Just Go Sideways
Sometimes you don't want to build up, and sometimes you're prevented from doing that by the 50% rule (unless the floor level is at today's required elevation, you can't built up more than 50% of the value of the house without the land - so logically, you couldn't add a full floor above what you have!) so sometimes you're able to buy the house next door, tear it down, and then add on to the side of your house - at today's elevation, of course. More often we see two adjacent homes torn down and a new one built in the middle of the two lots. It's far more unusual to see one take the lot to expand sideways.
Sideways is expensive to build - some people buy small homes and build a big monster home on the same lot :)