A site plan shows where proposed work sits on the property. It becomes especially important when setbacks, structures, equipment, property lines, easements or access affect the review.
Building departments use their own terminology, deadlines and procedures. Treat this as a practical orientation and verify the current rule with the jurisdiction that issued — or would issue — the permit.
No. The need depends on the project and jurisdiction, but exterior additions, structures and equipment locations commonly make site information more important.
For some simple projects a jurisdiction may accept an owner-prepared drawing; other projects require professional or survey information. Follow the city's instructions.