The general process for amending the FEMA Flood Insurance Rate Map (F.I.R.M. or FIRM) is as follows:
Determine the Base Flood Elevation (BFE) for your property. You will find the BFE listed on the Property Information System's "Site" tab. The number(s) listed there is the number of feet above mean low water that FEMA has determined as the base flood level for your area. The sub-floor of your house needs to be above that elevation in order to qualify for a map amendment. A surveyor can also determine this for you.
Contact a licensed surveyor to get an "Elevation Certificate" or "Certificate of Elevation." They are listed in the Yellow Pages under "Land Surveyors" or "Surveyors-Land." The surveyor will precisely measure the elevation of your house and compare it to the Base Flood Elevation for your area.
Submit the Elevation Certificate along with a Letter of Map Amendment (LOMA) request to FEMA. The surveyor may do this for you.
If FEMA concurs with the surveyor's assessment, they will issue a formal LOMA letter both to you, and to the County's Flood Plain Administrator, and the FIRM (Flood Insurance Rate Map) will be amended.
Submit a copy of the LOMA letter to your insurance company - you may get a reduction in your insurance rate.