A detached ADU is a separate small house in the backyard. An attached ADU shares at least one wall with the main home. Detached units usually cost more to build but rent for more, sell for more and are easier to manage as a rental. Attached units can be cheaper and fit narrow lots, but they bring shared walls, fire-separation rules and less privacy.
For investors buying in Orange County or LA County, the choice often decides which lots are worth buying in the first place. Here’s how the two compare on rules, cost, rent and resale.
Detached vs Attached ADU at a Glance
| Detached ADU | Attached ADU | |
|---|---|---|
| What it is | Freestanding unit, usually in the backyard | Shares a wall, floor or roof with the main home |
| State-guaranteed size | At least 800 sq ft; up to 1,200 where allowed | Up to 50% of the main home, but never less than 850 / 1,000 sq ft floors |
| Height (state minimum) | 16 ft; 18 ft near transit (+2 ft for roof pitch) | 25 ft or the zone limit, whichever is lower |
| Side and rear setbacks | 4 ft | 4 ft |
| Typical build cost (OC/LA) | $250K–$350K for an 800–1,000 sq ft 2-bed | Often 5–15% less for the same size |
| Privacy | High | Lower (shared walls, sound) |
| Rent | Higher | Usually a little lower |
| Resale appeal | Strongest | Good |
| Disruption to the main home | Minimal | Construction ties into the house |
The size and height rules come from California Government Code §§ 66321 and 66323, summarized in the HCD ADU handbook. Your city can allow more, but not less.
What Is a Detached ADU?
A detached ADU is a separate building with its own kitchen, bathroom, entrance and utilities, either shared or separately metered. It’s the classic “backyard cottage.” In California, a city has to approve a detached ADU of at least 800 square feet with 4-foot side and rear setbacks on most single-family lots, regardless of lot coverage or floor-area limits.
A detached unit can be one or two stories where the city allows the extra height. I break down that trade-off in Two-Story vs Single-Story ADU.
What Is an Attached ADU?
An attached ADU shares at least one wall with the main house. Common versions:
- A rear or side addition with its own entrance.
- An attached garage conversion, sometimes with an expansion.
- A second-story unit over an attached garage, where height rules allow.
An attached ADU is different from a junior ADU. A JADU is carved out of the existing home, capped at 500 square feet, and can share a bathroom. An attached ADU is a full unit with its own kitchen and bath. I compare all three in In-Law Suite vs JADU vs Detached ADU.
Cost: Which Is Cheaper to Build?
Attached ADUs are often cheaper per square foot because they can share a foundation edge, a wall, and shorter utility runs. That saving shrinks once you add the work of tying into an older house:
- Fire-rated separation between the units.
- Structural tie-ins and roof transitions on an older home.
- Sound insulation so tenants and owners don’t hear each other.
- Bringing parts of the old house up to code where the work touches it.
In Orange County, a typical 850-square-foot, 2-bed, 2-bath detached ADU runs about $250,000 to $290,000 all in, including design, permits, school fees and utility work. I cover that budget in What City in Orange County Is Best for ADU Investing. An attached unit of the same size is often 5% to 15% less, but get bids on both before you assume it.
Fees can also shift the math. Units of 500 square feet or less skip school fees under SB 543. Units under 750 square feet pay no impact fees. Starting in 2027, SB 1117 charges impact fees only on the square footage above 750.
Rent: Which Earns More?
With the same bedroom count, a detached ADU usually rents for more. Tenants pay for privacy: no shared walls, their own yard space, their own entrance. In my experience the gap is often $100 to $300 a month for comparable units in central Orange County.
There are exceptions. In some pockets, an attached unit with a street-side entrance competes well, especially on narrow lots. I found that in West Garden Grove, where attached units win. Costa Mesa runs the other way. See Detached vs Attached ADU in Costa Mesa.
For citywide rent ranges by size, use my Orange County ADU rent guide by square footage.
Resale Value and Appraisal
Buyers of homes with ADUs fall into two groups: families who want space for a relative, and investors who want rent. A detached unit appeals to both. An attached unit appeals to families but can feel like “part of the house” to an investor.
Appraisers will also look harder at how the unit is permitted and used. I explain how ADUs are valued in How Much Value Does an ADU Add?
Which Lot Fits Which Type?
| Lot situation | Usually better | Why |
|---|---|---|
| Deep lot, 6,000+ sq ft, backyard access | Detached | Room for setbacks, separation and a private yard |
| Corner lot or alley access | Detached | Separate entrance and easier construction access |
| Narrow or shallow lot | Attached | May be the only way to fit 800+ sq ft |
| Large existing house with an attached garage | Attached conversion or JADU | Lowest cost path |
| Main home you plan to live in | Detached | Privacy for both households |
Corner lots are my favorite for detached units. Here’s why: Corner Lots Are the Best Layout for ADUs and SB 9.
Can You Have Both?
Often, yes. State law lets many single-family lots combine an ADU within or converted from existing space, a new detached ADU and a junior ADU. Starting January 1, 2027, AB 956 allows two detached ADUs on a single-family lot. That makes the detached path even more valuable for buyers who want multiple rents on one lot.
Frequently Asked Questions
What is a detached ADU?
A separate, freestanding dwelling on the same lot as the main home, with its own kitchen, bathroom and entrance.
Is an attached ADU cheaper than a detached ADU?
Usually by about 5% to 15% for the same size, but fire separation and tie-ins to an older home can close that gap.
Which rents for more, attached or detached?
A detached ADU usually does, often by $100 to $300 a month in central Orange County, because tenants value privacy.
How big can a detached ADU be in California?
Cities must allow at least 800 square feet, and many allow up to 1,200 square feet.
Do attached ADUs need a separate entrance?
Yes. An ADU needs its own exterior access, separate from the main home’s entrance. Check your city’s rules before designing any interior connecting door.
Pick the Lot Before You Pick the ADU
The lot decides whether detached or attached makes sense. I look at depth, access, setbacks and utility paths on every property before you write an offer, then run the rent and cost for each option.
👉 Book a buyer ADU consult call to go through your options with Dylan
Prefer to talk now? Call Dylan Serna, ADU Realtor at 714-860-2868.
Dylan Serna | The ADU Realtor | eXp Realty | DRE #02217359
This post is general information. Costs and rents are illustrative 2026 estimates; confirm rules for any specific parcel with the city.
