potential for an ADU is an important first step before proceeding with the design and construction process. Our team of experienced builders will conduct a comprehensive site evaluation, analyzing key factors that impact ADU feasibility and planning.

We examine property setbacks, grading, drainage, existing structures, vegetation, easements, and utilities to determine the optimal location and size of the ADU. Careful site analysis allows us to maximize the ADU square footage while working within zoning regulations and property constraints.

Understanding the terrain and contours of your land is crucial. We evaluate the slope and topography, looking at elevations and areas requiring minimal grading. Minimizing grading helps reduce site work costs and environmental impact.

The site evaluation also looks at accessibility. We determine the best access point to the ADU, considering pathways from the street and connections to the main house. Optimizing accessibility facilitates construction and enhances the ADU’s functionality.

Existing structures and vegetation on your property also impact ADU development. Our team will identify how to best situate the ADU around mature trees, gardens, pools, or other built features you want to preserve. Integrating the ADU into your property is our specialty.

A meticulous site analysis ensures we can develop an ADU design tailored to your unique property. Our feasibility studies determine ADU potential so we can maximize livable space and create a seamless ADU-to-home flow.

ADU Floor Plans

The floor plan is one of the most important elements in designing your ADU. It determines how the space will function and flow. Here are some key aspects to consider for your ADU floor plan:

  • Entryways – The entryway to the ADU can be separate from the main home or connected through an existing doorway. Consider accessibility and privacy.
  • Main living area – This is typically an open-concept kitchen, dining and living room. Consider how this space will be used day-to-day.
  • Bedrooms – Determine how many bedrooms are needed and the ideal size. Include closets for storage.
  • Bathrooms – Include a full bathroom with a shower at minimum. Consider adding a half bath if there are multiple bedrooms.
  • Utility connections – Plan to connect utilities like electricity, plumbing, heating, and AC to the main home.
  • Outdoor access – Incorporate doors leading outside from the main rooms. This expands the living space.
  • Storage – Add storage closets and cabinets throughout. This maximizes usable space.
  • Multi-purpose rooms – An extra room can be a home office, gym, workshop, or guest room.
  • Natural light – Position windows to let in ample natural light. Skylights are another option.

Carefully considering the floor plan is crucial for creating a functional and livable ADU that is suited to your needs. An experienced ADU builder can help optimize the layout.

ADU Materials

When constructing an ADU, choosing the right materials is crucial to ensure durability, aesthetics, and integration with your existing home. Here are some of the most common materials used for ADUs and their key pros and cons:

Wood Framing


  • Cost-effective
  • Flexible for design
  • Easy to work with
  • Renewable resource


  • Less durable than other materials
  • Risk of rotting or attracting pests if not properly protected
  • Generally requires additional siding material

Steel Framing


  • Very strong and durable
  • Termite/pest resistant
  • Non-combustible
  • Consistent quality


  • More expensive than wood
  • Conducts heat (needs insulation)
  • Can be complicated to modify

Concrete Masonry Units (CMUs)


  • Extremely durable
  • Fire resistant
  • Pest/mold resistant
  • Good thermal mass


  • Heavy, requiring a deep foundation
  • Can be cold and damp if not insulated
  • Difficult to modify after construction

Structural Insulated Panels (SIPs)


  • Very energy efficient
  • Fast installation
  • Consistent quality
  • Minimal waste


  • Inflexible for complex designs
  • Expensive
  • Requires precision in installation

Brick and Stone Masonry


  • Attractive aesthetic
  • Very durable
  • Fire resistant
  • Low maintenance


  • Expensive
  • Heavy, requiring a robust foundation
  • Masonry work is labor-intensive
  • Difficult to modify after construction

Factors like budget, climate, design, and integration with the main house should be considered when selecting ADU materials. Wood framing offers flexibility at a lower cost but needs protection. Steel and masonry are extremely durable options but come at a higher price. Consulting an experienced builder is key to choosing the right materials for your ADU.

ADU Amenities

When designing your ADU, you’ll want to consider what amenities to include to enhance the living experience for your tenants or family members. Here are some of the must-have amenities to think about:

Kitchen – Include basic amenities like a sink, refrigerator, stove/oven, cabinets, and countertops. Consider adding extras like a dishwasher, microwave, pantry, and breakfast bar. Make sure there is adequate space for food prep and dining.

Bathroom – Include at least one full bathroom with a shower/tub, toilet, and sink. Add extras like tile floors, recessed lighting, and heated floors. Proper ventilation is a must.

Living Area – Allow a minimum of space for a couch, chairs, TV, and coffee table. Built-ins like bookshelves, desks, or Murphy beds can optimize the living space. Consider how to make the space multifunctional.

Bedroom – At a minimum, allow space for a bed, nightstand, and dresser. Consider extras like a walk-in closet, blackout curtains, and in-wall storage. Think about including space for a desk or lounging chair.

Laundry – Having a washer/dryer in the unit adds a lot of convenience. A laundry closet with hookups works if space is tight, and a stacking unit can save space.

Outdoor Space – If possible, incorporate outdoor space, such as a small deck, patio, or balcony, for tenants to enjoy. This adds livability and appeal.

Storage – Add closets, cabinets, and shelving to maximize storage space. Tenants will appreciate having places to keep their belongings organized.

Smart Home Features – Consider adding smart home amenities like programmable thermostats, smart locks, and wireless speakers. These add modern convenience.

Utilities – Make sure to meter utilities like electricity and water separately. High-speed internet is also a must-have nowadays.

ADU Costs

The cost of building an ADU can vary significantly based on several factors. Some key considerations that impact ADU costs include:

  • Size – The total square footage of the ADU is a major cost driver. Larger ADUs with more bedrooms, bathrooms, and amenities will be more expensive. Smaller, simpler ADUs can cost much less.
  • Type – Detached ADUs that are entirely separate structures tend to cost more than attached ADUs that are part of the existing home. Conversion ADUs and internal ADUs that use existing space also tend to be more affordable.
  • Location – Construction costs can vary greatly by region and even neighborhood. ADUs built in high-cost, dense urban areas will generally cost more.
  • Materials – Using higher-end finishes and luxury materials will increase costs. Opting for more affordable siding, flooring, cabinetry, and fixtures reduces costs.
  • Amenities – Adding a full kitchen, washer/dryer, HVAC system, and higher-end appliances increases costs. Keeping amenities simple and minimal is a good way to control costs.
  • Labor – Hiring a general contractor usually costs more than managing the project yourself. However, DIY projects take substantially more time and coordination.
  • Permitting – Navigating the permitting and regulatory requirements can add soft costs. Most areas have fees for plan review, permits, and inspections.
  • Foundation – ADUs that require a new foundation will be pricier. However, an existing slab converted to a garage or a building above a garage can eliminate foundation costs.
  • Customization – Unique, custom designs and architectural features will make an ADU costlier than a simple, rectangular footprint.

ADU costs can range from as little as $50,000 for a basic unit to $200,000+ for a luxurious, custom-designed ADU. Carefully evaluating your needs and budget considerations allows you to balance affordability and the features you value most.

Finding an ADU Builder

When building an ADU, finding the right builder is key to ensuring your project is completed on time, on budget, and to your specifications. Here are some tips for hiring a qualified ADU builder:

Hiring a Qualified Builder

  • Look for a builder who specializes in ADUs and has extensive experience with ADU projects specifically. They will understand the unique requirements and regulations.
  • Make sure they are licensed, bonded, and insured. This protects you if anything goes wrong.
  • Ask them to provide examples of past ADU projects they have completed. This will give you a sense of their work quality.
  • Request referrals from past clients. Speaking with previous customers will provide insight into their reputation.
  • Interview multiple builders before selecting one. Compare experience, pricing, and communication style.
  • Prioritize clear communication. Your builder should listen to your needs, explain the process, and answer all questions.
  • Look for a builder who uses high-quality materials and craftsmanship. Don’t sacrifice quality to save money.
  • Make sure they handle all the required permits for you. An experienced builder will be familiar with the permit process.
  • Ask about their timeline for completion. Look for a reasonable schedule based on the project scope.
  • Before starting, get all pricing, timelines, and expectations in writing. This will protect you down the road.

Taking the time to select your ADU builder carefully will pay off tremendously in the long run. Partnering with an experienced professional you trust will result in a smooth, stress-free ADU project from start to finish.

Marin County

All About Marin County

Marin County, though not a city, is an affluent region located just north of San Francisco, across the iconic Golden Gate Bridge. Known for its picturesque landscapes, Marin County offers a blend of natural beauty and suburban charm. It is home to an array of outdoor parks, historic landmarks, and educational institutions, providing residents and visitors with a diverse range of activities and attractions.


  • U.S. Route 101
  • State Route 1
  • State Route 131 (Tiburon Boulevard)


  • College of Marin, 835 College Ave, Kentfield, CA 94904


  • Mount Tamalpais State Park, 3801 Panoramic Hwy, Mill Valley, CA 94941
  • China Camp State Park, 101 Peacock Gap Trail, San Rafael, CA 94901
  • Marinwood Community Park, 775 Miller Creek Rd, San Rafael, CA 94903
  • McNears Beach Park, 201 Cantera Way, San Rafael, CA 94901
  • Old Mill Park, 352 Throckmorton Ave, Mill Valley, CA 94941


  • Gnoss Field Airport, 451 Airport Rd, Novato, CA 94945
  • San Rafael Airport, 400 Smith Ranch Rd, San Rafael, CA 94903


  • Bon Tempe Lake, Fairfax, CA 94930
  • Alpine Lake, Fairfax, CA 94930


  • Corte Madera Creek
  • Novato Creek

Points of Interest

  • Muir Woods National Monument, 1 Muir Woods Rd, Mill Valley, CA 94941
  • Point Reyes National Seashore, Point Reyes Station, CA 94956
  • Marin Museum of Contemporary Art, 500 Palm Dr, Novato, CA 94949
  • Mission San Rafael Arcángel, 1104 5th Ave, San Rafael, CA 94901
  • Marin County Civic Center, 3501 Civic Center Dr, San Rafael, CA 94903


  • Tiburon, Belvedere, CA 94920
  • Sausalito, Sausalito, CA 94965
  • Mill Valley, Mill Valley, CA 94941
  • San Anselmo, San Anselmo, CA 94960
  • Novato, Novato, CA 94945