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Cost to build an addition in the Bay Area

July 2, 2021

If you’re thinking of building an additional room onto your property anywhere in the Bay Area, we’ll go over all the considerations and incentives that cities have started to offer for ADUs and additional living units.

But before you try to get a quote from your local builder and architect, the wisest first step is to call your local building department and tell them your address and what you exactly intend to do. It is their job to help you comply with the local building regulations by directing you to the best resources. 

Here’s a list of things to cover with your city building department representative:

  • What you plan on doing to your property
  • The size of the addition
  • The exterior aesthetic qualities of your building

Ask them the following questions too:

  • Is my property zoned for an addition? If so, what are the square footage restrictions?
  • Where and what type of addition can I do on my property?
  • What will the city’s building department need from me and my builder to get permits for this addition?

The main cost drivers of building an addition in the Bay Area

Plans & designs 

After talking with your local building department, it is time to link up with a local general contractor who has experience building ADUs in your city. As you might have gathered, every city has unique requirements so working with a local builder is key to having a smooth experience.

Based on your square footage and design needs, your builder will either work with a licensed architect or a drafting team to create plans for the addition. If you’re doing a detached structure that will have its own foundation or modifying any walls of the existing home, your builder will have to work with a local architect to get plans drawn up. If the addition is merely converting a space into a livable area, like a garage, then plans can be drawn up by a drafter. Architects plans will run from $7,000 to $10,000. Drafters will typically charge 50% less.

A cost-effective alternative is to call your local building department and ask them if they have pre-approved ADU & addition plans that they can give you. In recent years, the state of California has tried to tackle the issue of unaffordable housing by incentivizing the creation of ADUs. Many cities like San Jose, Fremont, and Danville give out free pre-approved plans. This not only means you’re getting plans that are worth over $7,000 but they’re also pre-approved, so you won’t have to wait for plans reviews which can take 1 - 3 months.


Permits and inspection fees differ from city to city and are based on the size of the ADU being built. But to give you a ballpark estimate, you can expect to pay $10 per square foot for plans and inspection fees. You can save time and money if you decide to go with plans that your city offers.

Rough materials & labor

Moving on to the cost associated with your builder, rough materials and labor cover everything from the foundation preparation, framing of the structure, and connecting the structure to the utilities. Rough materials include the cost of the basic materials such as lumber and wood products, insulation, standard roofing, drywall, conduit, and plumbing. It does not include finish materials which we will go over in the next section.

You can expect to pay $400 to $500 per square foot for rough materials and labor for an addition in the Bay Area.

Finish materials

What you spend on finish materials is entirely up to you. If you’re going for higher end products then that will naturally drive the cost up. Check out our resources page to see a list of our vendors who offer discounts to Home Quality Remodeling customers.

Other considerations

Upgrades on your structure like a state-of-the art HVAC system or solar panels aren’t accounted for in the above mentioned numbers but it is typically cheaper to install these while the structure is being built rather than installing it later down the road. So ask your builder about these early on.

Budget for 10% more than the quoted price in case you run into unexpected issues with the process. Working with an experienced team like ours will lessen the probability of issues but sometimes, things just come up. An example of this is that your utility lines are damaged along the way and need to be repaired.

It can take roughly 4 to 8 months to build your addition so take into account that you’ll have a construction team on your property for that time. 

Lastly, if you’re interested in financing your addition, visit our financing page. We work with Renofi who can finance home renovations up to the mid $500s.

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