More than three-fourths of the repeat home buyers in a survey conducted here put less equity into the purchases than they took out of their previous homes, and the average equity kept by the sellers was $21,000, the National Association of Realtors reported.

Commenting on the results of the survey it commissioned, the association said the results indicated that buyers were taking on larger mortgage indebtedness -- and using the extra money for savings, furnishings or other purchases -- rather than buying less expensive houses.

The survey covered approximately 800 home purchases in the metropolitan area last May.

Other findings: 30 percent of those questioned had annual household incomes of $25,000 or less; 43 percent of the first-time buyers were in that income range. The first-time buyers were more likely to buy existing houses, town houses and condominium apartments and less likely to buy detached and new homes, which were favored by repeat buyers.

More than half the buyers interviewed chose detached houses, and one-fourth bought town houses. (In most statistical reports, the attached town house is included as a "single house" for tabular purposes.) Apartment condominium purchases were made by 16 percent of those interviewed.

Price obviously was a factor in the choices. The median purchase price paid for detached houses was $80,000, for town houses $60,000 and for condominiums $43,000. The figures support the generally accepted view that most young, first-time buyers turn to less-expensive, garden apartment conversions and town houses that have been a significant segment of the housing offered for sale here in the past five years.

Persons whose households had incomes of less $15,000 purchased condominium apartments in more than 45 percent of the cases tabulated, while those with incomes between $15,000 and $20,000 purchased apartment condos in almost 40 percent of the transactions.

The national Realtor organization's survey also disclosed that the median price paid for existing houses was $73,500 compared with $90,500 for new houses.

Not unexpectedly, married couples predominated among buyers, accounting for 70 percent of those surveyed. However, 26 percent of the transactions were made to single persons -- supporting the general market wisdom that they are becoming an increasingly important segment of the buying public.

Unmarried couples accounted for 4 percent of all the tabulated purchases. While that percentage is relatively small, it probably would have been much smaller 10 years ago, when sales of houses to unmarried couples were rare.

Young buyers in the 25-to-34 age group -- a major segment of the buying public nationally -- made up half the survey here. More than 17 percent of the young first-time buyers indicated that they relied on loans (probably from their families) for down payments, and more than 19 percent said they relied of gifts for an average of more than half their down payments.

They survey showed that 79 percent of the buyers already lived in this area and that most purchased in the same county or city.