Depending on how much of the property is in the flood plain, you may want to cancel the deal. Find out what the county flood plain ordinance says. The FEMA website has a section which will tell you if your area has such an ordinance. If it does, you will be required to get a building permit. Keep in mind a 100 year flood ocurrs more often than once every 100 years. It is also an arbitrary flood that was selected by Congress as a minimum to pass the National Flood Insurance Program. The reason I mention that is there are floods which greatly exceed the 100 year flood. You can build to be safe from a 100 year flood and get wiped out by those that exceed that criteria. Building a new house in a flood plain is not a good idea especially if you can avoid buying the land to start with.