ADU Builders in San Jose & the South Bay
Detached ADUs, garage conversions and junior ADUs — designed, permitted and built by a licensed general contractor.
Accessory Dwelling Units, Built Properly
An ADU is the rare project that pays for itself. Across San Jose, Gilroy, Morgan Hill and the wider South Bay, property owners are adding accessory dwelling units for rental income, for aging parents, for adult kids who cannot afford Silicon Valley rent, and for the flexibility of having a second unit on a lot they already own.
Also useful: our South Bay building permits guide.
D One Builders is a licensed general contractor (CSLB #1028049) based in Gilroy. We build ADUs the same way we build custom homes — proper foundations, proper framing, proper inspections. An ADU is a house. It is smaller, but nothing about the standard is smaller.
What an ADU Costs in San Jose and the South Bay
Most contractors will not publish numbers. Here are ours: ADU construction typically runs $180,000 to $350,000, depending on size, finishes and how much site work your lot demands. A garage conversion generally lands at the lower end of that range because the shell already exists. A new detached unit with its own foundation, utility runs and full kitchen lands at the upper end.
Against that, ADUs in this market commonly rent for $2,500 to $4,000 per month. That is the entire reason the category has grown so quickly here. We are happy to walk through the payback math on your specific lot before you spend a dollar on drawings.
The variable that surprises people is site work. Two identical units on two different lots can differ by tens of thousands of dollars once you account for utility distance, grading, drainage and sewer connection. We look at that first, because it is the number most likely to change your decision.
The Four Types We Build
Detached ADU. A standalone unit on your lot with its own entrance, kitchen and bath. The most private option, the most flexible to design, and typically the strongest rental performer.
Garage conversion. Converting an existing attached or detached garage into a legal dwelling. Usually the fastest and least expensive route to a second unit, since the foundation, walls and roof are already standing. Popular throughout San Jose and Morgan Hill.
Attached ADU. An addition that shares a wall with your existing home but functions as an independent unit. A good fit on narrower lots where a detached structure will not clear setbacks.
Junior ADU (JADU). A smaller unit created inside the existing footprint of your home, typically up to 500 square feet. The lowest-cost entry point, and often the simplest to permit.
Permits and California ADU Law
California has spent the last several years steadily making ADUs easier to build. The most useful change for homeowners: cities are required to review compliant ADU applications ministerially — as an over-the-counter approval rather than a discretionary hearing where neighbors can object. In practice that makes an ADU more predictable to permit than many ordinary remodels.
That said, every city still applies its own zoning, setback, height and design standards, and those differ meaningfully between Gilroy, Morgan Hill, San Jose, Los Gatos and unincorporated Santa Clara County. We handle the permitting end to end and deal with the planning departments directly — we work in these offices constantly and know how each one operates.
How the Process Works
1. Site walk. We look at your lot, your setbacks, your utilities and what you are trying to accomplish. You get a straight answer on feasibility and a realistic cost range before you commit to anything.
2. Design. We develop the plans, size the unit to your lot and your budget, and make the trade-offs explicit rather than burying them.
3. Permitting. We submit, respond to plan-check comments and manage the city relationship.
4. Construction. Foundation through final inspection, with a real schedule and regular updates. You always know what is happening this week and what is coming next.
Where We Build ADUs
We build ADUs throughout the South Bay from our base in Gilroy, including San Jose, Morgan Hill, Gilroy, Los Gatos, San Martin, Hollister and San Benito County.
If you are weighing an ADU against a home addition, we are glad to talk through both — they solve different problems, and the right answer depends on whether you want rental income or more room in the house you already have. You can also read our California ADU construction guide for a deeper walkthrough, or browse recent projects.
ADU Questions We Get Constantly
How much does an ADU cost in San Jose?
ADU construction typically ranges from $180,000 to $350,000 depending on size and finishes. A garage conversion is usually the lowest-cost route, while a new detached ADU with its own foundation, utilities and full kitchen sits at the higher end. Site conditions matter more than most people expect: utility runs, grading and sewer connections can move a budget by tens of thousands before a single wall goes up.
How much rental income can an ADU generate in the South Bay?
ADUs in this market often generate $2,500 to $4,000 per month in rental income. That range is why the payback math works for so many South Bay property owners, and it is the single biggest reason ADU demand has grown across Gilroy, Morgan Hill and San Jose.
How long does it take to build an ADU?
Most ADU projects run six to twelve months from first drawing to final inspection. Design and permitting typically take two to four months, and construction takes four to eight. Garage conversions are usually faster than new detached builds because the foundation and shell already exist.
What types of ADUs do you build?
We build detached ADUs, attached ADUs, garage conversions and junior ADUs (JADUs) created within the footprint of an existing home. Which one makes sense depends on your lot, your setbacks, your budget and what you want the unit to do — rental income, multigenerational housing or a home office.
Do you handle ADU permits?
Yes. We handle design, permitting and construction, and we work directly with the planning departments in Gilroy, Morgan Hill, San Jose, Hollister and the surrounding South Bay cities. California law requires cities to review compliant ADU applications ministerially rather than through discretionary hearings, which makes the process more predictable than a typical remodel — but each city still applies its own zoning, setback and design standards.
Can I convert my garage into an ADU?
In most cases yes, and it is often the most cost-effective path to a legal second unit because the slab, walls and roof already exist. The work usually involves insulation, electrical and plumbing runs, egress windows, and bringing ceiling height and fire separation up to code. We will tell you honestly if your garage is a poor candidate before you spend money on drawings.
Is an ADU a good investment in Silicon Valley?
For most South Bay property owners, yes. Between rental income and the value added to the property, ADUs are one of the strongest returns available in residential construction right now. The honest caveat is that returns depend on your lot, your financing and how long you plan to hold the property — we would rather walk you through that math up front than after you have committed.
Talk to a Licensed Builder
No pressure and no sales pitch — just a straight answer on what your project takes and what it costs. D One Builders, CSLB #1028049.
Request a Free EstimateOr call 831-318-5896