In FEMA's publication "Mandatory Purchase Of Flood Insurance Guidelines" available at http://www.fema.gov/nfip/mpurfi.htm
it states on page 25 in a heading titled "Buildings in the Course of Construction" the following:
When a structure is to be built in an SFHA
that, when completed, will be a walled and
roofed building that will be eligible for
coverage, flood insurance must be
purchased to provide coverage during the
construction period. Therefore, when a
development or interim loan is made to
construct insurable improve-ments on land,
flood insurance coverage must be
purchased. The only practical way of
implementing the flood insurance coverage is
to require the purchase of the policy at the
time that the development loan is made, to
become effective at the time the construction
phase is commenced, and in an amount to
meet the mandatory purchase requirement.
Material to be used on a building in the
course of construction, but yet to be walled
and roofed, is eligible for flood insurance,
subject to certain underwriting restrictions.
The NFIP, to the extent possible, conforms
its practices with those of fire insurers by
providing insurance coverage that begins
during the period of time when construction
is taking place.
For new construction in Regular Program
communities, the Elevation Certificate and
the premium will be based on an elevation
figure derived from construction drawings.
However, the policy will not be renewed
until a new Elevation Certificate, based on
actual construction, has been submitted.
Coverage under the policy becomes
available immediately when the construction
starts, and is not delayed until the building
has reached a roofed and walled condition.
Hope this helps.
Opinions expressed are my own, and do not necessarily reflect those of my employer.