Detached ADU vs Garage Conversion: Which One Should You Build?
For most Orange County lots these are the only two real options. Here is what each costs, how long each takes, and what you give up — with the numbers we actually publish.
A garage conversion turns space you already own into a legal ADU for $90K–$150K in roughly 4–5 months. A detached ADU builds new in the backyard for $200K–$450K+ over 6–12 months. Conversion wins on budget and speed; detached wins on size, privacy and rent. Both are approved ministerially — and state law forbids your city from making you replace the parking a converted garage removes.
The numbers at a glance
| Garage conversion (2-car) | $90K–$150K · about 4–5 months |
| Custom detached ADU | $200K–$450K+ · about 6–12 months |
| Parking you lose | None replaced — cities cannot require replacement parking when a garage converts |
| Size ceiling | Conversion: your garage footprint · Detached: 800 sq ft guaranteed, ~1,200 sq ft typical local max |
| Impact fees | Neither pays them under 750 sq ft |
What each path costs in 2026
| Build type | Typical 2026 range |
|---|---|
| Garage conversion — 2026 range | $90K–$150K |
| Detached ADU — 2026 range | $200K–$450K+ |
| Junior ADU — the third option, inside the house | $60K–$130K |
Ranges at our published 2026 rates; final pricing depends on site conditions — panel, utilities, slope, access. Run your exact size in the cost calculator.
The comparison, side by side
The honest summary is that these two options answer different questions. A conversion asks “what is the cheapest legal way to add a rentable unit?” A detached build asks “what is the best second home this lot can hold?” Neither answer is wrong — but the lot, the budget and what you plan to do with the unit decide which question you are really asking.
| Garage conversion | Detached ADU | |
|---|---|---|
| 2026 cost | $90K–$150K (about $150–$300/sq ft) | $200K–$450K+ |
| Typical timeline | About 4–5 months, first site visit to final approvals | About 6–12 months, design to final approvals |
| Size you get | The garage footprint — a 2-car garage is roughly 400 sq ft | Up to 800 sq ft by right; ~1,200 sq ft is the typical local ceiling |
| What you give up | Garage parking and storage | Yard space |
| Privacy | Attached to, or right beside, the main house | A separate building with its own entry and outlook |
| Where it rents | Lower half of the county’s $1,800–$4,000 band | Upper half, especially with two bedrooms |
| Approval path | Ministerial — no hearing, 60-day clock | Ministerial — no hearing, 60-day clock |
| Parking rules | No replacement parking can be required | One space maximum, waived near transit |
Ranges are our published 2026 figures; timelines are early-planning estimates, not guarantees. Site conditions — panel capacity, utility runs, slope, access — move both. Run your size in the cost calculator.
When the garage conversion is the right answer
Choose the conversion when budget is the binding constraint, when you want a tenant or a family member in place this year rather than next, or when the yard is genuinely too small for a detached unit after 4-ft setbacks. It is also the right call when the garage is already sound: a level slab, adequate ceiling height and utilities nearby are most of the cost saved. The trade you are making is real — you lose the parking and the storage, and the unit is only as big as the footprint you started with. What you are not losing is legality: state law bars cities from requiring you to replace that parking, which is the objection homeowners most often expect and most often never face.
When detached wins
Choose detached when the unit has to hold more than one bedroom, when privacy between households matters (parents, adult children, a long-term tenant), or when you want the strongest rent position the property can support. It is also the better answer on lots with room to spare, where a standalone building leaves both homes with usable outdoor space. And it is the only path to the sizes state law protects most generously — 800 sq ft guaranteed on qualifying lots, with roughly 1,200 sq ft available in most cities. On tight lots that ceiling is still reachable by going up: our Rosemead build put 1,188 sq ft on about half the ground footprint.
The option most people miss: do both
A single-family lot may add one ADU and one Junior ADU. That means the attached garage can become a JADU (up to 500 sq ft, inside the existing home) while a detached ADU goes in the backyard — two additional units on one property, each on its own ministerial path. Owners planning for aging parents now and rental income later often find this is the sequence that actually fits: convert first at $60K–$130K, build detached when the budget is ready.
What we check before you choose
The recommendation comes out of the site visit, not a spreadsheet. On the garage side: slab condition and level, ceiling height, fire separation from the house, and how far utilities have to travel. On the detached side: what the yard genuinely holds after setbacks, panel capacity, the sewer or septic path, and access for equipment. We put both options in writing with the real numbers for your address before any design money is spent — and if the conversion is the better deal, we say so.
Frequently asked questions
Is a garage conversion cheaper than a detached ADU?
Do I have to replace the parking I lose when I convert my garage?
Which one takes longer to build?
Which one rents for more?
Can I build a detached ADU and convert my garage?
How big can each one be?
Check what your property allows
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