FEMA API: https://www.fema.gov/openfema-api-documentation
As part of the OpenFEMA initiative, FEMA provides read-only API based access to data sets (Entities). The data is exposed using a RESTful interface that uses querystring parameters to manage the query.
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Thank you - here is the really comprehensive website https://blogs.bing.com/maps/2018-06/microsoft-releases-125-million-building-footprints-in-the-us-as-open-data/
data download available here: https://github.com/Microsoft/USBuildingFootprints