Westchester County Executive Rob Astorino announced Tuesday that the 750th affordable housing unit has been developed, a major milestone in the county’s federal agreement with the U.S. Department of Housing and Urban Development. Under the terms of the 2009 settlement reached between HUD and county government, Westchester is required to spend at least $51.6 million to develop 750 units of affordable housing by the end of this year in 31 mostly white communities.