Private flood insurance will likely provide you the most affordable rate if you have a basement. In Oregon, some private insurers offer private flood insurance that costs 50% less than FEMA’s mandatory flood insurance for structures located within the 100-year floodplain, and those savings are probably even higher for homes with basements.
Since basements in the floodplain are often several feet below the projected 100-year flood, the National Flood Insurance Program (NFIP) sees them as an expensive liability, providing coverage for a limited number of items in the basement while also charging a higher premium. In contrast, many private insurers offer more competitive rates and broader coverage for homes with basements. Property owners can locate private flood insurers online, or ask for referrals from friends and neighbors. Shop around though because some companies charge agency fees whereas others don't, which could save you even more money. As always, buyer beware – pay attention to the terms and conditions, and seek out good advice from a qualified professional who knows flood insurance (see Find the right flood insurance agent).
Also, in most cases someone with a basement will not benefit from getting an Elevation Certificate because private insurers don’t use ECs to determine their insurance rates. The exception is if you think your home is on natural high ground that is elevated above the anticipated 100-year flood (see In some instances, an elevation certificate may show you are “outside” the floodplain for more).