Module 2 · Lesson 6
Existing Garages and Structures: Asset or Obstacle?
Your existing garage can be the cheapest path to an ADU — or a money pit. Slab, footings, framing, and attached-vs-detached, explained.
An existing garage or backyard structure is either your project's biggest head start or its most expensive surprise. The difference comes down to condition and configuration.
Why existing structures are valuable
A structure that already exists gets two huge advantages: it usually keeps its existing setbacks (even right on the property line), and its walls, roof, and slab are already built and paid for. That's why garage conversions are typically the lowest-cost ADUs.
What determines whether yours is a good candidate
The slab and footings. This is the big one. Many older garages were built on a thin slab with no real footings — fine for parking a car, not fine for a dwelling. If footings are missing or shallow, you'll need foundation work, from perimeter underpinning to (in bad cases) a full new slab. This single issue explains most of the spread between cheap and expensive garage conversions.
Framing and roof. Dwellings carry requirements garages don't: insulation depth, shear strength, sometimes new headers where the garage door was. A sagging roof or rotted framing means replacement, not reuse.
Height and size. Living space needs adequate ceiling height (7'6" in most habitable rooms under California code — check locally). A standard two-car garage gives you roughly 400 SF: a solid studio or small one-bedroom.
Attached vs. detached. An attached garage conversion turns part of your house into the unit — shorter utility runs, but you'll need to seal the connection between the two dwellings properly (fire separation, sound). A detached garage conversion behaves more like a small standalone home.
The honest math
If your garage is structurally sound, conversion is usually the cheapest ADU you can build. If it needs a new foundation, new framing, and a new roof — you're essentially building new construction inside an old shell, and it may be smarter to demolish and build the ADU you actually want. A feasibility review should tell you which side of that line you're on before you commit.
Key takeaways
- Existing structures keep their setbacks and their shell — the two reasons conversions are cheap.
- The foundation is the make-or-break item: garages without proper footings need real foundation work.
- A standard two-car garage yields roughly 400 SF — studio or small one-bedroom territory.
- If the slab, framing, and roof all need replacement, compare against demolishing and building new before committing.