WASHINGTON (AP) _ More than 2 million of the nation's 5.1 million substandard housing units are located in rural areas, an Agriculture Department report said Tuesday.

''Since 1970, at least a third of the nation's substandard housing has been in rural areas, although rural areas have only a quarter of total occupied housing,'' the report said.

More than half of the substandard homes are in the South, which accounts for only 41 percent of the nation's rural households.

Donald L. Lerman of the department's Economic Research Servic, said most of the research was based on 1980 census information. Lerman, who wrote the report, said some of the findings were made available last year to another USDA agency, the Farmers Home Administration, to help with make decisions about rural housing programs.

Housing was defined substandard if had more people than rooms or if it lacked complete plumbing - hot and cold running water, a flush toilet and a bathtub or shower - for the exclusive use of occupants.

''Texas leads all states, with nearly 143,000 rural occupied substandard units, 7.1 percent of the national total, followed by North Carolina with 121,000, 6 percent; and Kentucky with 103,000, 5.1 percent - but these states have very high rural populations,'' the report said.