Friend built his house on a mound of the (then) maximum flood level, as were several blocks of flats Several years later flood defences were built for the rest of the town, up to that level. A couple of years later, the flood level was less than one brick below the ptop of the defences (which leaked anyway but not too badly!)
Another 18" was added at a cost more than that of the original defences.
I'd never buy anywhere near flood level!
When I moved to one place it was suggested not to buy in certain areas (which had flooded in the previous 10 years). 30 years later they flooded, residents claimed they had never flooded before (in their lifetine perhaps).
I'd never buy anywhere near flood level!
Best flood defence is height above water. Not guaranteed though.