Most homeowners only hire a structural engineer once or twice in their life. The questions are the same every time: what is this person's job, what am I paying for, and how do I know I got what I paid for? This FAQ answers them in plain English.

What does a structural engineer actually do?

A structural engineer designs the parts of a building that carry load — the foundation, columns, beams, walls, and roof framing — and proves that the design will not collapse under any expected combination of gravity, wind, snow, and earthquake. The engineer produces two deliverables: a plan set (drawings the contractor builds from) and structural calculations (the math that justifies every member size and connection on those drawings). Both are stamped and signed by a state-licensed Professional Engineer (PE) and submitted to the building department for permit.

When do I need one?

In California, a structural engineer's stamp is required for:

  • Any new home, ADU, or addition over 500 sq ft
  • Any structure with a roof span over 24 ft or with significant cantilevers
  • Any wall removal, beam replacement, or significant alteration to load path
  • Any retaining wall over 4 ft high (measured from the bottom of the footing)
  • Any new foundation, including conversions of garages or basements to living space
  • Any project in a Special Wind Region or Seismic Design Category D, E, or F (most of urban California)

For a small interior remodel that doesn't touch a load-bearing wall, you typically don't need one. The line is: if the project changes how loads travel through the building, hire a structural engineer.

What does a plan set actually contain?

A complete structural plan set typically has:

  • Cover sheet — project info, code references, design loads, soil bearing capacity
  • General notes — material grades, fastener specs, special inspection requirements
  • Foundation plan — footing sizes, anchor bolts, holdowns, slab details
  • Framing plans — one per floor and one for roof, showing every joist, beam, post, and shear wall
  • Shear wall schedule — table of wall types, panel spec, nailing pattern
  • Section / detail sheets — connection details, shear transfer, beam pockets, post bases
  • Calculation package — usually 30–80 pages of analysis backing every member size

PDF set vs. CAD set — what's the difference?

A PDF set is a print-ready, finalized plan. The contractor builds from it; the city reviews it. You can't edit it, but you can print as many copies as you need.

A CAD set includes the editable source files (DWG or RVT). Useful if you need to modify the plan for a specific site, adapt it to a different foundation, or hand it to another engineer for stamping in a different state. Most homeowners don't need a CAD set; most builders and developers prefer to have one available.

What is a "shear wall," and why does my engineer keep talking about it?

A shear wall is a wall designed to resist horizontal forces — wind pushing on the side of the building, or earthquake shaking the foundation. Most California houses use sheathed wood-frame shear walls: plywood or OSB nailed to the studs in a specific pattern that the calculations require. The shear wall schedule on the plans tells the framer exactly which nails, which spacing, and which holdown hardware to use at each wall.

Shear walls are also where most field errors happen. If a framer over-spaces nails or skips a holdown, the wall's capacity drops by 30–60%. This is why structural inspections matter.

What is a "holdown"?

A bolted bracket that ties the bottom of a shear wall directly to the foundation, preventing the wall from rotating off its sill plate during an earthquake. Common products are Simpson HDU and HD series. The holdown size is sized in the calculations and called out on the plans.

Slab on grade, raised, or basement — which is right for my project?

It depends on the soil, the lot grade, and your priorities:

  • Slab on grade is cheapest, simplest to build, and works on most flat lots with stable soils. Best for tract-style ADUs and houses.
  • Raised crawl-space foundation with stem walls and floor joists works better on sloped lots, expansive soils, and freeze-prone areas. Slightly more expensive but easier to access plumbing later.
  • Basement is rare in California outside of very specific markets (older Bay Area neighborhoods). Adds 30–80% to foundation cost. Useful when you have a steep lot and want to extract maximum livable area.

See our deeper write-up: Slab vs. crawl space for ADUs.

What does "structural observation" mean on my plans?

Structural observation is when the engineer of record visits the construction site at key points (foundation, framing, roof) and confirms the work matches the plans. It's required in California for most ADUs and houses in Seismic Design Category D, E, or F. Observation is not the same as the city building inspector's visits — those check code compliance generally; structural observation checks the structural specifics of your plan set.

My contractor wants to substitute a different beam — is that okay?

Sometimes. A "value engineering" substitution is normal: the framer found that the GLB beam you specified is back-ordered, and they want to use an LVL of the same depth. The right answer is to email your structural engineer and ask. The PE will check that the substitute carries the load and either approve it (often within an hour) or specify what does work. Never approve a substitution yourself unless you've checked the calculations.

How do I know my plans are good?

Three quick checks:

  1. The cover sheet lists the design loads. You should see roof, floor, wind, and seismic numbers, plus a soil bearing pressure. If those are missing, the plan set is incomplete.
  2. Every beam, post, and shear wall has a callout. Walk the plan with a marker and circle every member that has no size next to it. If you find any, the plan isn't ready for permit.
  3. The PE stamp is current. Look up the engineer's license at the California Board for Professional Engineers. The license must be active and the discipline must be Structural or Civil.

Why is XE Engineering's pre-engineered plan store useful?

Custom structural design takes 4–8 weeks and costs $3K–$8K for a typical ADU. A pre-engineered plan in our plan store covers the same code with the same engineer's stamp, available immediately, for a flat fee. The structural calculations are already done; the connection details are already drawn; the foundation alternates (slab and crawl) are both included. If your lot fits one of our standard footprints, you save 6+ weeks and roughly $5K compared with custom.

For lots that don't fit a standard plan, our structural engineering services deliver a custom design within the state's 60-day permit window. Contact us with your project details for a fixed-fee proposal.