ADU 101 — the four types, explained

The first wall most homeowners hit is not money or permits. It is language. Every builder website assumes you already know the difference between an attached ADU and a junior ADU, and none of them stop to explain it, because explaining it does not book the estimate.

There are only four things to learn. Once you have them, most of what you read about ADUs stops being confusing.

Detached ADU

A standalone building somewhere on the lot, usually in the rear yard. It has its own walls, its own roof, its own foundation, and typically its own utility connections. This is the type most people are picturing when they say backyard cottage.

Why people choose it: real separation. Nobody hears anybody. It is the easiest type to rent, the easiest to hand to an aging parent, and the only type where you get to design a whole building rather than remodel around what already exists. It is also the type most likely to add clearly attributable resale value, because a buyer can see a second house.

What it costs you: the most money per square foot, and a meaningful chunk of your yard. You are paying for a foundation, a full building envelope, and a roof before you have a single finished room.

Best for: rental income, multigenerational living, or anyone who wants a building that stands on its own architecturally. This is also the type where design quality matters most — a detached unit is a small building in plain sight, so it either looks like architecture or it looks like a shed. That is the whole subject of the designer ADU guide.

Attached ADU

New square footage built onto the existing house, sharing at least one wall, with its own exterior entrance. Think of a bump-out on the side or rear that happens to be a complete little home rather than a bigger family room.

Why people choose it: it usually costs less than detached because one wall, part of the roof structure, and often the utility runs already exist. It also eats less yard, which matters on a tight South Bay lot.

What it costs you: sound and privacy. A shared wall between a rental and your living room is a real thing you will notice. It also constrains design — the addition has to work with the existing roof geometry, which is the single hardest thing to get right visually. Done badly, an attached ADU reads as a lump on the side of the house.

Best for: narrow or small lots, and households who want a family member close rather than separate.

Garage conversion

An existing garage becomes a dwelling. The footprint and usually the roof stay put; the slab, insulation, windows, plumbing, and electrical get brought up to habitable standards.

Why people choose it: the shell already exists and is already approved to sit where it sits, which can simplify the setback conversation. It is often the fastest route to a finished unit.

What it costs you: your garage, first of all — and in most Bay Area neighborhoods that means committing to street parking permanently. There are also real technical costs people underestimate: garage slabs are usually sloped to drain and are often not insulated underneath, ceiling heights can be tight, and the existing walls may not be built to the standard a dwelling requires. A cheap conversion is very easy to spot from the street, because the giant blank wall where the garage door used to be never gets resolved.

Best for: budget-conscious projects on lots where the garage is already detached at the rear — which is often the best-value designer ADU on the board, if the front elevation gets properly redesigned rather than patched.

JADU (junior ADU)

Up to 500 square feet carved out of the existing house — very often a bedroom with an en suite, plus a new efficiency kitchen and its own exterior door. A JADU may share a bathroom with the main house, which is the clearest legal difference from a full ADU.

Why people choose it: it is by far the cheapest and quickest path to a legal second unit, and state law generally allows one JADU in addition to one ADU on a single-family lot.

What it costs you: interior space you currently use, and privacy inside your own home. There is no meaningful architectural upside — a JADU is an interior remodel, not a building.

Best for: a first step, a home office with a door to the street, or housing a family member with a small budget.

So which one should you build?

The honest answer is that the lot usually decides more than you do. Rear yard depth, existing setbacks, tree protections, sewer location, and where the panel sits will narrow the list fast — often before you have spent a dollar. That is why the useful first step is not picking a type, it is getting someone competent to look at the lot.

What you can decide up front is the standard you are holding the result to. If the goal is a building that looks deliberate rather than delivered, read how to build an ADU that doesn't look like an ADU before you talk to anyone, then use the builder checklist to test who is worth your time.