If you are thinking about adding an ADU to your LA property, the first decision is usually whether to convert an existing garage or build a detached unit from scratch. Both paths are allowed under California ADU law. They differ on cost drivers, timing, and the kind of lot each is best suited for.
What each path actually involves
A garage conversion re-uses the existing slab, walls, and roof of an attached or detached garage and rebuilds the interior as a code-compliant accessory dwelling — full kitchen, bath, sleeping area, and separate entry. The structure is already there; the work is in new openings, insulation, MEP, finish-out, and sound/fire separation from the main home.
A detached ADU is a standalone structure built fresh on the lot — foundation, framing, roof, MEP, weatherproofing, finish-out. Typically 600–1,200 sq ft. The work is more like building a small new home than remodeling an existing space.
When a garage conversion usually makes more sense
Garage conversions usually win when:
- The lot is dense and a detached unit would consume yard you want to keep
- The existing garage is in good structural shape (the slab, walls, and roof do not need to be rebuilt)
- Speed matters — garage conversions typically run 3–5 months of active construction vs. 5–9 months for detached new construction
- Budget is the binding constraint — re-using existing structure usually costs less than building from scratch
When a detached ADU usually makes more sense
Detached ADUs usually win when:
- You want more square footage than the existing garage offers
- You want full physical separation from the main home (for rental, for family, for guest use)
- The existing garage is too small or in poor structural condition
- Lot setbacks and coverage allow a meaningful detached footprint
- You want the unit to feel like a separate home, not a converted outbuilding
Surprises that come up in both paths
On garage conversions, the slab is the variable that most often surprises. Older garage slabs are sometimes too thin to carry a new partition wall, or not graded for indoor floor finishes, and need to be re-poured. On detached ADUs, soils reports on hillside lots and setback interpretations on tight lots are the most common timeline shifters.
Both paths typically need separate electrical subpanels. Separate water and gas service is sometimes worth it, sometimes not — depends on whether you plan to rent the unit and want submetering.
Next steps
The right path depends on your lot, your budget, your intended use, and what your existing garage actually looks like structurally. A feasibility review on your specific property is the right first step. We do this as part of a first consultation across LA County — Culver City, Calabasas, West LA, the San Fernando Valley, Pasadena, Glendale, Burbank, and surrounding areas.
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