Understanding Flood Zones
In 1968, Congress created the National Flood Insurance program. This is under the jurisdiction of the Federal Emergency Management Agency (FEMA). As such, FEMA has issued Flood Insurance Rate Maps (FIRM) covering most of the Santa Clara Valley and San Francisco peninsula homes. If the property is projected to be subject in the event of a 1% (or 100 year) flood, it is classified as being in Special Flood Hazard Areas (SFHA). What people will identify as a "flood zone" is usually a special flood hazard area.
The difference is significant because, using Palo Alto as an example, essentially all of Palo Alto is in a flood zone. Quoting the City of Palo Alto web site:
All of Palo Alto that is not in a Special Flood Hazard Area is in "ZONE X", which is described as an area of moderate risk of flooding (roughly speaking, outside the 100-year flood but inside the 500-year flood limits), or, for A0 zones, where the 100-year flood will be less than a foot deep. Thus, all of Palo Alto has been determined to be subject to some risk of flooding, and it is inaccurate to say that a given property is "not in a flood zone" simply because it is not in a Special Flood Hazard Area.
Palo Alto Flood Zone Descriptions
Zone X
The special floodplain construction rules are not applicable to structures in an "X" zone, and federal regulations do not require that flood insurance be purchased to protect an equity loan on structures in an "X" zone. But flood insurance is available for structures in "X" zones, and at rather attractive prices.
Zone ACr
A stream by definition is flooded. If the stream is predicted to be contained in its channel during a 100 year flood FEMA notes this on the flood maps as ZONE A CONTAINED IN CHANNEL.
Zone A0
Predicted shallow sheet-flow flooding between one and three feet depth from an overflowing creek.
Zone AH
Area of flooding less than three feet depth.
Zone AE
Potential tidal water flooding due to extremely high tide and or storm activity. FEMA has estimated that some properties that are not 10.5 feet above sea level are at risk.
Zone A
Areas that are flood-prone in the event of a 100-year flood but for which the flooding depth is not predicted from the available data
Click HERE for the City of Palo Alto flood information.
Construction in a Special Flood Hazard Area
The National Flood Insurance Program imposes requirements on construction. The most important one is that the lowest floor be elevated above the flood level. This effectively prohibits basements. Additionally, zoning requirements which limit building height and impose a daylight plane can prevent the building of a second floor if the first floor must be raised too much.