How much does it cost to build an ADU in California?

How much does it cost to build an ADU in California? That’s a common question for homeowners looking to maximize their property value. As real estate costs continue to rise, ADUs are quickly becoming one of the most popular forms of housing in California. ADUs provide additional livable space in areas where smaller, more affordable housing is hard to find.

Many factors affect the final price of an ADU construction including design costs, site preparation, construction, utilities, permitting, and interior finishing. We’ll walk you through the steps needed to build an ADU and break down what goes into each of these costs to help you better understand and prepare for your investment.

In This Article:

How much does an ADU cost in California?

Overall, the cost of an ADU can range from $300,000 to more than $400,000, depending on how they’re constructed and furnished. Most modestly-equipped ADUs in California cost around $275,000. ADUs have all the same amenities as single-family homes but cost significantly less.

The cost of building an ADU can vary depending on a variety of factors including where you’re building, design, and finishing. Three types of costs go into building an ADU:

  1. Hard costs: Include the construction, materials, utilities, and interior furnishing.
  2. Soft costs: Include 3D design, architectural drafting, engineering checks, etc.
  3. Fees & Permitting

Breakdown of ADU Costs

Here’s a broad timeline and breakdown of costs for a typical detached ADU project in California.


Find out if you’re eligible

Before you explore your options for adding an ADU, make sure your property is eligible. Check with your local planning department to find out if you can add an ADU to your property, the maximum square footage allowed, and the required property setbacks that will influence the location of your ADU.

If your property is eligible for an ADU, schedule an initial consultation with a trusted developer to review a satellite inspection of your address, conduct a general feasibility analysis, and provide you with a ballpark quote.


Design & Drafting

So far you haven’t paid a cent, yet you’ve verified that you can build an ADU on your property and have received an estimate of the total cost for the project. To get an accurate, finalized cost, our team will visit your property to take detailed measurements to provide our designer with the information required to create a full 3D model with a round of revisions. View our popular ADU models for design inspiration.

After the creation of the 3D model of your ADU, we’ll transform it into the drafts and engineer-approved drawings required for the permitting process. This step includes engineering analysis & stamp, Title 24 energy analysis & certification, and solar design & plans.

Typically, architectural costs can range from 8-15% of your total project cost. This cost usually depends on the size of the ADU, the experience of the architect, the complexity of the project, and the local economy. For example, if your average cost per square foot is $300 or $400, then an average 600 sq.ft. ADU would cost between $180,000 and $240,000. If your architect charges 8% of the total cost, that comes out to a fee of $14,000 to $19,500.

At Backyard Unlimited, total costs for Design and Drafting will run around $5,500 (~$2,500 for 3D design and ~$3,000 for drafting and engineering). At this point, you will have a contract with finalized pricing for your ADU build. That budget will include a detailed list of all materials, labor, and responsibilities included and excluded. All our material will come with manufacturer produced cutsheets and installation information, so you can rest assured you know exactly what your new ADU is being built with.


Permitting Your ADU

Once this detailed and engineered set of drawings is created, the city/county permitting process begins. How much does an ADU permit cost in California? Each city is different, and many have specific requirements for drawings, details, and engineering that differ from other localities. Depending on your location, you may need both zoning and building permits. Zoning permits can range from $25 to $4,000 and building permits can run from $450 to $15,000. With Backyard Unlimited, we include the cost of permitting in your contract, which comes to about 5% of the direct cost of your project.

The “plan-check” process starts by submitting these detailed drawings into the city/county for their review. The drawings will then be circulated around all relevant departments in the city to make sure they meet the requirements of each department. You will be required to make changes as necessary regarding comments from the city/county, until they are satisfied with the set of drawings. At this point, your stamped set of construction drawings will be issued. Now you are ready to build!


Site Preparation

Once we get the permits in hand, we can start preparing your property for the ADU. A detached accessory dwelling unit needs a new foundation, requiring our builders to dig into the ground. This often includes grading the land and installing the foundation, as well as any utility work. The cost of sitework may increase depending on what likes below the surface (rocks, piping, wiring, etc). You may think to cut costs by doing some of this work yourself however, you’ll still need someone to inspect the site and ensure that the soil is stable enough for construction, so it’s better to do it right the first time and let our team handle it – especially since this is included in your contract price.


ADU Construction

Construction costs take up the largest portion of your project cost. Building an accessory dwelling unit can cost anywhere from $300 to $500 per square foot. This pricing can fluctuate depending on your location, plans, overall design, size of the ADU, and the quality of the building materials. Your ADU will arrive in prefabricated sections, which we will assemble and finish. Backyard Unlimited schedules and manages all subcontractors, as well as scheduling for all inspections.


Utility Upgrades & Interior Finishing

Finishing the interior of your ADU includes electrical and plumbing work, following by installing drywall. Once this is completed, our team handles the painting, trim, flooring, installing fixtures, and everything else that needs to happen to make the structure livable. Using the design-build approach lets us pick and choose exactly what you do and do not want in your ADU design. Depending on your selections, finishing and furnishing your ADU can cost between 30% and 50% of your initial investment.

  • Utilities & System Connections – ADUs are completely independent from the primary residence. They differ from tiny homes or finished sheds because ADU construction includes the installation of electrical, plumbing, and HVAC systems. Depending on the project, the increased electrical demands of an ADU may overwhelm your current electrical supply system, which will require upgrading your system. You can offset some of this demand by installing solar on your ADU – a popular choice. Factor this into your decision-making and discuss any concerns with a contractor.
  • Interior Finishes & Materials – Like any other type of housing, you have endless options for customizing the interior of your ADU. Flooring, paint, light fixtures, faucets, countertops – all need to be selected for your ADU. The level of quality for each of these can dramatically alter your budget. Make sure you have estimates for a variety of options before wrapping up your project to avoid sticker shock at the finished product.
  • Furniture & Appliances – Before you can sit back an enjoy your new ADU, you need to have somewhere to sit. Since an ADU is smaller than a normal home, you won’t likely have as much furniture to purchase. In addition to getting furniture for your living room and bedroom, you’ll also need to worry about your bathroom and getting appliances, bar stools, and tables for your kitchen.

Featured ADU Project: Petaluma, CA

ADU Project in Petaluma, CA

Explore the details for a recent ADU project we completed in Petaluma, California. This 2 bed, 1 bath detached ADU is 747 square feet.
See Project Breakdown

Designing and building an Accessory Dwelling Unit is a large, complex investment. You need someone who knows the ADU laws and has expertise in designing and installing smaller structures.

Here at Backyard Unlimited, we have spent the last decade installing accessory structures throughout California. Our team knows the ADU laws and what it takes to get them permitted and approved. We only use our own employees to design, assemble, and finish our structures. It’s not surprising that discerning property owners choose Backyard Unlimited as their ADU design/build contractor.

Design My ADU
Adu Guide 2024

Have ADU Questions? Get Answers from the Experts

We’ve compiled our years of experience building prefab structures into The Ultimate Guide to ADUs in California. In this guide, you’ll learn everything you need to know about the ADU process – from planning & permitting to designing & construction.

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