FEMA shows the flooding based on existing conditions (ie no sea level rise, climate change, ect). Zones X means it is in the 1 in 500 year flood zone, and lenders generally only require flood insurance if you are in the 1 in 100 year flood zone. Flood Factor shows future flood risk (ie their models will incorporate increases in flooding/rain due to climate change). FEMA flooding also only shows flooding from rivers or oceans, while Flood Factor shows flooding from rivers, oceans and rainfall.
In terms of adding another unit to the basement, that is 100% based on your local zoning and codes.
I would not stress too much about the Flood Factor score. Any sort of climate change model, no matter how sophisticated is still a best guess on what may happen. They are good for large scale and long-term planning, but loose their effectiveness when you zoom down to the parcel level.
For mitigation, I would:
- Talk to neighbors/city/floodplain administrator to see if there has been any flooding in the past
- Manage the drainage on your property by installing french drains, ensuring the ground slopes away from the house, install a sump pump for the basement in case you do get flooding.
- Be aware of storms passing through so you be prepared in case you do get flooding.