Families buying a home for the first time in 1975 had a more difficult time affording it than did families who bought their first home back in 1970.

This is the principal conclusion of a study by the Congressional Budget Office on costs, income and home ownership released this week. It also examined the relative difficulties of families selling one home and moving to another, and those who choose to stay in their present home.

The reason first-time buyers had so much difficulty financially lies in the discrepancy between increases in housing and incomes, the study noted. In the years between 1970 and 1975, the median price of a house rose 63 per cent while the median family income rose only 39 per cent.Put another way, the costs of home ownership rose one and a half times faster than did the income of this group.

The first-time buyer had to assume the full burden of cost increases: sales price, interest rates, property taxes, insurance, maintenance and repairs, and heating and utilities. By comparison, the repurchaser faced only increases in the interest rates and operating expenses, the study said. The rise in value of the first house tended to offset the increases in prive of the second, to the extent that the repurchaser faced cost increases only two-thirds as rapid as his rate of income growth.

Thus, in the period 1970 to 1975, while the repurchaser's income rose 39 per cent, his housing costs rose only 27 per cent, the budget office noted. For the homeowner who stayed put during those years, the increase in operating expenses amounted to only half the increase in his salary, 22.8 per cent versus 39 per cent. This pattern held true for all income groups.

Under the category of federal action to improve the affordability of housing, especially for the poor, study director Neil S. Mayer suggested that subsidy programs could be narrowed to first time buyers, and targeted on existing housing, which is less expensive than new.

The study also proposed alternative mortgage instrument, such as graduated payments. Deferred payments would involve partial payment of mortgage costs by the federal government in the early years, with later repayment with interest.

It also suggested subsidies to lower the price of the downpayment for lower income people, but warned that the lower downpayment would mean higher monthly payments which the poor probably wouldn't be able to afford.