Multifamily ADUs: Adding Units to Apartment and Duplex Properties

The numbers at a glance

What state law entitles a multifamily owner to build

Two separate entitlements stack. First, detached units: as of 2024 law, a lot with an existing multifamily dwelling may add up to eight detached ADUs, provided the number of ADUs does not exceed the number of existing units — a duplex can add two, a fourplex four, an eight-unit building eight. Second, conversion units: non-livable space inside the existing building — storage areas, boiler rooms, attached garages — can become ADUs, at least one and up to 25% of the existing unit count. Cities may not impose development standards on these units beyond what the statute authorizes.

Why the per-door math is different

A homeowner builds one unit and pays for one mobilization. A multifamily owner building four detached units pays one design package, one permit effort, one site setup — spread across four doors. Construction itself is priced per unit at the same published ranges as any detached build (run a unit’s size through the calculator), but the soft costs and site costs divide. Conversion units go further still: the shell, roof and slab already exist. This is why ADUs have quietly become the fastest way for small landlords to add doors without buying another building. Weighing a lot split instead? See SB9 vs ADU.

What we check on a multifamily feasibility visit

Utility capacity first — panel and service sizing, water and sewer laterals, whether units meter separately. Then siting: setbacks, fire access between buildings, and how many detached units the yard genuinely carries versus what the statute nominally allows. Then the conversion inventory: which storage or garage spaces qualify as non-livable space eligible for conversion. Parking is rarely the obstacle owners expect — replacement parking cannot be required when garages convert, and requirements are waived near transit. We put all of it in a written feasibility summary before any design money is spent.