There are many things to ask such as How old is the structure.? Can it support another story? Does it have a pitched roof or flat? etc...These are questions for a Builder but first you need to go to the Monroe County Building Department and speak with them. I would go in person to Plantation Key behind the police station on the bayside and bring your tax Id #. You may be able to only have an entrance and storage on the first level if it is below flood and grandfathered in. Then there is the question of cost and percentage you can improve. Definitely you need to speak to the County. No Agent can answer these questions for you without more information.
The first place to start may be the Monroe County Building & Zoning Department. The link can be found below. Are there 2 story properties in the area now? - If so, it is safe to assume that you too would be eligible.