How Much Does It Cost to Build an ADU (Accessory Dwelling Unit) in California?

So, you’ve been pondering on the idea of maximizing your property’s potential in the Golden State, and the thought that keeps popping up is, “how much does it cost to build an ADU in California?” There’s no straightforward answer, as ADU costs vary greatly. But worry not, in this blog post we’ll break down the cost to build an ADU, covering every crucial aspect from construction to permit fees, giving you a comprehensive understanding of the costs involved. 

Moreover, we will help you find the cheapest way for new construction of a dream ADU, which will save a significant amount of your funds.

Deciphering the ADU Construction Puzzle: The Costs Explained

Ever looked at your property and thought, “Ah, the potential! If only I knew what it costs to build an ADU in California?” Don’t worry; you’re not alone! Building an Accessory Dwelling Unit (ADU) in California is an exciting yet daunting venture for many. 

So let’s roll up our sleeves, dig into the details and demystify the costs associated with such on site construction as ADU.

Unraveling the Three Paths to ADU Construction

There’s no one-size-fits-all in ADU construction. You can venture on three different paths – each one with its unique appeal and cost implications:

1. The Path of Isolation: Detached Accessory Dwelling Unit

Picture a cozy, standalone house right in your backyard. That’s a detached ADU! They’re completely separate from your primary residence and popular for providing ultimate privacy. There are a huge number of variations of detached ADUs from backyard cottages and guest houses to granny flats and home offices. However, these new structures carry the heaviest cost tag due to their independent nature.

2. The Path of Connection: Attached ADU

Imagine an extension to your house, a space that breathes the same air but has its distinct charm. That’s an attached ADU for you! As they are part of your existing structure, they are usually less expensive than detached ADUs. We can easily explain this by the fact that the attached ADU has at least a common wall and sometimes a foundation with the main house, which means the reduction of total cost. For this reason, specialists consider the construction of an attached ADU as the cost-effective way to expand existing space.

3. The Path of Transformation: Garage Conversion

What if the garage you rarely use could turn into a beautiful, functional living space? Yes, you guessed it – that’s the essence of a garage conversion. This form of ADU is often the most cost-efficient, thanks to the pre-existing structure.

Understanding ADU Costs

Cost estimation for building an ADU in California involves two key components – fixed costs and variable costs.

Fixed Costs

Your ADU dream will incur some fixed costs – the unavoidable expenses like permitting fees and design fees. Permits can range anywhere from $1,000 to $5,000, while design fees usually account for 10-15% of your total project cost.

Variable Costs

Then comes the variable costs – the fluctuating expenses tied to the unique aspects of your project. Construction costs, for instance, can range from $200 to $400 per square foot. Other costs like site work, inspection fees, and material costs also fall into this category and can significantly change the final numbers of an ADU cost per square foot – read more about here –

Location Matters: ADU Costs Across California

If you’re contemplating the cost of building an ADU in California, remember that the location matters. Busy urban regions like Los Angeles or San Diego might have steeper construction costs than their more rural counterparts. However, they also offer higher rental income potential, so it might just be worth the investment. 

On par with this, no one has canceled the fact that the construction of an accessory dwelling unit on the one site with a regular house will increase the future value of the whole property in any city, because demand never disappears in the real estate market.

Calculating the Final Figure: ADU Costs in California

So how do you tie it all together to answer the big question, “How much does it cost to build an ADU in California?” You add up all the costs! As an example, for a 600 square foot detached ADU in Los Angeles, with construction costs at $250 per square foot, design fees at $20,000, permit costs at $2,000, and miscellaneous costs at $10,000, you’re looking at an approximate total cost of $182,000. Remember, this figure can vary based on multiple factors like your specific location and market conditions. 

Furthermore, the complexity of your project has a direct impact on the cost per square foot, because building an accessory dwelling unit with a unique design, additional outdoor living space and an integrated Smart Home system will cost much more than a tiny house with a standard design and layout.

Pros and Cons of Building an ADU

Many homeowners do not risk exposing existing structures to changes and building an ADU, so before you embark on your ADU-building journey, it’s important to weigh the benefits and the challenges.


  • An ADU can substantially enhance your property’s value, so much that the resale value increases by at least several hundred thousand dollars.
  • It serves as a constant source of rental income.
  • It provides additional space for family or guests.
  • ADU construction is the only win-win and good investment that pays off with a 101% probability.
  • ADU prices vary from $30,000 to $200,000+, which means you can always pick a project for your budget.


  • The upfront costs are hefty.
  • You’ll have to navigate through permitting and inspection processes.
  • Financing the construction can be a challenge.

California ADU Cost Takeaway

So, to answer the question, “How much does it cost to build an ADU in California?” – It’s a complex equation. But with the insights we’ve shared on the ADU construction process, the comprehensive cost breakdown, and the impact of regional differences, you’re well-equipped to start your ADU project. Here’s to creating spaces that add value and joy!

Frequently Asked Questions — FAQ

Can I qualify for a California ADU grant?

Eligible property owners can receive financial aid from California to build an ADU. Criteria include meeting certain income limits and occupancy requirements. In this regard, we recommend you sign up for a free consultation with a lawyer, as the wording in the law is varying depending on the city, e.g., in San Diego and Los Angeles the rules are different. The lawyer will also help you arrange all the documents for the property so that the California Department of Housing and Community Development or any other authorized body will accept and approve your application.

How many ADUs am I allowed in California?

Typically, single-family properties in California can host one attached and one detached ADU. However, this rule applies only to accessory dwelling units in the single family home format, because in case of multi-living properties the situation is different.

What’s the maximum size for an ADU in California?

Detached ADUs in California can be as large as 1,200 square feet, while attached ADUs can’t exceed 50% of the primary residence’s living area. In order not to violate the law on the maximum allowable square footage of a new building, it is best to discuss this point with your lawyer.

Is it wise to invest in building an ADU?

Yes, indeed! Despite the initial costs, ADUs can boost property value and generate continuous rental income. In addition, always keep in mind that with the help of the new accessory dwelling unit, you will receive passive rental income and in a short period will completely beat off the final cost, moreover, you will also be able to pay property taxes and impact fees from this amount.

Do I need a construction loan to build an ADU?

It’s not mandatory, but it can be helpful. Other options could include a home equity line of credit, personal loan, cash-out refinance, or you can borrow money from friends or relatives, because investment in ADU is worth it.

Can my garage be converted into an ADU?

Absolutely! In fact, garage conversions are a popular, cost-efficient way to create an ADU in California. Furthermore, this project is the most variable, because from the entire structure of the old garage, experienced general contractors can make a new guest house, a home gym, or a home theater for relatively small development costs.