A garage conversion and a new detached ADU are both legal second units in San Diego County. They are not the same project. One reuses a building that already has a slab, walls, and a roof. The other plants a new structure in the yard. Pick the structure first. The floor plan comes second.
Two different buildings, not two finishes
Garage Convert SD is a referral service. We match homeowners with licensed, independent local pros. We don’t convert garages, we don’t build detached ADUs, and we don’t hold a contractor license. The crew you hire does the licensed work. You contract with that pro directly.
A garage conversion starts with the existing garage, attached or detached. The permit changes the use from vehicle storage to a dwelling. The envelope is mostly already there. The work is insulation, utilities, openings, and finishes that meet residential code.
A new detached ADU is new construction in the yard. Foundation, framing, roof, and utility trenches are all in the bid. You get a layout that is not forced into two car-stalls. You also spend backyard area the garage conversion would have left alone.
A modular or prefab ADU is a delivery method for new construction, not a third legal product. That comparison lives on our prefab versus garage conversion post. A room addition grows the main house. That comparison lives on garage conversion versus room addition. This page is only convert-the-garage versus build-a-new-detached-unit.
CRS Builders, Groysman, ProStruct, and BluPlan all publish this same fork because San Diego homeowners actually ask it. The honest answer is site-specific. A blog cannot pick the winner for your lot.
When the garage you already have is the better shell
Keep the conversion path on the table when the garage is already in the right place: a rear detached garage with its own approach, or an attached garage whose door wall can become an entry without wrecking the street face.
State ADU law helps this path. Gov. Code §66314(d)(11) says the local agency shall not require replacement of garage parking converted to an ADU. The City of San Diego still has a Beach Impact Area parking path, and other coastal cities write their own exceptions. Confirm the city on the tax bill, not the neighborhood nickname. Conversions inside the existing footprint usually skip the setback fight that a new backyard building can trigger. ADUs at or under 750 square feet of interior livable space are exempt from impact fees under Gov. Code §66311.5. Most two-car garage conversions land under that line. Confirm the current text and your city’s fee sheet.
The conversion still has to become a dwelling. Slab moisture, ceiling height, egress, panel capacity, and sewer distance are the usual surprises. Our two-car floor plans post covers how the width splits. Our ceiling height and egress post covers when the shell cannot sleep anyone.
If you still need a car on site, a conversion spends that stall. A new detached ADU can leave the garage as a garage. That is often the whole decision on a street with no other parking.
When a new backyard unit is the cleaner project
Choose new detached construction when the garage is doing a job you cannot lose, when the garage sits on the front and you do not want to close the door, or when the layout you want will not fit two bays.
A new unit can face a side yard, set its own windows, and keep a private patio. A garage conversion inherits the driveway, the door wall, and whatever privacy the existing location allows.
New construction pays for foundation and roof that a conversion already has. The licensed pro you hire writes the number after seeing the lot. Garage Convert SD does not publish a conversion or detached-ADU price.
New detached ADUs also face local height, setback, and lot-coverage rules that conversions inside an existing garage often avoid. Confirm the jurisdiction. City of San Diego, another city, and County PDS are different counters.
Pre-approved ADU plan libraries, where a city or the County publishes them, are almost always for new detached units, not for converting an existing garage. A garage conversion still needs a custom floor plan of the shell you have.
How to decide on your lot
Walk the property with four questions.
Where do the cars go if the garage disappears. Where does the new unit sit if the garage stays. How far is sewer and power from each option. Which product is an ADU, a JADU, or only a habitable room.
A JADU cannot be a new detached backyard unit. It has to be within the primary residence, which is why an attached garage can be a JADU and a backyard cottage cannot (Gov. Code §66333(d)).
Qualifying ADU applications are ministerial. Gov. Code §66317 gives the local agency 60 days from a completed application. That clock is a decision deadline, not construction time, and it applies to both paths once the file is complete.
A full ADU conversion and a new detached build are both multi-trade jobs. If the price exceeds $500, California requires a written home improvement contract. The downpayment may not exceed $1,000 or 10 percent of the contract price, whichever is less (Business and Professions Code §7159). Verify the license at cslb.ca.gov.
Call (858) 400-4935 to get matched with a licensed independent local pro who will compare both options on your lot, not on a catalog page.