Do I need an engineer for my LA addition or ADU?

It is one of the first questions homeowners ask, because the answer changes the design team, the budget, and the schedule. The short version is that state law sets a broad baseline, and Los Angeles draws a tighter line because of its earthquake risk.

What state law allows

California's architect licensing law lets people who are not licensed architects or engineers prepare plans for wood-frame single-family homes of up to two stories and a basement, multifamily buildings of up to four units of the same size and construction, and garages and similar structures that go with them. That is why many homes are drawn by building designers.

The same law has a limit built in. If any part of the structure "deviates from substantial compliance with conventional framing requirements," the building official must require plans and calculations for that part from a licensed architect or registered engineer, with their stamp and signature (Business and Professions Code 5537(b)).

LA's one-story line

The Los Angeles Department of Building and Safety (LADBS) goes further. Its policy on stamped plans says the conventional framing exemption cannot be used for structures more than one story tall in Seismic Design Categories D0, D1, D2, and E, the high-seismic categories this rule is written for. When a building is one story with a basement, or more than one story, structural plans and calculations must be submitted and signed by a California licensed engineer or architect. An unlicensed designer may still prepare the architectural drawings for a home of up to two stories and a basement.

Two details catch people. A second story added to a one-story house turns the whole building into a two-story building, so the engineering requirement applies. And a one-story ADU is not automatically exempt: large openings, long spans, or walls that do not stack can make part of it irregular, and that part has to be engineered. For how big and how tall an ADU can be in the first place, see ADU rules in Los Angeles.

Additions tied to the house

An addition can be built as a structurally independent piece or tied into the existing house. Under the LA code, if the addition is structurally independent, the existing house's lateral system can stay as it is. If it is not independent, the house and the addition are evaluated together as one structure for wind and earthquake forces (LAMC 91.2.1103.2). The code includes exceptions, including one for residential buildings of five or fewer units, used only as residences, where the existing building and the addition together still meet conventional framing rules, and one that lets an existing element stay when the addition raises its load only slightly, counting earlier additions too, and creates no new structural irregularity. Whether those exceptions fit is a question for the engineer, and it shapes how the new and old structure connect.

Structural observation and inspection

Engineering does not end at the permit. LADBS requires structural observation, site visits by the engineer or architect of record or their designee, when a home's lateral design is engineered. One-story wood-frame homes and garages (R-3 and U occupancies) under 2,000 square feet, in Seismic Design Category D, on ground no steeper than 10 percent, are generally exempt from that trigger, though LADBS can still require observation for other elements. The owner hires the observer, and LADBS requires the observer to be independent of the contractor.

Steep lots add more. On slopes steeper than 33 percent, LADBS requires structural observation plus continuous inspection by a City licensed deputy inspector of structural connections, footings, grade beams, and retaining walls. If you are planning permits for an ADU, see how long LA ADU permits take, and for the addition itself, our home additions page.

Common questions