ADU vs. Garage Conversion: Which Is Actually Right for You?

If you’ve been reading ADU sites for more than fifteen minutes, you’ve probably noticed a pattern. The contractor-marketing pages all push the same answer: build new. Bigger ticket, longer engagement, more flexibility on design, more for them to sell you.

The honest answer is: it depends, and the math is rarely close.

This is a decision people get wrong because they make it on the wrong axis. They compare what the finished spaces will look like and pick the prettier picture. The right comparison is between two different kinds of project. One costs less and finishes sooner but boxes you into the geometry you have. The other costs more and takes longer but gives you a building shaped for the use you actually want.

Here’s what actually changes between the two.

What you’re really choosing between

A garage conversion turns the existing structure on your lot into livable space. The walls and roof are already there. You’re adding insulation, drywall, plumbing, HVAC, electrical, and finishes, plus whatever you have to do to bring the structure up to current building code for habitable space. In most cities, the legal status of the converted unit is “ADU” or “JADU” (junior ADU). Same regulatory bucket as a new build, but with a head start on the structure.

A detached ADU (often called a DADU, or just “the backyard cottage”) is a new building from the foundation up. You pour concrete, frame, roof, finish, and connect to utilities. The footprint, height, and orientation are yours to choose, within whatever your city’s setback and size rules allow.

Both end with a permitted, livable, secondary unit. The path to get there is what differs, and it differs in ways that matter.

The cost gap

Across the US in 2026, the typical ranges look like this:

Build type	Typical 2026 cost (turnkey, permitted)
Garage conversion (1-car, ~250-300 sq ft)	$80,000 to $150,000
Garage conversion (2-car, ~400-500 sq ft)	$120,000 to $220,000
Detached ADU (400 sq ft)	$180,000 to $320,000
Detached ADU (600 sq ft)	$240,000 to $400,000
Detached ADU (800-1,000 sq ft)	$300,000 to $500,000+

These are wide ranges because regional labor and material costs are wide. Coastal California, Seattle, and the Northeast skew to the top. The Midwest and most of the South skew to the bottom. (Numbers triangulated from contractor cost-guide pages including aduwizard.com, angi.com, and maisonremodeling.com as of May 2026. Confirm with two or three local builders before you budget.)

The conversion is roughly 40-60% the cost of an equivalent-size detached build. That’s the headline. But the headline isn’t the whole story, because the equivalent-size comparison is doing some work. Your existing garage is probably not the size you’d want a unit to be.

The size trap

Most attached garages were built between 1950 and 1990, when the standard was 20×20 (400 sq ft) for a two-car or about 12×22 (264 sq ft) for a one-car. That’s the box you have. You can’t push a wall out without it becoming an addition (different permit, different cost). You can’t change the ceiling height without re-doing the roof. The orientation is whatever the original architect picked, which was almost always “doors face the street.”

If 264 sq ft works for what you’re trying to do (a guest studio, a home office, a rental for a single person), the conversion is a screaming bargain. If you wanted 600 sq ft with a separate bedroom and a real kitchen, you can’t get there from a one-car garage without doing both a conversion and an addition, and at that point the cost converges on a detached ADU anyway.

The size trap is the single most common reason garage conversions disappoint. People plan for what fits before they plan for what they need, and they end up with a unit that costs $130k and feels too small.

The permit and timeline difference

Permitting a garage conversion is usually faster than a new build, because the structure already exists and the planning department is reviewing what you’re changing rather than starting from a blank lot. Typical timelines:

Garage conversion: 4-12 weeks for permit approval, 8-16 weeks of construction. Total 3-7 months.
Detached ADU: 8-20 weeks for permit approval (foundation, structural, fire, planning all involved), 16-28 weeks of construction. Total 6-12 months.

The permit timelines have tightened in many states recently. California’s SB-9 and follow-on legislation forced cities to adopt by-right ADU permitting with statutory review windows. Washington’s HB-1337, Oregon’s HB-2001, and similar bills in other states have similar effect. Where these laws bind, both paths are faster than they were five years ago. Conversion is still the faster path, mostly because there’s less to review.

The hidden costs of conversion

Here’s what the cost-guide tables don’t tell you.

Garages were built to a different code than living spaces. To make yours legal as habitable area, you typically need to:

Add insulation to walls, ceiling, and (often) the slab. The slab insulation alone can mean cutting and re-pouring concrete, which is not cheap.
Replace or upgrade the slab and footings if they don’t meet today’s residential code (older garages often have 4” slabs that are technically fine but were poured for cars, not for the loads of habitable structures with full furnishings).
Add or upgrade the electrical service. Your garage probably has a 20-amp circuit for lights and a door opener. A unit with a kitchen, HVAC, and a bathroom needs its own panel or a serious upgrade to the house panel.
Run plumbing from the house, usually trenching across the driveway or yard. If your sewer line is on the wrong side of the house, this gets expensive fast.
Bring HVAC to the space. Mini-splits are the usual answer; they’re efficient but the install runs $4,000-$8,000 for a single-zone setup.
Replace the garage door with a code-legal exterior wall, including egress windows or a real exterior door.
Address ventilation, egress, and fire-separation to current residential standards. If your garage is attached to the house, fire-separation between the unit and the main dwelling is usually required and not always cheap.

None of this is exotic. All of it is real money. The cost-range for conversions assumes a garage in reasonable condition: slab is sound, no foundation work needed, electrical service has headroom, and the sewer line is on the right side of the house. Where any of those assumptions breaks, conversion costs trend up toward detached-ADU territory.

When the conversion is the right call

The garage conversion is the right answer when all of these are true:

The space you need fits in the footprint you have. (250-500 sq ft, depending on garage.)
The garage is structurally sound: slab, foundation, walls, roof.
Your house electrical and sewer can support a second unit without a major overhaul.
You don’t actually need the garage for cars. (This sounds obvious. People underestimate it. If you have one parking space and you live somewhere with on-street parking enforcement or HOA restrictions, losing the garage may cost you more than the conversion saves.)
The use case is single-person or rental: guest studio, home office, rental to a single tenant, or a “room-for-now” for an aging parent.

When all five are true, you can be moved in under $130k and inside six months. That’s a powerful combination and it’s why conversions are popular.

When the detached ADU is the right call

The detached build is the right answer when any of these are true:

You need more space than the garage gives you. (600+ sq ft, separate bedroom, full kitchen.)
You’re planning for a couple, a parent who needs an aide, or a family rental.
Your garage is in poor condition (cracked slab, sagging roof, foundation issues).
You’d lose meaningful resale value or daily utility by giving up the garage.
You want any of: separate entrance from a different side of the lot, a full second bath, vaulted ceilings, a private outdoor space attached to the unit, two-story.
You plan to sell within 5 years and you’re optimizing for resale appraisal value (a permitted detached ADU adds more to appraisal in most markets than a permitted conversion of equal square footage).

The cost is real. Figure $250k-$400k turnkey for something most people would want to live in long-term. But you get a building shaped for what you actually want, and you keep the garage.

The category most people miss: garage conversion plus a small detached structure

If you have a deep lot, a real third option exists. Convert the garage into a 250-sq-ft studio (cheap, fast). Build a small detached storage/workshop/parking structure separately (also relatively cheap, no habitability code burden). You get a usable rental or family unit for around $100k and you keep covered parking and storage.

Most people don’t consider this because they’re choosing between “convert” and “new build” rather than asking what the lot actually wants. Worth thinking about, especially in colder climates or HOAs where uncovered parking is a hassle.

The decision framework

Forget the marketing pages. The decision turns on three questions, in this order:

What’s the smallest space that solves your real problem? Not your aspirational problem. The actual one. Is it a studio for a single occupant, or is it a 1-bedroom for a couple? If it’s the studio, the garage conversion is on the table. If it’s the 1-bedroom, you’re probably in detached ADU territory.
What does your existing garage actually look like? Get a contractor or structural engineer to walk it before you budget. Cracked slab and a roof that needs replacing kills the conversion math fast.
What’s your real timeline? “I need this person moved in by Christmas” is a different project than “we’re 18 months out from my mom moving up here.” A 9-month detached build is fine on the second timeline. On the first, it’s the conversion or nothing.

If you can answer those three honestly, the right path is usually obvious. The cost difference is large, but it’s not the deciding number. The deciding number is what fits your situation.

What to do next

Walk your lot. Measure your garage (interior dimensions, ceiling height, where the electrical panel is, where the sewer cleanout is). Get two contractors out for estimates on each path, not one. Ask them to break out hidden-cost line items: slab work, panel upgrade, sewer trenching, fire separation. The estimates that don’t have those line items are the ones that will surprise you later.

When you’re ready to dig in further:

ADU cost guide: what $300k actually buys you in 2026.
Permit walkthrough: the steps in the order they happen.
Layouts that work in 400 sq ft: design ideas for tight units.

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