Garage Conversion Cost: What You'll Really Pay

A garage conversion is the cheapest way to create living space or a rentable ADU — the walls, roof, and slab already exist, and you're not paying for any of them. That part of the sales pitch is true. What the "starting at" prices hide is that garage conversions have the widest cost variance of any small project I've supervised, because everything depends on what your particular garage is hiding.

The realistic range

For a standard two-car garage (roughly 400 square feet) converted to a permitted ADU in Southern California, realistic all-in budgets generally run from the high five figures to low six figures — commonly somewhere between $80,000 and $150,000 as of 2026, with simple living-space conversions (no kitchen, not a legal dwelling) coming in meaningfully below that. Anyone quoting far below this range is either excluding items you'll pay for anyway or planning an unpermitted conversion — which creates resale, insurance, and legalization problems that cost more than the permit ever would.

What actually drives the price

• The slab. Garage floors slope toward the door and often sit lower than the house. Leveling, moisture barriers, or partial re-pours are common — and invisible in low bids until the "unforeseen conditions" change order arrives.
• Plumbing runs. A bathroom and kitchenette mean sewer and water lines to a building that never had them. Distance from the main line is one of the biggest single variables — trenching through a driveway is not cheap.
• The electrical panel. An ADU's load frequently pushes older homes past their panel capacity. A main panel upgrade adds real cost, and utility coordination adds real time.
• Insulation and openings. Garages aren't built to dwelling code: walls and ceiling need insulation, the garage door becomes a framed wall (usually with new windows and egress), and Title 24 energy compliance applies.
• HVAC. Usually a mini-split — modest cost, but it has to be in the plans, not an afterthought.
• Parking rules. Good news: California law generally prohibits cities from requiring replacement parking when a garage converts to an ADU. You lose the storage, not the project.

The fee-waiver advantage

Nearly every garage conversion lands at 750 square feet or less, which means state law waives development impact fees — one of several reasons conversions pencil better than new construction per rentable square foot. Plan check and permit fees still apply; the permit process is the same sequence as any ADU, minus foundation inspections you mostly won't need.

How to read garage-conversion bids

Because the variance lives underground and behind walls, this project type is where bid leveling earns its keep. Three rules: First, any bid without a line item for slab condition and sewer distance is a bid that will grow. Second, allowances (fixtures, finishes) should be realistic numbers, not $500 placeholders. Third, ask each bidder the same question — "what happens to the price if the slab needs work?" — and compare the answers. The honest ones have an answer; the cheap ones have a change order waiting.

Is it the right ADU path for you?

If your goal is maximum rental return on minimum investment, a garage conversion is usually the strongest play in San Diego — typical one-car and two-car footprints match studio and one-bed layouts that rent reliably (see ADU floor plans that make rental sense). If you need more space or want to keep the garage, compare against prefab and site-built options with real all-in numbers — the full San Diego ADU cost breakdown shows how the paths compare. And if you want those numbers gathered, leveled, and argued over on your behalf, that's what I do.