If your project changes how your house carries its weight, Building Control will ask to see structural calculations. For most extensions and loft conversions, that means at least one steel beam or new lintel.
When you will need them
Typical triggers are removing or opening up a load-bearing wall, a new opening in an outside wall for bi-fold or sliding doors, a loft conversion with new floor joists, steel beams or a dormer, and a two-storey extension.
What they include
A structural engineer sizes each beam, lintel and padstone, checks the bearings and the loads coming down onto them, and sets out what the builder needs to support during the works. The results are drawn up as a structural overlay on your drawings, so the builder can see exactly where each element goes.
Why it matters
Correctly sized steels avoid costly changes on site, help builders quote accurately, and are needed for Building Control to sign the work off.
This guide is general information about the rules in England, not advice on your property. Always check with your council before you build.