ADU Cost Per Square Foot

ADU Cost Per Square Foot

Please see our updated post on ADU Cost Here!

The ADU Cost Per Square Foot is a highly sought-after number that you hear thrown out by contractors, homeowners, and architects all the time.  However, you’ve probably found out by now that the ADU Cost Per Square Foot number varies based on who you ask.  “Why is that?” you may wonder.   Well, there’s actually a lot more to it than most people think.

First, what’s included?  Many people naively think that any contractor can build an ADU.  They ask their contractor buddy about how much an ADU will cost and are often met with an ADU Cost Per Square Foot around $200.  They falsely believe that they’ll be able to build an 800 square foot unit for $160,000 all in, only to find out later, that’s not the case.

Here’s why.

Building an ADU requires more than just the materials and construction.  Building an ADU also requires formal plans that must comply with local and state laws and codes.  These plans require an architect or draftsman and engineers, often structural and electrical, in order to complete them and get them passed through the city.  

The city will also often require an application fee, and you will have to pay for permits and any impact fees required.  Often when contractors throw out an ADU Cost Per Square Foot, they are not including these expenses, all of which are necessary before you can begin constructing the ADU.  

Once you’ve paid your architect or draftsman, the engineers, and the city and public works, you’re then ready to move into the construction cost.  Certainly now, there’s a settled ADU Cost Per Square Foot…

Unfortunately, it’s not that simple.  Take for example a 2-bed, 2-bath, 800 square foot ADU with high-end materials and compare it to a modest 1-bed, 1-bath, 800 square foot ADU with low-end materials.  The square footage is the same, but do you think it costs the same?

No, the modest, 1-bed, 1-bath will cost less.    

With that in mind, here’s an ADU Construction Cost Per Square Foot range to go by, but please remember, this is only the construction cost.  You must factor in the cost of the plans, permits, engineering, design, utility costs, and impact fees – most of which must be paid for before you begin construction:

Converting an existing space or garage into an ADU$150 – $250+
Building a detached ADU (requires solar in CA)$180 – $300+
Building an attached ADU $185 – $300+
Building an ADU over an existing structure$200 – $350+

Now, whether you’ll be on the low end of the range or the high end will depend on several factors:

  • Where are the utilities located?  How far are they away from the ADU?  Do you want to tie into the primary residence’s utilities, or have separate utilities for the ADU?  
  • How many bedrooms and baths?  Kitchens and baths are the most expensive parts of an ADU.
  • Do you want laundry facilities in the ADU?  How about a dishwasher?  Do you want full-size appliances or apartment size?  Do you want low-end appliances or high-end appliances?  
  • Low-end materials or high-end materials?
  • For existing spaces or garages, will structural work need to be completed?  Will the foundation need to be reinforced or new footings installed?

Most people usually end up with a construction cost per square foot somewhere in the middle of the range.  However, let’s now go ahead and add in all of the other ADU Costs that we’ve covered up to this point:

ADU Plans$2,000 – $40,000+
ADU Engineering (often structural and electrical are needed)$0 – $5,000+
ADU Permits & Application Fee (varies by city)$1,500 – $8,000+
ADU Impacts Fees – Only for ADUs over 750 square feet (varies by city)$2,000 – $14,000+
ADU Utility Fees$0 – $20,000+
ADU Design Costs$2,000 – $30,000+
ADU Project Management$2,000 – $25,000+

As you can see, the costs vary, and a lot of that variance is based on the size of the ADU, how difficult it will be to build, and also the city that you live in.  

Let’s now go through an example to calculate the Total ADU Cost Per Square Foot:

Example 1 – ADU Garage Conversion (400 square feet) – No structural work, utility or impact fees

ADU Plans: $2,500 – $10,000+ (Let’s use $3,000)

ADU Engineering (Title 24): $500+ (Let’s use $500)

ADU Permits: This varies greatly, but let’s use $2,500

ADU Design Costs: This varies greatly, but let’s use $2,000

Even without project management, you’re up to $8,000.  Let’s say you’re able to get your construction cost around $200 per square foot for $80,000 total.  Once you add on the $8,000, you’re at $88,000 for 400 square feet or $220 a square foot.  

Example 2 – Detached ADU (800 square feet, 2-bed, 2-bath) – Will require impact fees

ADU Plans: $4,000 – $40,000+ (Let’s use $12,000)

ADU Engineering (Structural & Electrical): $2,000+ (Let’s use $2,000)

ADU Permits: This varies greatly, but let’s use $5,000

ADU Impact Fees: This varies per city, but let’s use $5,000

ADU Utility Fees: Varies per property, but let’s use $10,000

ADU Design Costs: This varies greatly, but let’s use $10,000

Without project management, you’re up to $44,000 before construction.  Since you want 2-bathrooms and nice finishes, your construction cost comes in around $220 per square foot or $176,000.  Once you add on the $44,000, you’re up to $220,000 total.  $220,000 divided by 800 square feet now comes out to a Total ADU Cost Per Square Foot of $275.  

You can see that the Total ADU Cost Per Square Foot is much higher than just the construction cost.  This is what’s important to consider and keep in mind when having contractors bid the job.  Ask if plans, engineering, design, permits, and utility costs are included.

The Easier Way 

If you want to know how much it will cost to build an ADU on your property, the best option is going to be reaching out to an expert.  While you can calculate the ADU Cost Per Square Foot and try to account for the engineering, plans, and design, among other things, yourself, it’s best to consult a professional.  

At Superior ADUs, we assess what kind of ADU our clients want to build, where they want to put it on their property, and how they’re planning to use the ADU.  After analyzing their personal situation, we make recommendations on what to build and give them a cost estimate.  This cost estimate will be much more accurate than an ADU Cost Per Square Foot calculation, because the estimate is tailored to exactly what our clients want.  

To schedule a free consultation, please fill out the web form, call 949.216.0686, or email   

Disclaimer: Please note that all numbers and estimates on this page are only approximations based on current building costs in our area of Orange County, CA, and are intended for learning purposes only.  It is always best to consult an ADU builder in your area, have them see and assess your property and project requirements, and then provide you with an accurate cost, which could be lower or higher than the examples provided.

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