An ADU — an “accessory dwelling unit,” or what a lot of people just call a granny flat or backyard home — is one of the smartest things you can add to a San Diego property. It gives your family space, or gives you rental income, and it usually adds real value to your home.
The first question is always the same: what does it cost?
The honest numbers
There’s no single price, because there are two very different paths:
- Detached ADU — a brand-new standalone unit in your yard. These generally start around $150,000 and go up with size and finish level. You’re paying for a new foundation, walls, roof, and full utilities.
- Garage conversion — turning your existing garage into a living space. This is usually less expensive, because the foundation and shell already exist. You’re mostly paying to insulate, finish, and add a kitchen and bath.
Be careful with “per square foot” quotes you find online. San Diego’s costs, your lot, and your finishes all move the number. A real builder gives you a price based on your actual property, not a generic guess.
Detached ADU vs. garage conversion
Here’s the simple way to think about it:
Choose a garage conversion if you want to spend less, you don’t use the garage for much anyway, and the location and size work for your needs.
Choose a detached ADU if you want better light, more privacy, a purpose-built layout, and more long-term value — and your budget and lot can support it.
Neither is “better.” It depends on your goals. A good contractor will tell you honestly which one fits — even if it’s the cheaper option.
What’s included in a real ADU project
A legal, well-built ADU isn’t just four walls. It includes:
- Feasibility and planning — checking setbacks, utilities, and what your lot allows before you spend on drawings.
- Permits and engineering — so it’s legal, safe, and financeable.
- Full utilities — electrical, plumbing, and HVAC done to code.
- A real finished interior — a proper kitchen or kitchenette, a full bath, and durable finishes that feel like a home.
Why “built legal” matters so much
The biggest ADU mistake we see is an unpermitted conversion someone did on the cheap. It can come back to bite you at resale, at refinance, and with the city. An ADU built by the book is an asset. One built off the books is a liability.
We only build ADUs the right way — permitted, engineered, and inspected. If you’re weighing one for aging parents, adult kids, or rental income, let’s talk it through, or read more on our ADU page.