An ADU is a second home. An addition is more of the one you have.

Questions worth asking.

Is an ADU or a home addition better?

Neither is better in the abstract; they answer different questions. An addition makes the house you live in bigger, and it is the right call when you need another bedroom or a larger kitchen inside your own front door. An ADU is a second, separate home on the same lot, and it is the right call when the space needs its own entrance, its own kitchen and the option of rent. The legal differences follow from that one distinction: an ADU is reviewed ministerially on a 60-day clock under Government Code §66317, gets four-foot side and rear setbacks under §66314(d)(7), pays no impact fees at 750 square feet or less under §66311.5(c)(1), and may be rented separately under §66314(d)(1). An addition gets none of those, because it is not a separate dwelling.

Which one is approved faster?

An ADU has the stronger guarantee. State law makes the review ministerial — no hearing, no design board — and gives the city 60 days from a completed application, after which the application is deemed approved (§66317(a)(1) and (a)(3)). The clock starts at completeness, not at first submission, so correction rounds delay when it begins. A home addition runs the ordinary zoning path with no statutory deadline. In practice the schedules can still land close together, because the ADU's clock only covers the city's review, not design and engineering.

Which one costs more?

A detached ADU is a whole small building — new foundation, full kitchen and bath, its own utility runs — and Avorino publishes it at typically $245K – $440K+ for 500 – 1,200 sqft over 5–8+ mo. An attached ADU shares a wall and often a service line, at typically $200K – $350K+ for 450 – 1,000 sqft. An addition is priced per square foot against the existing structure, and the published range for that sits on our additions page. Compare the two on what you get: the ADU can be rented, the addition cannot.

Do impact fees really disappear on an ADU?

On a unit of 750 square feet of interior livable space or less, yes: §66311.5(c)(1) bars a local agency, special district or water corporation from imposing an impact fee at all. Above that size the fee is charged proportionately to the square footage of the primary dwelling. Utility connection fees and capacity charges are a different thing and sit outside the exemption, though §66311.5(b) requires them to be proportionate to the unit's burden on the system. An addition carries the city's ordinary fees either way.

Can I do both?

Often, yes — they are separate permits and separate scopes, and state law does not make one exclude the other. What decides it is the lot: how much clear side and rear yard is left after the four-foot setbacks, what the city's lot-coverage limit leaves you, and where the sewer lateral and the electrical panel sit. That is a site question, and it is the first thing we check.

Does an ADU add more value than an addition?

We do not publish a valuation claim, and anyone who gives you a single number for it is guessing at your market. What we can say plainly is the structural difference: an ADU can be rented separately from the house under §66314(d)(1) and an addition cannot, so only one of the two can produce rent. Whether that matters more than extra space inside your own front door is your call, and the ROI calculator runs your own numbers rather than ours.

Related: detached ADU, home additions, what is an ADU, and ADU design ideas.