CALIFORNIA STATE – If you’ve heard someone say, “You can build an ADU for $100K,” they might be telling the truth — but only about a very specific kind of project (and often leaving out major line items).
In California, the real, all-in price can swing wildly based on what youre building, where you live, and how many “surprises” your property is hiding.
Below is what experts, public guidance, and recent real-world pricing suggest homeowners should actually plan for.
A quick disclaimer: we’re journalists, not realtors or builders. Nothing in here is meant as financial advice. Always consult a realtor or advisor before making decisions about your own home or finances.
1) The realistic statewide range (and why the range is so wide)
Across the U.S., ADU costs are often quoted in a broad “per square foot” range. Angi, for example, cites roughly $150 to $300 per square foot on average, with some projects going over $600 per square foot. (Angi)
California can run higher than national averages, especially in expensive labor markets. UC Berkeley’s Terner Center has noted that Bay Area ADU construction costs can exceed $800 per square foot in some cases (their example: a 500 sq ft ADU reaching $400,000). (Terner Center)
Translation: A “normal” build in a lower-cost California market might land in the mid-to-high six figures, while a complicated Bay Area project can push into the $300K-$500K+ zone.
2) What many homeowners actually mean when they quote a low number
A lot of the lowest numbers floating around are missing one (or more) of these:
- Site work (grading, retaining walls, drainage, tree removal)
- Utility upgrades (electrical panel, trenching, new meters, sewer laterals)
- Permits and plan check
- Design + engineering
- Finishes (the difference between “builder basic” and “nice” adds up fast)
Prefab pricing is a classic example. One widely covered factory-built ADU company has advertised units with prices starting around $95,000 — but reporting also notes that this is not the full installed, permitted, utility-connected cost. (Business Insider)
3) A practical “all-in” budget by ADU type (California reality check)
These are common budgeting buckets homeowners use to sanity-check early planning. Think of them as planning ranges, not quotes:
- Garage conversion / conversion ADU: Often the least expensive because you already have a slab, walls, and a roof. Still, bringing it up to code and adding plumbing/kitchen can be significant.
- Attached ADU: Usually cheaper than detached because you can share walls and sometimes shorten utility runs.
- Detached ADU: Typically the most expensive per square foot because youre building an entire small house from scratch (foundation, framing, roof, exterior finishes, separate systems).
Local builder guides in major California markets frequently place many detached builds in the roughly $150,000 to $400,000+ band depending on size/finish level, with high-cost areas pushing higher. (ladu.co)
4) The fees California law can reduce (and the fees it doesnt)
California policy can meaningfully change your budget — but only in certain categories.
- Impact fees: The state ADU handbook says an ADU under 750 sq ft is exempt from impact fees from local agencies, special districts, and water corporations. (California Housing Department)
- Utility connection/capacity charges: Guidance also says connection fees or capacity charges should be proportionate to the burden of the ADU relative to the primary home. (ahcd.assembly.ca.gov)
That said, proportionate does not mean cheap. Utility work (especially electrical upgrades or long trench runs) can still be one of the biggest “how did it get so expensive?” line items.
5) The sneaky cost drivers that blow up budgets
If you want to know whether your ADU will be “normal expensive” or “painfully expensive,” experts and builders tend to look at these first:
- How far utilities have to run (and whether the main panel/sewer line can handle it)
- Soil, slope, and drainage (flat lots are a gift)
- Access (can crews and equipment reach the backyard easily?)
- Foundation type (new slab vs. raised floor vs. hillside solutions)
- Local labor market (Bay Area and coastal metros generally cost more)
- Finish level (a few upgrades repeated across kitchen, bath, flooring, windows adds up fast)
Terner Centers Bay Area cost-per-square-foot warning is basically a summary of what happens when multiple cost drivers stack up at once. (Terner Center)
6) How to get a better estimate fast (without wasting months)
If you want a solid early estimate, the fastest path is usually:
- Pick the ADU type and target size (example: 600-750 sq ft is popular because of livability and, under many scenarios, helps avoid impact fees under 750 sq ft). (California Housing Department)
- Get a site walk from a design-build ADU contractor (theyll flag utility/panel/sewer/slope risks quickly).
- Ask for a line-item budget that separates:
- Hard construction
- Site work
- Utilities
- Soft costs (design, engineering, permitting)
- Contingency
Pro tip: insist on a contingency line. Even well-planned ADUs can uncover surprises once demolition or trenching starts.
7) Ways homeowners actually keep costs down
The strategies that show up again and again:
- Build smaller (the minimum size that still works for your needs)
- Choose a simpler footprint (rectangles beat complicated shapes)
- Use pre-approved plans when available (can reduce design time and uncertainty)
- Avoid relocating major utilities unless you have to
- Consider conversion ADUs when your existing structure is in good shape
Prefab can also help in the right scenario, but remember to compare “unit price” vs. “installed, permitted, turnkey total.” (Business Insider)
Bottom line
A realistic California ADU budget is often less about “price per square foot” and more about (1) site conditions + (2) utilities + (3) local labor + (4) how custom you go. In many parts of the state, homeowners commonly end up in the mid-to-high six figures all-in, and in high-cost markets (especially the Bay Area) it can climb much higher. (Angi)
