|Sorry for the bad quality, it's just a quick pen-sketch after all...|
1.) the building itself has to be lightweight, most probably some kind of prefab with sandwich walls, or shipping container structures, or even full tiny houses sitting on the platform.
2.) tall anchor-poles embedded deep in the ground keep the platform and the building with it in place in case of the flood being higher than the supporting poles. The building can move up and down along these poles, but they they prevent it from drifting away.
3.) segmented pontoon system, that would be filled with some kind of expanded concrete or plastic foam. this makes them strong and sturdy, so they can support, and make the whole platform with the building float. with this segmented pontoons and the platform we could build whole neighborhoods with pathways and bike lanes(I don't think it's a good idea to put cars on a platform system that moves with the water from time to time...)
4.) supporting poles hold the weight of the whole structure. they would be about 3m high(depends on the area and the regular moves of the river), to keep the building away from the water in most of the year. the platform and the building is not attached to these poles, so if a huge flood is coming, the structure can rise with the water. But after the water's gone, the platform can sit right back on these poles.
Note: the whole system can be prefabricated, and assembled on the spot really fast most probably in the dry summer, and if the house is a container-house, it could be erected in a matter of days...