If you've spent any time researching backyard housing in California, you've run into the term ADU. It stands for Accessory Dwelling Unit — a secondary, independent living space on a property that already has a primary home. ADUs go by a lot of names depending on who's talking: granny flats, in-law units, backyard cottages, casitas. They all mean the same thing.
What makes ADUs different from a home addition or a converted bonus room is independence: a legal ADU has its own kitchen, bathroom, and entrance, and can function as a fully separate residence — whether that's for a family member, a long-term tenant, or a short-term rental (subject to local rules).
The four main ways to build one
Junior ADU (JADU)
A JADU is the smallest and most affordable option, created entirely within the existing walls of your home — typically capped at 500 square feet of interior living space under state law. Because it shares structure with the main house, permitting and construction are usually faster and cheaper than any other ADU type. JADUs often share a bathroom with the main home and use an efficiency kitchen.
Garage Conversion
Exactly what it sounds like: your existing garage becomes a livable unit. Since the structure, foundation, and roof are usually already there, garage conversions tend to be the fastest and least expensive full ADU option — you're primarily paying for interior build-out, utilities, and finishes rather than new structure from the ground up.
Detached ADU
A fully separate, new structure built elsewhere on your lot — most commonly in the backyard. This is the most flexible option in terms of layout and design, since you're not constrained by an existing structure's footprint, but it's also the most expensive because everything is new construction: foundation, framing, roof, and all.
House Extension
Rather than a standalone unit, an extension adds new square footage directly onto your existing home. This can work well when local setback rules make a detached unit difficult, or when you want the new space to feel like a true part of the main house rather than a separate structure.
Which one is right for you?
It usually comes down to three things: your lot size and shape, your budget, and what you want the space for. A rental-income-focused homeowner often prefers a detached ADU for privacy and separation from the main home. Someone housing an aging parent might prefer a JADU for proximity. Someone on a tighter budget with an underused garage often finds a garage conversion delivers the best value per dollar.
Whatever direction you're leaning, the fastest way to get real numbers is to run your address and preferences through a quote — square footage options and pricing vary meaningfully by type, and it's easier to compare once you see your actual numbers side by side.
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