Garage Conversion vs Detached ADU: Which Should You Build?

Almost every ADU project starts with the same fork in the road: convert the garage you already have, or build a new detached unit in the yard. The honest answer is that neither is "better" — they solve different problems at very different price points, and the right choice usually becomes obvious once you understand what you're actually paying for in each.

The cost gap is real, but smaller than it looks

A garage conversion typically runs $75,000–$200,000 depending on your city and scope, while a detached new build runs $150,000–$450,000 in California's major markets. The conversion is cheaper for one simple reason: the most expensive parts of a building — foundation, framing, roof — already exist. You're paying for insulation, drywall, electrical, HVAC, and, if you're making a full ADU, a kitchen and bathroom.

But the gap narrows fast when the garage isn't cooperative. A cracked or sloped slab, low ceilings, or a structure built before modern code can add tens of thousands in remediation before the "conversion" part even starts. A garage in poor condition can cost nearly as much to convert as a small detached unit costs to build — with a worse result. Get the slab and structure assessed before committing; it's the single most important inspection in a conversion.

What each option actually gets you

A detached ADU is a small house: better privacy, better light on all four sides, a layout designed from scratch, and typically stronger rental appeal and rent. It also preserves your garage — parking, storage, and resale buyers who want one. The price for all that is the full construction stack: excavation, foundation, utility trenching (often the hidden budget-killer if the unit sits far from your main lines), and a longer build.

A conversion gets you livable space at the lowest cost per square foot of any ADU path, on the fastest timeline — often 3–6 months of construction versus 5–8 for detached. The compromises: you lose the garage, the footprint is fixed at whatever your garage is (a 1-car garage at ~240 sq ft makes a studio, not an apartment), and attached conversions share walls and sound with your house.

The 750 square foot rule changes the math

In California, ADUs under 750 sq ft are exempt from development impact fees under state law — a saving of $15,000–$25,000 in many cities. This quietly favors conversions and modest detached units: a 2-car garage conversion (~440 sq ft) sails under the threshold, while a maxed-out 1,200 sq ft detached build pays full freight. If your goals fit in 749 square feet, the state is effectively paying you to stay small.

How to decide in practice

Choose the conversion if your primary goal is space on a budget — a home office, a room for a parent, a modest rental — and your garage is structurally sound and you can live without it. Choose detached if you're optimizing for rental income or long-term property value, need real separation, or your garage fails the inspection. And if you're between them, price both: our garage conversion calculator and ADU cost calculator use the same 2026 regional data, so the comparison is apples to apples for your state and finish level.

Run the numbers for your project

Every project is different — size, scope, and city move the total by six figures. Get an itemized estimate in under a minute:

Written and reviewed by ADUCostGuide. Cost figures reflect 2026 data and typical projects; rules and prices vary by city — see our city guides and confirm details with your local planning department and licensed contractors.