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Can You Build an ADU on a Duplex

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As the housing market evolves, so do the opportunities for property owners to maximize their spaces. One such opportunity that has gained significant attention in recent years is the addition of Accessory Dwelling Units (ADUs). ADUs are small, self-contained living spaces that can be built on the same property as a single-family home or multi-family property. While initially limited to single-family homes, recent changes to zoning laws have expanded the potential for ADUs on multi-family properties, including duplexes, triplexes, and apartment buildings. If you’re a property owner or investor, understanding how to capitalize on these new opportunities can provide a valuable avenue for increasing income, expanding living space, or creating a family compound.


In this article, we’ll explore everything you need to know about “Can You Build an ADU on a Duplex,” and how local zoning regulations affect your options.


What is a Multi-Family Property?


Can You Build an ADU on a Duplex

A multi-family property is defined as a structure with two or more residential units that are attached to one another. Common examples of multi-family properties include:

  • Duplexes

  • Triplexes

  • Quadplexes

  • Apartment buildings


While single-family homes were historically the only properties eligible for ADUs, regulations have evolved to include multi-family properties, opening the door for property owners to build additional living spaces.


If you own a duplex, triplex, or apartment building, you may now be able to add multiple ADUs to your property, creating more rental income or additional space for family use. The first step is understanding the requirements that apply to your property and the type of ADUs that can be built.


Zoning Regulations for Multi-Family Properties

Zoning laws determine what kind of construction is allowed on a specific property. For ADUs to be built on a multi-family property, the lot must be in a residential zone. This means that properties in industrial or other non-residential zones will not be eligible for ADU development. The specific zoning laws for each city or county vary, so it’s essential to check with your local planning department for details.


Some cities have particularly favorable regulations for multi-family properties, allowing for more than one ADU to be built on a single lot. Understanding these regulations is crucial because local ordinances can vary widely, and what is permitted in one area may not be allowed in another.


ADU Opportunities for Multi-Family Properties

When it comes to adding ADUs to multi-family properties, there are two primary types of ADUs: accessory space conversions and detached ADUs. Each of these options comes with its own set of regulations and possibilities.


Accessory Space Conversions

An accessory space refers to a non-living space on your property that can be converted into a dwelling unit. These spaces may include:

  • Garages

  • Utility rooms

  • Storage rooms

  • Basements

  • Attics

  • Community rooms

  • Detached accessory buildings (e.g., carports, clubhouses)


These areas must not already be considered living spaces, meaning you cannot convert existing apartments into ADUs. However, converting a garage or basement is allowed, provided the space was never intended to be a residential unit.


Detached ADUs

A detached ADU refers to a fully separate structure that is built on the same property but is not physically attached to the main building. In some jurisdictions, you may be able to build two detached ADUs on your property. These detached ADUs could be stand-alone or attached to each other, depending on the local regulations.


The beauty of detached ADUs is their flexibility in terms of design and location on your property. Whether you want to create a small guest house, a rental unit, or a family compound, detached ADUs offer a great solution.


The Famous Formula for Larger Multi-Family Buildings

One of the most exciting developments for multi-family property owners is the ability to convert larger areas of their properties into ADUs. For buildings with eight or more units, there’s a special formula that allows owners to convert up to 25% of their total units into ADUs.

  • Example: If you own an apartment building with 20 units, you may be able to convert five of those units into ADUs.

  • In addition to converting existing space, you may also be allowed to build up to two detached ADUs, further increasing the potential for additional rental income or living space.


This formula can provide significant opportunities for property owners to maximize the use of their land and the value of their property.


Size and Design of Multi-Family ADUs

ADUs come in a variety of sizes, and the size of the unit depends on the regulations in your area. In California, for example:

  • Cities must allow at least 850 square feet for one-bedroom ADUs.

  • For two or more bedrooms, the minimum allowed size is 1,000 square feet.

  • While cities cannot impose smaller maximum sizes, they are allowed to impose larger ones.

For multi-family properties, the flexibility of accessory space conversions and the variety of sizes available allow property owners to maximize the potential of their property. Whether you're converting a garage, basement, or utility room, or building a detached ADU, you can design your space to meet your needs while adhering to local guidelines.


Parking Considerations for ADUs

One of the main concerns for property owners adding ADUs to their properties is parking. In some areas, zoning laws may require that a certain number of parking spaces be provided for each new ADU. However, if your property is:


  • Within ½ mile of public transportation, or

  • One block of a car-share vehicle parking may not be required.


This exemption is particularly helpful for multi-family properties in urban areas where parking can be scarce. When planning your ADUs, it’s essential to consider the parking requirements in your area and whether your property has adequate space for additional vehicles.


Cost and Feasibility of Building ADUs on Multi-Family Properties

The cost of building ADUs on a multi-family property can vary widely depending on factors such as:

  • Location

  • The scope of the project

  • Whether you need to upgrade utilities or parking spaces

However, one of the advantages of building ADUs on a multi-family property is the potential for economies of scale.


  • If you own multiple units, you may be able to save on construction costs by completing the project all at once rather than building individual ADUs on separate properties.

  • Additionally, some properties may have existing infrastructure (e.g., utilities and parking spaces) that can be utilized for the new ADUs, reducing the overall cost.


How SB9 Affects Multi-Family ADUs

California’s SB9 legislation allows homeowners to split their lots into two parcels and turn single-family homes into duplexes. This law, however, currently does not allow for the addition of ADUs on properties that have been split into multiple parcels or turned into duplexes under SB9.


This limitation exists while lawmakers evaluate the potential impact of ADUs on these newly split properties.


While SB9 offers unique opportunities for homeowners, it’s important to understand its limitations in regard to ADUs, especially if you plan to use both provisions of the law.


ADUs on Duplexes

Many property owners with duplexes are interested in adding ADUs, and the good news is that it is possible, as long as local zoning laws allow for it. In many cities, duplexes are eligible for ADUs, but factors like:

  • Lot size

  • Existing infrastructure can influence how many ADUs can be added.


Common Duplex Configurations for ADUs

Duplexes come in several configurations, including:

  • Side-by-side duplexes

  • Front-to-back duplexes

  • Stacked duplexes

  • Split-level duplexes

The configuration of your duplex will affect how you can build ADUs. Each layout offers unique possibilities for creating efficient, functional ADUs.


Can an ADU Be a Duplex in California?

In California, it is possible to build a duplex-style ADU, subject to local zoning regulations. While ADUs are typically small, self-contained units, a duplex ADU is simply a larger ADU that contains two living spaces within one structure. This type of ADU could be ideal for larger properties or for owners seeking to create two rental units.


How Many ADUs Can You Have on One Property in California?

In California, the number of ADUs you can have on one property depends on:

  • The size of your property

  • Local regulations

In general, local jurisdictions must allow at least:

  • One ADU from the accessory space, or

  • Two detached ADUs


For larger multi-family properties, such as apartment buildings, the number of ADUs allowed may increase, with some cities allowing up to 25% of the total units to be converted into ADUs.


Conclusion

Maximizing the potential of multi-family properties with ADUs is an excellent way to increase rental income, expand living space, or create a family compound. Whether you own a duplex, triplex, or apartment building, the opportunity to build additional units is within your reach.


The key to success lies in:

  • Understanding local zoning laws and regulations

  • Ensuring your property meets the necessary criteria

  • Working with experienced professionals who can guide you through the process


If you're ready to explore your ADU options and start your construction project, contact Result Construction for all your Construction Services Antioch needs. Our expert team is here to help you navigate the complexities of ADU development, ensuring your project is executed smoothly and efficiently. Contact us today to get started!


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