3 Ways to Cost Accessory Dwelling Units (ADUs)

If you're researching how much it costs to build an ADU, then you can use the 3 simple techniques in this article to get a ballpark “all in” construction cost.

How much it costs to build an ADU varies based on the building’s design and finish, the accessibility and slope of the property, and the cost of labor and material where the property is located. Costs commonly vary between $200/square foot to $600/square foot, based on these variables.

The following three techniques help you narrow down to a more precise and more useful estimate of your ADU costs.

If you’re early in the process, these three techniques will help you get a rough idea of the cost of building an ADU - and you can use them to explore different options. Each technique takes less than 10 minutes, so I recommend using all three and seeing how close they end up to each other…

• Use these example quotes from a contractor

• ADU cost calculator websites

• Average costs of finished ADUs in a developed market

ADU Sample Costs for “Typical Projects”

Contractors are happy to tell you what a project typically costs, as long as you understand this is a rough estimate.

For example, BuildZig have given us a sample ADU Cost Sheet that explains how much it costs to build a 500 square foot Accessory Dwelling Unit in Oakland. Another great resource in the East Bay is architect Todd Jersey who has written this guide on budgeting and pricing your ADU.

How to use it best

To use this Cost Sheet, simply skim the left-hand column of the sheet and find the rows that are most similar to your project. You can ignore the right hand of the sheet for now (that side focuses on estimating the potential value you’re creating and your return on investment).

There are different rows about the cost of basement and garage conversions, prefab or modular ADU solutions, and custom construction of a detached or attached ADU.

High end and low end ADU costs

To illustrate that the project price can vary a lot based on the details of my property, BuildZig have included multiple rows for some projects.

For example, if you’re interested in custom construction (sometimes called “Stick Build”) of a detached ADU, you can use the bottom two rows.

BuildZig says that a typical project with “easy access“ is going to run about $300/sq. ft. while the same design and floor plan on a lot with “difficult slope” might run about $450/sq. ft.

Note these prices are for Oakland which is pretty pricey even by California standards. I’ll write a separate guide about converting costs from one city to another. But for now, you can just treat this as a relatively high end estimate for California construction.

Please remember, you cannot order one of these ADUs at this price like you’re shopping on Amazon - the prices aren’t binding…

It’s simply a very easy way to get a rough ballpark cost of a typical ADU, at a glance.

An example of typical costs for Los Angeles ADUs

Here’s another resource from southern California where a contractor quotes price ranges for typical ADU projects around Los Angeles and explains some of the largest costs driving those price ranges. I haven’t worked with Bay Cities Constructions personally, but I think the post is very helpful to form a rough idea of what drives your ADU costs.

ADU Cost Calculator websites

This is the easiest way to play around with multiple ideas.

The cost calculators I’m linking to are run by county websites and non profits so they’re not trying to profit off of you.

Use the ADU cost calculator for San Mateo County or the ADU cost calculator for Napa County and Sonoma County.

If you don’t live in one of those three counties, it’s okay — use the San Mateo calculator and I’ll give you a way to convert it to your own part of California

How ADU Cost Calculators work

These calculators let you plug in several variables like the square footage of your ADU, the type of finish you want, the type of construction (new build, converting part of the house, or modular), and the lot’s terrain and accessibility.

There are more variables in the calculator but these four have the biggest effect.

The calculator has a lot of assumptions built in. For example, there are some costs that you’ll incur no matter how big or small your project is (fixed costs) while there are other costs that vary with the square footage of your project (variable costs).

These cost calculators are a little harsh on garage conversions because typical California garages need a lot of retrofitting and have some surprise costs. For that reason, they’ve built in an average surprise factor for garage conversions.

The great thing about cost calculators is that they are dynamically calculating the answer in real time when you change variables. So you can tweak the square footage and watch your costs change by square foot.

How to Use ADU Cost Calculators

The websites are pretty straightforward. On the left-hand side, you can input variables about your potential ADU project. On the right hand side, in the lower part of the screen, the website will calculate your all in costs.

These calculators also break down

• hard costs (construction, material, labor)

• soft costs (architect, engineering, foundation specialists, and so on)

• fees (permit and inspection fees)

In the top part of the right hand of your screen, these calculators try to assess your potential rent income. That’s very helpful but you can ignore it for now while you try to figure out your costs, and come back to it later when you’re ready to think about financing and return on investment.

How to adjust ADU Costs for your city

Use the County of San Mateo cost calculator, say you’re in South San Francisco, and then you can use this construction cost multiplier sheet to adjust your ADU costs.

Take the cost that the calculator gives you for South San Francisco, and multiply it by the multiplier for a city near you, and you’ll get a rough idea of construction costs in your area.

Broadly, if you live somewhere where construction labor costs are high, you should assume the estimate needs to be ramped up a little. If you live somewhere with construction labor costs lower than San Mateo, you can assume there’s headroom in the estimate.

If you’re using the Napa/Sonoma calculator, the cheapest area is Rohnert Park or unincorporated Sonoma County. The most expensive estimate is St Helena. You can use these to see the high end and low end of the calculator’s assumptions.

And now you’re ready for the third technique…

How much it cost to build actual ADUs

While the first two methods focus on hypothetical “typical” projects, this third technique is about using actual construction costs.

Kol Peterson is an ADU advocate and designer/builder in Portland, Oregon — he’s compiled a lot of self-reported data from Portland homeowners. And he’s published the actual construction costs they reported.

There are a few important things to keep in mind when you look at this data:

• Portland costs are not as high as many parts of California

• These include an estimate of the self-reported “sweat equity” investment of the homeowner

Nothing beats a good graph of actual costs, in my opinion, even when the numbers are less relevant to your location. If nothing else, it lets you know that these projects are real and they can be done by real homeowners like you!

How much will it cost to build your ADU?

These three techniques are great to get an estimate, but you may want to know how to get more precise about your specific ADU project costs.

I’d say you have three great options (and you can do any combination of them simultaneously):

1. Get professional expert advice

When you’re ready to get real about how much it will cost to build your specific ADU, I recommend you have an expert visit your property in person to suss out the details. BuildZig, the company that provided the example cost sheet above, offers professional ADU site assessments to do just that.

2. Take an online course to learn about building ADUs

If you’d like to learn more about Accessory Dwelling Units before proceeding, you can save $100 on this comprehensive ADU course organized and taught by Kol Peterson, the expert designer who put together all that useful data about actual ADU projects in Portland.

This course will help you wrap your head around what you can do to control your costs, finance your project, and be as successful as possible.

I highly recommend Kol’s classes, and I have signed up for his ADU Academy for professionals in the past.

3. Keep learning about ADUs on your own

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