The short answer
For any RCC or steel structure, for approvals in most jurisdictions, and whenever an existing building is altered.
A structural engineer designs the foundation, columns, beams and slabs and certifies the structural drawings submitted for approval. Most local bodies require structural drawings and a stability certificate for multi-storey work.
They are also essential for renovations, additional floors, cantilevers and any change in use that increases load. Skipping this step shifts the risk onto the contractor — and onto you.
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