WASHINGTON (AP) _ Even though prices went up, falling interest rates made homes more affordable in November, the National Association of Realtors reported Wednesday.

Although other interest rates have been falling for about three months, November saw the first drop in home loan rates since April.

Using personal income, interest rates and home costs, the association came up with a housing affordability index of 85.6 for November, meaning the typical family has 85.6 percent of the income necessary to buy a typical home.

The Realtors used a 12.89 percent mortgage interest rate to do their computation. Rates continued to go down in December.

Jack Carlson, chief economist for the Realtors, predicted the affordability index would reach 90 by the third quarter of 1985. The index has not been at the 100 mark since December 1978, when mortgage rates were in the 10 percent range.

''Lower mortgage interest rates and rising family income were more than enough to offset the increase in resale home prices in November,'' according to David D. Roberts, president of the association.

The median income of American families was $26,157 in November, meaning half the families had more than that amount and half less. That income would be enough to meet the requirements to get a standard 80 percent mortgage loan on a $62,100 home.

The Realtors said about 35 percent of all homes available for sale in November had price tags below $60,000, while half of all homes cost more than $72,500.

By the trade group's calculations, the typical family's income fell $4,400 short of that needed to qualify for a standard 80 percent loan on a $72,500 house.