Does a Duplex Need Title 24?

Yes — but under the single-family standards, not the multifamily ones. A duplex is classified Group R-3 regardless of how many storeys it has, so it is documented as single-family.

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The misconception worth clearing up first

A duplex has two dwelling units, so it sounds like multifamily. Under the Energy Code it is not. A two-dwelling building of any number of storeys is Group R-3 and is treated as single-family — in effect as two single-family homes. That means the single-family standards apply and the CF1R family of forms is used, not the low-rise multifamily LMCC set.

Getting this wrong is expensive in both directions. A duplex documented on multifamily forms will be questioned at plan check, and a designer who assumes multifamily rules will be working to the wrong requirements from the start.

Townhouses sit on the same side of the line

A townhouse is a single-family dwelling unit built in a group of three or more attached units, and townhouses are covered by the single-family standards regardless of how many habitable storeys they have. So a row of six townhouses is not a multifamily building for Energy Code purposes, even though six units in a single stacked building would be.

Where multifamily actually starts

At three or more dwelling units, townhouses excepted. That makes the real dividing line the one between a duplex and a triplex, not the one between a house and an apartment block. A duplex and a fourplex on the same street are assessed under different standards and produce different paperwork.

What this means in practice

A duplex is documented much like a house: the same form family, the same additions and alterations provisions if the work is not new construction, and the same treatment of windows, insulation and equipment. If the project is a new duplex, it is new single-family construction, with the requirements that go with it.