The Cost to Build ADU in Northern California

The Cost to Build ADU in Northern California

In recent years, the rising cost of housing has become a pressing issue for many people. As a result, alternative housing options such as Accessory Dwelling Units (ADUs) have gained popularity. An accessory dwelling unit is a small secondary residential unit that can be built on an existing property, providing additional living space and potentially generating rental income for homeowners.

While ADUs offer numerous benefits, the cost to build one can vary greatly depending on various factors. In this article, we will explore the different components that contribute to the cost of building an ADU and provide some tips on how to keep costs under control.

What Is An ADU?


An Accessory dwelling unit, also known as a granny flat, in-law suite, or backyard cottage, is a self-contained living unit that can be built on the same lot as a single-family home. These units typically have their own entrance, kitchen, bathroom, and living space.

ADUs can take various forms including detached structures like tiny houses or converted garages or basements. They can also be attached to the main house, such as an apartment above a garage or a basement conversion with a separate entrance.

Average Cost For an ADU

ADU costs can range from $50,000 to over $200,000, depending on the size, design, materials used, and location. According to a study by HomeAdvisor, the average construction costs for building a 600-square-foot ADU in the United States is approximately $100,000.

Average Cost for an ADU in California

In California, the cost of ADUs is generally higher due to the expensive cost of living and stringent building codes. Additionally, most California ADUs must include solar panels, further increasing costs. According to a report by Buildzoom, a 600-square-foot ADU in Los Angeles averages around $150,000, while in San Francisco, it can rise to $200,000.

Average Cost of an ADU Per Square Foot

The cost per square foot can range from $150 to $400, depending on the unique characteristics of the project. For example, a simple garage conversion may have lower costs compared to building a detached ADU with full amenities.

Factors that Affect the Cost to Build an ADU

A team building an ADU

Several factors contribute to the overall cost of building an ADU. Understanding these factors can help you estimate the cost and make informed decisions during the planning process.

Permits and fees

This includes the cost of obtaining permits, local building codes, any associated fees from the local government, and potential expenses for inspections, environmental impact assessments, or other regulatory requirements that may be necessary for the project.

Design and planning

This refers to the cost of hiring an architect or designer to create detailed plans for the ADU, including architectural drawings, structural engineering, and potentially consultations for zoning compliance and permits.


The largest portion of the cost will go towards the actual construction of the ADU, including materials and labor. Additionally, permits and design fees will also contribute to the overall expense.


Additional utility connections may be required for the ADU, including water, electricity, gas, and sewage lines. These installations can add significant expenses to the overall cost, impacting both the initial investment and ongoing maintenance.

Landscaping and exterior features

If the ADU is built on an existing property, landscaping may need to be adjusted to accommodate it. The cost of any additional exterior features such as a patio or deck should also be considered.

Interior and Exterior Finishes

This includes items such as flooring, cabinets, and fixtures. The cost can vary greatly depending on the quality of materials chosen.

The exterior finishes of an ADU can range from a simple paint job to more expensive options such as siding, brick, or stone. A basic paint job is the most budget-friendly option, offering a fresh look without a hefty price tag.

ADU Cost by Type


The type of ADU you choose to build can also have a significant impact on the overall cost. Here are some common types of ADUs and their approximate costs:

Detached ADU

A standalone unit built on its own foundation with separate utilities, offering complete independence and privacy. This option typically includes features such as a private yard, garage, and additional living space, making it ideal for those seeking a more spacious and self-sufficient living arrangement. However, it is the most expensive option, with costs ranging from $150,000 to $250,000.

Attached ADU

A unit that is attached to an existing structure such as the main house and shares utilities, such as water, electricity, and heating. This option can cost anywhere from $80,000 to $150,000, depending on factors like size, materials used, and local labor rates. It typically includes a private entrance, one or more rooms, and possibly a small kitchen or bathroom.

Garage conversion

Converting an existing garage into an ADU can be a more affordable option, with total cost ranging from $50,000 to $100,000. This price often includes structural modifications, electrical and plumbing upgrades, insulation, and interior finishes. It's important to also consider permitting fees and potential zoning regulations that could impact the project. Additionally, the overall cost may vary based on the level of customization and finishes chosen.

ADU Cost Factors

A close up of an ADU

There are several key factors that influence the overall cost of building an ADU:


The size of the ADU is one of the most significant factors in determining its cost. Generally, the larger the unit, the more expensive it will be to build due to higher material costs, increased labor, and potentially more complex design and permitting requirements. Larger ADUs may also require more advanced plumbing, electrical work, and additional fixtures, all of which can contribute to the overall expense.

Type of ADU

There are several types of ADUs, including detached, attached, and interior conversion units. Detached units tend to be more costly due to additional construction and utility connections required.

Design and features

The complexity of the design and desired features such as high-end finishes or custom elements can significantly impact the cost. For instance, intricate architectural details often require specialized craftsmanship. Additionally, the choice of premium materials can drive up the budget.


The cost of building an ADU can also vary depending on the location. Factors such as labor costs, availability of materials, and local regulations can affect the overall cost.

Existing infrastructure

If your property already has existing utilities and connections in place, the cost of building an ADU may be lower compared to a property without existing infrastructure.

Permitting and fees

Before starting any construction, you will need to obtain permits and pay applicable permit fees. These costs can vary depending on your location and the type of ADU being built.

ADU Alternatives

A garage conversion

If the cost of building an ADU is out of your budget, there are some alternative options to consider:

  • Renting a room: If you have extra space in your home, you could consider renting out a room for additional income.
  • Tiny homes: A tiny home can be a more affordable alternative to an ADU. These small and often portable dwellings can be built for a fraction of the cost of an ADU.
  • Co-living spaces: Co-living spaces allow multiple individuals to share a larger home, reducing individual living costs.
  • Garage conversion ADU: This option involves converting an existing garage into a livable unit, which can be more cost-effective than building from scratch.

Tips for Controlling Costs

A man budgeting his costs

As with any construction project, there are ways to keep costs in check when building an ADU. Here are a few tips to consider:

Plan and research

Before starting any construction, take the time to plan and research your options carefully. This will help you identify potential cost-saving measures.

Use standardized designs

Custom designs can be expensive, so opting for a pre-approved or standardized design can save you money. Many cities and counties offer pre-approved ADU plans that can be modified to fit your needs.

DIY where possible

If you have experience with construction or are willing to learn, taking on some tasks yourself can save money on labor costs.

Explore financing options

There are various financing options available for building an ADU, such as construction loans or home equity loans. Consider all your options to find the most cost-effective solution for your project.

Frequently Asked Cost to Build ADU Questions


How long does it take to build an ADU?

The timeline for building an ADU can vary depending on factors such as the type of unit, location, and any potential delays during the construction process. On average, it takes between 4-6 months to complete an ADU.

Can I build an ADU on a small lot?

Yes, you can build an ADU on a small lot. However, there are specific requirements and limitations that may vary depending on your location.

Will building an ADU increase my property taxes?

Adding an ADU to your property can potentially increase your property taxes. However, some jurisdictions offer tax incentives or exemptions for ADUs, so it's essential to research the tax implications in your area.

Can I rent out my ADU on Airbnb?

Some cities and counties have regulations in place for short-term rentals, so it's important to research the rules and restrictions before listing your ADU on platforms like Airbnb. Additionally, you may need to obtain a separate permit or license for short-term rentals depending on your location. Consider consulting with professionals such as contractors or financial advisors to help guide you through the process and make informed decisions about your ADU project.

Are there any additional costs besides the ones mentioned above?

Yes, depending on your unique situation, there may be other costs to consider, such as demolition of existing structures, site preparation, and landscaping. It's important to carefully plan and budget for all potential costs before beginning construction.



Building an ADU can be a significant financial investment, but the potential benefits and added value to your property make it a worthwhile consideration. By understanding the key cost factors, exploring alternative options, and implementing cost-saving measures, you can successfully build an ADU that fits your budget and meets your needs. Remember to research local regulations and consult with professionals for guidance throughout the process. With careful planning and budgeting, you can create a functional and valuable ADU for your property.


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