The Short Answer
It depends on which ADU you're building — and the difference is big enough to change your bid. As one Bay Area architect put it in a thread that made the rounds: conversions are easier, but new builds need a structural engineer. That's roughly right. Here's the full ladder.
The ADU Risk Ladder
| ADU Type | Structural scope | Engineer needed? |
|---|---|---|
| Garage conversion (no footprint change) | New openings, garage-door infill, floor details | Usually — light scope |
| Conversion + addition | Above, plus new foundation & framing for added area | Yes |
| New detached, single-story | Full gravity + lateral design, foundation | Yes — always |
| Two-story / above-garage | Stacked loads, full lateral system, drag/collector design | Yes — heavier scope |
| Hillside ADU | All of the above + soils-driven foundation design | Yes — plus geotech input |
Why New Builds Always Need Engineering
A new detached ADU is a complete building. In most of California that means design for high seismic loads — Seismic Design Category D in much of the state — with a full lateral system: shear walls, holdowns, diaphragms, and collectors, all calculated and detailed. Plan checkers in SDC D jurisdictions look for the lateral package specifically; a set with framing plans but no lateral analysis is a guaranteed correction cycle.
Garage Conversions: Lighter, Not Zero
Conversions dodge the biggest costs — the foundation and roof exist. But engineering commonly still enters for:
- New window/door openings in bearing or braced walls — header design and king/trimmer studs.
- Garage door infill — the new wall's framing, connection to the existing foundation, and its role in the lateral system.
- Lateral check — garage fronts are often the weakest wall line in the house; changing them can require upgrades.
- Floor changes — sleepers or new slabs over the existing slab, and moisture detailing the city will ask about.
What About Pre-Approved Plans?
Pre-approved ADU programs are real time-savers — but they cover the standard building, not your site. Foundations still have to match your soil conditions, hillside lots take you out of the standard program entirely, and local amendments can add requirements. Budget for site-specific engineering even with a pre-approved set; it's a fraction of full design.
Hillside ADUs: Read the Soil Before the Bid
If the lot slopes, foundation cost is driven by soils, not square footage. Get soils information and a structural foundation concept before committing a price — this is common enough that we wrote a separate guide on hillside lots.
The Sequence That Keeps ADU Bids Honest
- Classify the ADU on the ladder above.
- Scope the engineering — a 30-minute review of the layout and site photos is enough to define it.
- Get a fixed engineering fee before you finalize your bid to the owner.
- Submit one complete package — architectural + structural coordinated, lateral included, calcs attached.