Want to build ADU in Bayarea CA

8 Replies


I am planning on building a ADU in Union City, CA . I have 8750 sq ft lot and I can build upto 800 sqft of ADU. Here are some of the questions

1. I spoke to the city and got confused on how city permit works. Not sure , if I need to hire a Architect and structural engineer or either will work?

2. regular construction / modular /prefab or premade panels?

3. My property is not square or rectangle shape . It is longer on one side and shorter on other side. Can I still go to modular constructions ? 

4. Is $225 /sq ft norma for modular ?

5. Should I get a architect /structural engineer (atleast 3k) , get a plan made and then get some quotes for regular build ?

6. Any idea on how is prefab / sq ft?


Each city had different requirements . You need to speak to an a city planner and ask all your questions . You can also make an appointment and go down there in person .


An ADU is a simple Structure. A licensed Architect in CA could probably design this for you and also add structural information that the GC can follow. In most Residential construction, a Registered Architect is not required, but please check with your City about this requirement.

The construction type - regular, Modular or Premade depends mostly on Time and Money. A modular is quicker but could be a bit more expensive. The regular could be cheaper but could take longer to build. So make notes of what is that which is important to you, Money or Time? although they both are interdependent.

The best way to understand costs is to get a Floor plan drawn and get quotes from at least 3 people in the area who have done ADU's

It's always a good idea to involves a professional right from the start, especially someone who is experienced as they will suggest techniques that could bring value and save money in the long run. The 3K is will be well spent.

Make sure you have your requirements in hand when contacting an Architect:

2 beds, 1 bath, etc, Daylighting, etc etc

Good luck!


I am building one in San Mateo and planning one in Hayward.

Spending about 20k on soft cost in San Mateo, for 640 sf ADU. Close to getting final approval, about 4 revisions, involving architect, civil engineer, arborist, gas line designer, and some other folks. Each city gives you different headaches, forcing you to shed money like crazy.

Plan about $200k for site built, expect rent of around $2500/month, plus appreciation of about $400k, just estimate.

For Hayward, doing a main house expansion (900 to 2000 sf) and an ADU (1000 sf) at the same time. Expect similar soft cost. Architect is talking to city for initial planning and floor plan, etc.

Thank you all for the replies . I was wondering if I should get a general contractor to do all of the work or get independent sub contractors like for pouring foundation and framing etc...

I was also looking at prefab walls made by wood or steel. I am confused on which route to take . Should I get a framing guy and ask him to do my framing from scratch or get pre paneled walls ?

Which one is better, steel or wood paneled wall? 

Should I look at panel walls or do the entire framing from scratch ?

Sorry If my questions are all over . I am very confused . I want to save money but at the same time do not want to take up stuff, which I will not be able to finish

I'm looking to build an ADU in Castro Valley. We're limited to 640 sq ft. It would've been nice if I was able to go up to 800 sq ft.

I'm working with a design/build firm that takes care of everything  for the most part.  Design fees were about $10k in which they get everything ready to submit to the city for approval.  You'd  still need to pay for the permit fee.  

I'm in the process of getting my plans approved now. 

I don't  know if I'm allowed to list any company names on the post but PM me and I'll let you know who I'm working with.  You should check them out and see if that's something you're interested in.  ADUs are their specialty and they've got a active sites in the bay area that you might be able to visit.  They do prefab and stick builds.  I'm told general build time is about 3 months which is pretty quick.  They've got a Facebook page as well.

I considered acting as a project manager  but I'm no expert and didn't want to deal with the hassle of managing a bunch of contractors.   I also don't have a lot of time.

Goodluck and let us know what you decide.

@Vipul Parikh exciting! Your number one priority should be to talk with an architect/general contractor that has done it before to just get their feedback. You should not have to pay them for just a call, and I am almost certain that they will be able to help you streamline the process quite a bit!

Hi @Vipul Parikh, I am also considering building a smaller ADU in the union city area and I am interested in knowing if you completed this project and would appreciate any referral or advice you could share. Thanks!