How Much Does an Accessory Dwelling Unit Cost To Build?

Thanks to California’s new ADU laws, more homeowners are taking advantage of opportunities to increase their homes’ footprints. Sometimes referred to as granny flats or in-law suites, an ADU – accessory dwelling unit is a secondary living space attached to or situated near an existing home. 

Building an ADU is a significant project, and it’s not inexpensive. So here’s what you need to know about accessory dwelling unit costs before starting your project.

ADUs can be used as guest suites, housing for aging relatives or live-in nannies, home offices, workout spaces, or even studios for hobbies like art or recording music — the possibilities are endless. 

It’s also possible to rent the new structure to long-term tenants. Depending on where you live, a small ADU can earn between $1,500 to $2,200 in rent each month while a larger unit with two or more bedrooms can fetch between $2,500 and $3,300 per month.  

Here at GreatBuildz, a free service that connects homeowners in Los Angeles, Orange County, Ventura & San Diego with reliable general contractors, we help people every day to better understand the accessory dwelling unit costs, scope, and details for their prospective projects – below is an overview of what costs to expect when building an ADU.

Cost To Build a Garage Conversion ADU

Accessory dwelling unit cost in my backyard

For a garage conversion, you can expect to pay between $95,000 and $125,000, though total costs will vary depending on where you live, the size of the project, who you hire, and the materials you choose. Garage conversions are typically the least expensive option because you’re building within a structure that already exists.

Accessory Dwelling Unit Cost

Typically, a one-story detached accessory dwelling unit costs between $200,000 and $400,000. Accessory dwelling unit costs for a standalone structure, usually called a detached ADU, are higher than garage conversion costs because you’re essentially building a small house. It will need its own foundation and support structure as well as all-new electric, plumbing, and HVAC. 

ADUs range in size from approx. 400sf to 1200sf, so the costs will greatly depend on the size you’re building. A large 3-bedroom ADU at 1,200sf is literally three times bigger than a 400sf studio ADU. One thing to keep in mind is that as you build bigger, the price per square foot goes down. So, for a large ADU, the cost may range from $250-$300/psf…while for a small ADU, the costs may be $350-$400/sf.

When you’re calculating the costs for a project like this, you’ll also want to budget for an architect or designer to draft your plans and submit them to the City Department of Building and Safety for a plancheck review. You can expect to pay anywhere from $6,000 to $15,000 depending on the complexity of your project. 

How much can I expect to pay for my ADU?

adu unit cost

One way to plan your ADU budget is to assume a cost of $250 to $400 per square foot. This range takes into account the fact that some contractors charge higher rates than others and material costs are somewhat volatile when there are supply chain disruptions. 

There are multiple different kinds of ADUs, so the only real way to get an understanding of the approx. costs are to speak to an experienced ADU contractor. For example, a two-story ADU or ADU above a garage will cost more to build than a single-story ADU. A detached ADU might cost more to build than an attached ADU. Let’s face it, every property is different and each one has certain conditions that could make it easier or harder for the contractor. If access or parking is difficult in your neighborhood, don’t be surprised if the costs go up. Sam applies if you are building on a hillside lot.

To help you get a clearer sense of what you can expect to pay, we provided you with an Accessory dwelling unit cost calculator, which asks you to enter the size of the ADU you’re planning to build, the county you live in, your home’s current value, the type of terrain you’re building on and whether you’ll be including any add-ons like solar panels

How can I get accessory dwelling unit cost savings?

Accessory Dwelling Unit cost savings

One of the simplest ways you can save money when building an ADU is to consult with an experienced ADU contractor early on in the planning process. He or she will be able to talk you through all your options when it comes to features and materials, which will help you come up with a plan that fits your needs, your tastes, and your budget.

For example, using all-electric appliances means you won’t need to run a gas line to the new unit. Or perhaps you’d be open to designing the layout so that the kitchen and bathroom share a wall, thus simplifying the plumbing. Instead of having hardwood floors installed, you might go with vinyl plank.

Still, it’s worth pointing out that many homeowners choose to finance their ADU projects by taking out a home equity line of credit, securing a construction loan, or using a home renovation loan. 

What do local ADU ordinances mean for me?

A number of requirements and regulations will dictate where you can build, how big you can build, and what your structure looks like.

Firstly, you can build an ADU only in the backyard or side area of your existing home. You cannot build in front of your home (as of mid-2022…however things may change!). Additionally, there must be at least 4 feet of space between the edge of the unit and your property line. 

An attached ADU can either be 1,200 square feet or half the square footage of your existing home — whichever is less. A detached ADU can be no larger than 1,200 square feet. In both instances, they can be one or two stories, with a maximum height of 16 feet or 25 feet. The roof must have a minimum pitch of 2:12 for more than 50% of the total roof area. 

cost to build ADU

The materials you use also matter. Builders cannot use laminates, single-piece composites, or interlocked metal sheeting. Roofs cannot be made with wood shingles. Windows must be at least double-pane glass and labeled for building use. 

Inside the unit, the living room/bedroom must be at least 70 square feet. The kitchen, hallways, and closet must total 70 square feet. And the bathroom should be at least 30 square feet and include a toilet, sink, and shower or bathtub. 

Some local governments have additional ADU ordinances, and it’s important to be aware of exactly what’s required in your area. If your ADU is not in full compliance, you might be forced to correct or even demolish your project — and that will drive your costs up considerably. An experienced ADU contractor will be well-versed in these and other regulations and will make sure everything is exactly as it should be.

How do I prepare to collect bids from ADU contractors?

A contractor bid is an itemized estimate of what a contractor thinks your project will cost to complete. Without your guidance, it’s highly unlikely that bids you receive from different contractors will include the same things. That’s why it’s important to send a scope checklist when you solicit bids.

A scope checklist includes everything you anticipate needing for your project. This can include everything from rough construction elements like foundation, framing, doors, and windows; installation of new sewer, water, and gas lines; kitchen and bathroom construction and appliances; and site considerations like fencing, hardscaping, and landscaping.

To help make sure nothing gets overlooked, we’ve created a handy scope checklist template. Remember, the more detailed your scope checklist the more accurate the contractors’ bids will be.

Accessory Dwelling Unit cost checklist

Typical Scope Checklist For Building An ADU – Contact Us For an Editable Version!

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From there, we recommend reaching out to collect at least three bids from different contractors. This will allow you to determine who is offering the best value for the work being completed. 

And connecting with these contractors doesn’t have to be hard. At GreatBuildz, we’ve built a robust network of licensed general contractors. Call and tell us about your ADU project and we’ll connect you with three pre-screened professionals taking new clients in your area. 

GreatBuildz is a free service that connects homeowners in Los Angeles, Orange County, Ventura & San Diego with reliable, thoroughly screened general contractors and provides project support from start to finish.


Submit your project to be connected with several fully vetted, trustworthy contractors who are experienced with accessory dwelling unit costs and projects. For more info, visit or call 818.317.3567 today.

Hi, we're Paul and Jon - Co-Founders of GreatBuildz. We believe everyone deserves to find a great contractor, have a stress-free renovation, and enjoy their beautiful new space. There are so many contractors out there and it's often hard to tell the good from the bad... until it's too late. We started our company to help simplify your contractor search and help you have a stress-free renovation experience. We're always available to help, no matter where you are in the process. Click here to learn more about our story.

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