Short answer: if your addition changes how the house carries load, you need a structural engineer — and your building department will require one's drawings before issuing a permit.
When you definitely need one
- Removing or altering a load-bearing wall — the classic case. The engineer sizes the replacement beam and its supports.
- Adding a second story — the existing foundation and framing must be verified for the new loads.
- New foundation or slab for the addition, especially on expansive or fill soils.
- Long spans: open-concept great rooms with 20ft+ clear spans need engineered beams or posts.
- Hillside or retaining conditions — lateral loads and drainage change the design completely.
When you might not
A small non-structural bump-out (like a bay window within existing framing) or purely cosmetic work sometimes goes through with architectural drawings only — but the final call belongs to your city's plan reviewer, not the contractor. When in doubt, ask before you build.
What the engineer actually delivers
- Structural calculations proving the new framing works (gravity + lateral).
- Framing plans, beam/post schedules and connection details.
- Foundation design for the new footprint.
- A PE-sealed set your city will accept — see how sealing works in Texas, Florida and all 50 states on our coverage page.
Planning an addition? Send the existing plans — we will tell you exactly what engineering your city will ask for.