Difficult Development Areas (DDA) are areas with high land, construction and utility costs relative to the area median income and are based on Fair Market Rents, income limits, the 2010 census counts, and 5-year American Community Survey (ACS) data.

Maps of Qualified Census Tracts (QCTs) and Difficult Development Areas (DDTs) are available on the official HUD site:
https://www.huduser.gov/portal/sadda/sadda_qct.html

Source: HUD