An ADU is one of the few things you can add to a property that pays you back every month. The main question on homeowners' minds is what it takes to get there in terms of investment.
In this blog, we'll cover what an ADU costs to build in Vacaville, what you get at each investment level, the site and permitting costs homeowners forget, and how California's ADU rules can work in your favor.
An ADU or garage conversion in Vacaville costs $150,000 to $450,000+, depending on size, layout, and finish level. Refresh projects run $150,000 to $225,000, mid-range projects run $225,000 to $325,000, and custom projects start at $325,000 and climb past $450,000.
What separates those tiers isn't just square footage. A 500-square-foot studio with a kitchenette and builder-grade fixtures is a different build than a 900-square-foot two-bedroom with a full kitchen, laundry room, and a spa-style bath. Both are ADUs. They're not the same project.
The fastest way to find your number is to start with who's going to live there and for how long.
Lermaxx Builders groups ADU and garage conversion projects into three levels, separated by size, finish quality, and how self-sufficient the space needs to be.
A comfortable, functional living space with practical finishes.
Best for guest space, a home office, or a family member who doesn't need a full second household.
More space and better materials, built for someone living there full-time. This is where most Vacaville ADU projects land.
Best for long-term family housing or a rental you want to hold for years.
A fully independent residence with the finish level of a custom home.
Best for a parent moving in permanently, a premium rental, or a property you're building long-term value into.
A garage conversion usually costs less than a comparable detached ADU, because the foundation, walls, and roof already exist. How much less comes down to what shape that garage is in.
A conversion skips the most expensive early-stage work, but it still needs insulation, drywall, a full kitchen and bathroom, plumbing and electrical runs, HVAC, egress windows, and often a new subfloor. The savings are real, but they're not dramatic, and a few conditions can erase them entirely:
The building isn't the whole budget. These are the line items that catch ADU owners off guard, and they can add tens of thousands to a project.
California has some of the most ADU-friendly laws in the country, and several provisions directly reduce what you pay. These are worth knowing before you settle on a size.
State law prohibits any impact fee on an ADU with 750 square feet or less of interior livable space. Go over that line and you can be charged impact fees proportional, by square foot, to what the primary house would pay.
That single threshold is why a 749-square-foot ADU sometimes pencils better than an 800-square-foot one, and it's worth pricing your project both ways before you commit.
Under a state-mandated program, the city maintains pre-approved plan sets you can build from:
An application for a detached ADU using one of these plans has to be approved or denied within 30 days of being deemed complete.
If one of those layouts works for your property, it's the shortest, most predictable path to a permitted ADU, and notice that two of the four come in under the 750-square-foot fee threshold.
Vacaville requires one parking space per ADU, but state law carves out a long list of exemptions the city has to honor, including properties within a half-mile walking distance of public transit.
Converting existing space into an ADU triggers no parking requirement at all, tandem parking on your existing driveway can satisfy the requirement, and as of 2025 the city can't make you replace an uncovered parking space you demolish to build.
Vacaville caps ADU size against the primary dwelling: up to 850 square feet for a home of 1,700 square feet or less, up to 1,000 for a home of 2,000 or less, up to 1,200 for a home of 2,400 or less, and 50% of the primary dwelling above that. It's worth checking early, since it can rule out a layout before you get attached to it.
Run the math on your own property before you build, using rent against total investment rather than construction cost alone. The formula is simple: annual rent divided by all-in cost, including design, permits, utilities, and site work.
Rental income isn't the only return, though, and for a lot of our clients it isn't the main one:
Property value:
A permitted, legal second dwelling adds value that an unpermitted converted space never will
Avoided cost:
For families weighing assisted living or a second mortgage for an adult child, the comparison isn't rent, it's what the alternative costs every month
Flexibility:
A space that's a rental this decade can be a parent's home the next
Legal standing:
Permitted square footage appraises, insures, and sells. Unpermitted square footage does none of those things
Work through these before you start collecting bids.
We price the whole project, not just the building. Lermaxx Builders is a licensed and insured California general contractor serving Vacaville and the surrounding Northern California communities.
We start with a conversation about how you plan to use the space, your budget, and your timeline. Then we walk the property, because an ADU budget is a site question as much as a building question. Where the utilities run, how equipment gets to the backyard, what the soil and drainage look like, whether the existing panel has room. Those answers are why two identical ADUs on two different lots carry different numbers.
What you get back is a written scope of work with specifications, an estimated investment, and an anticipated schedule. When design, engineering, or permitting support is needed, we coordinate architects, designers, structural engineers, and permit professionals directly, or we'll work with the plans you already have.
An ADU in Vacaville costs $150,000 to $450,000+. Refresh projects run $150,000 to $225,000, mid-range projects $225,000 to $325,000, and custom projects $325,000 to $450,000 or more, depending on size, existing site conditions, layout, and finish selections.
Usually, yes. The savings come from reusing the existing foundation, walls, and roof. Slab elevation problems, distance to plumbing, insufficient ceiling height, or framing that doesn't meet code for habitable space can narrow that gap considerably, which is why both options are worth pricing.
Yes. The City of Vacaville requires a building permit to construct an ADU or convert an existing structure into one. ADU applications are reviewed ministerially under state law, meaning a complete and compliant application gets approved without a public hearing.
Start with who's living there. A guest suite, a full-time home for a parent, and a two-bedroom rental have genuinely different space requirements. Then layer in your lot, setbacks, and the 750-square-foot fee threshold. Those three inputs usually narrow it to one or two good options.
An ADU is a rare kind of project. It solves something specific and immediate, whether that's a parent who shouldn't be living alone anymore or income you'd like your property to generate, and it keeps paying off long after the specific reason you built it has changed.
The homeowners who end up happiest are the ones who started with a clear purpose and a real number. We'd like to help you get both.
Tell us who's going to live in it and what your property looks like, and we'll walk it with you.
Schedule your ADU consultation with Lermaxx Builders.