They will build on flood planes as long as idiots are prepared to buy the houses.....
If you are buying a house, check the EA maps to see if it floods.
http://www.environment-agency.gov.uk/subjects/flood/826674/829803/858477/?version=1&lang=_e
If it does flood....err......then don't buy it. If you DO buy it and it floods, then don't squeel like a stuck pig about it.
If people wont buy them, they will stop building them.