When can an overlay be considered?
An overlay is a product- and condition-specific option. The existing roof layers, deck condition, flashing, edges and selected covering all affect whether the assembly can accept another layer; local review may also apply.
What changes when the old roof stays in place?
The old covering remains concealed, so the deck cannot be examined in the same way as during tear-off. Added layers can also change edge and flashing details. GAF describes these tradeoffs for asphalt shingles; other systems require their own manufacturer guidance.
What should the proposal say?
Name the existing layer count if known, the exact overlay product, the edge and flashing plan, and how hidden damage will be handled. Compare that scope with a full tear-off option before choosing.