Build an ADU if you want rental income, a separate space for family, or a second unit that widens your buyer pool. Build an addition if you want your existing house to be bigger, with the new space connected to the rooms you already use. They cost similar amounts per square foot in Los Angeles. The difference is not price, it is what you get.
Side by side
| ADU | Room addition | |
|---|---|---|
| All-in cost | $120,000 to $400,000 | $300 to $500 per sq ft |
| Typical project | 600 to 800 sq ft unit, $200k to $250k | 400 sq ft bedroom and bath, $150k to $250k |
| Produces rent | Yes, commonly $2,000+ a month | No |
| Connected to the house | No, separate entrance and systems | Yes, part of the floor plan |
| Needs its own kitchen and bath | Yes, by definition | Only if you want them |
| Permit path | Protected by state law, 60-day decision | Normal plan check, no statutory deadline |
| Can a city say no | Almost never on a standard unit | Yes, if zoning or floor area will not allow it |
| Property tax effect | New construction assessed | New construction assessed |
| Resale | Adds an income-producing unit | Adds square footage to the house |
| Build time | 5 to 8 months after permits | 4 to 7 months after permits |
Build the ADU if
- You want the property to produce income, now or later
- You need housing for a parent, an adult child or a caregiver who needs their own front door
- You might downsize into it and rent the main house
- Your lot has a usable rear yard and reasonable access
- You want the widest possible buyer pool when you eventually sell
Build the addition if
- The problem is that your house is too small, not that your lot is underused
- You need a bigger kitchen, another bedroom off the hallway, or a primary suite
- You want the new space to feel like part of the house, not a building in the garden
- Your rear yard is small, or access to it is poor
- You are not interested in being a landlord
The thing people get wrong
Owners frequently try to build an addition and call it an ADU, or build an ADU and use it as a bedroom for the main house. Both create problems.
An ADU is a legally separate dwelling with its own kitchen, bathroom and entrance. If you build one and then open a door into the main house and use it as a bedroom, you have spent ADU money for addition value, and you may have created a permit problem. If you build an addition and later want to rent it, you cannot without adding a kitchen, a separate entrance and going back through permitting.
Decide which one you are building before design starts, because converting between them afterwards is expensive.
Can you do both
Yes, and on the right lot it is the strongest play. An addition that fixes the main house plus a detached ADU in the rear yard gives you the space you want and the income you want, and the permits run on separate tracks.
It is also the most expensive option, and the two projects compete for the same site access. We sequence them rather than running them at once.
What about a garage conversion
A garage conversion is an ADU, just the cheapest kind, at $120,000 to $150,000. If your goal is income and you have a garage you do not love, it is usually the best return on the list.
It is not an option if you need the garage, or if the garage is in poor condition, in which case a purpose-built detached unit is the better buy even at a higher price.
Los Angeles County figures, reviewed 2026-10-29 by America General Contractors, California contractor licence CSLB 1083262. Planning ranges, not a quote, and not legal advice. We confirm the current rules for your parcel before anything is drawn.
Questions
Which adds more value in Los Angeles?
Usually the ADU, because it adds an income stream as well as square footage, and appraisers increasingly treat a permitted ADU as a real asset. An addition adds value too, particularly when it takes a two-bedroom house to three, which widens the buyer pool substantially.
Which is faster?
Roughly the same, with the ADU slightly more predictable because state law caps the decision window at 60 days for a complete application. Additions have no equivalent deadline.
Which is cheaper per square foot?
They are close. A large ADU is often cheaper per foot than a small addition, because the addition's fixed costs spread over fewer feet and the ADU's spread over more.
Can I convert an addition into an ADU later?
Only by adding a kitchen and a separate entrance and permitting it as a dwelling unit, which usually costs more than building it right the first time.
What if I am not sure?
Tell us what you are actually trying to solve rather than which one you want. We will walk the lot and run both numbers before anything is drawn.
Related reading
- How much does an ADU cost in Los Angeles?
- How much does a room addition cost in Los Angeles?
- Can I build an ADU on my property in Los Angeles?
- ADU Cost Per Square Foot in Los Angeles
- Garage Conversion Cost in Los Angeles
- Does an ADU Raise Your Property Taxes?
