Do I need a full-perimeter drainage system or just on the wall that leaks?

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This is a real and money-relevant decision, and the honest answer is that it depends on where water actually enters. A partial-perimeter interior drainage system protects one wall or a portion of the perimeter and costs less than a full-perimeter system; it is appropriate when the water source is genuinely localized, for example a single wall on the uphill side of a hillside lot, or one wall along which seepage consistently appears while the rest of the basement stays dry. A full-perimeter system protects the entire basement perimeter and is the reliable choice when water enters around multiple walls, when the source is general hydrostatic pressure (the water table rising under the whole footprint), or when the basement is or will be finished and you want comprehensive protection behind the finishes. The risk of under-scoping is that hydrostatic pressure finds the unprotected sections: install drainage on only the wall that happened to leak first, and the next heavy rain can push water up the cove joint on an adjacent, unprotected wall, leaving you to extend the system later at additional cost and disruption. The risk of over-scoping is paying for full perimeter when a documented single-wall source did not require it. We resolve this by diagnosing the actual entry pattern during the on-site assessment (ideally with the basement showing its behavior during or after a rain) rather than defaulting to the largest system, and we tell you the tradeoff plainly: where a partial system is genuinely sufficient we will quote it, and where the pattern indicates hydrostatic pressure around the footprint we will explain why full perimeter is the durable answer. For a finished basement specifically, full perimeter is usually worth it regardless, because retrofitting an extension later means tearing finishes out twice.