flood plains are usually more to do with elevations. If all of my home's runoff goes to my neighbor, its just runoff now. If their home doesn't have drainage (p

flood plains are usually more to do with elevations. If all of my home's runoff goes to my neighbor, its just runoff now. If their home doesn't have drainage (proper grade, etc), then its getting flooded regardless of your drainage. Your drainage may make it worse, but the reality is that the damage is already done. This may apply for more recent builds rather than homes 50-60yrs old.