Low/Free Standing Deck and Rodents

We are currently debating whether to go with a patio or deck in our backyard. Our back patio door is elevated at about 18 inches and considering a deck at that level or a "freestanding" slightly lower at 8-12 inches from the ground.

What are the best approaches for a deck to mitigate the potential for rodents, but also allow the deck underside to "breath". I'm skeptical of an enclosed underside of deck that in the event something gets in, we would have to rip open a part of the deck.

Would preparing the ground with rocks or concrete help mitigate the issue? How low can you have a deck while still it being open from the sides?

We previously had issues with voles/mice in the backyard. Thanks