Module 9 · Lesson 3
Structural Engineering: Making It Stand Up
Foundations, framing, shear walls, seismic design — what a structural engineer contributes to your ADU and why California demands it.
Architectural plans say what the building is; structural engineering proves it will stand — through gravity, wind, and (this being California) earthquakes. Here's what the engineer actually decides.
The foundation design. Slab thickness, footing depth and width, reinforcing steel — sized to your building's loads and your soil's capacity (which is why bad soil triggers lesson 9.6's soils report). For conversions, the engineer evaluates the existing foundation and specifies the retrofit — the very work that drives Module 6's cost tiers.
The framing plan. Every beam, header, joist, and rafter sized by calculation: the beam over your big glass door opening, the headers where the garage door was, the roof structure carrying solar panels. Undersized framing fails inspection; oversized framing wastes money. Engineering is precision spending.
Seismic design — the California specialty. Earthquakes push buildings sideways, and resisting that takes shear walls — walls sheathed and nailed to a specific schedule, anchored with hold-downs bolted into the foundation. The engineer decides which walls do this work, and here's why you care: shear walls constrain window placement. That wall of glass facing the view? It competes with the wall's seismic job. This tension is solvable — with steel frames, strategic wall placement, plywood schedules — but only when engineer and designer talk early (lesson 9.7's punchline).
Calculations the city checks. Behind the drawings sits a calculation package — the math proving every member works. Plan checkers review it; a licensed engineer's stamp attests to it.
When engineering gets bigger: hillsides (retaining, stepped foundations), two-story units (stacked loads, more shear), above-garage builds (Module 3.4's whole story), and large openings. When it stays light: single-story rectangles with modest windows on flat lots — the engineering equivalent of an easy commute.
Cost recap from Module 6: typically $3K–$8K+ for straightforward projects. It's a small line that controls big ones — foundation and framing are a third of your construction budget, and this is the document that sizes them.
Key takeaways
- Engineers design the foundation, size every framing member, and lay out California's seismic system.
- Shear walls compete with windows — the classic design-engineering tension, cheap to resolve early.
- Conversions get their retrofit scope (and much of their cost) from the engineer's evaluation.
- Hillsides, second stories, and big openings grow the engineering; simple boxes keep it light.