Best case, you get a topo survey that shows it isn't really floodplain. As folks said already, sometimes the maps are based on old or inaccurate information. You might be able to do a little mild grading work and get the portion you want to build on up out of the floodplain.
That assumes your locality plays ball and you're not creating a flood problem for a neighbor or other parts of your property.
I think your first stop should be at a place that does site work and can do the survey, drawings, and engineering to prove to FEMA and anyone else that it won't flood.
Of course, if it's just for a block wall shed full of landscaping material and equipment or something else like that and you don't care if it floods a little once in a while and you aren't going to insure it, none of this matters as long as it doesn't cause a bunch of runoff to go into your house or the neighbors.