On March 28 Montgomery County will become the first jurisdiction in this area to float a bond issue for the purpose of providing mortgage money for low-cost, single-family housing. In so doing this affluent suburban community is joining a growing number of municipalities across the country that are cutting the cost of real estae through tax-exempt financing.

The $57 million offering by the Housing Opportunities Commission will be made through a syndicate. Minimum participation for investors will be $5,000. Because these bonds are secured by mortgaged property they are ex pected to carry a higher rating than municipal bonds backed by the jurisdiction's tax base. The yield is expected to be similar to that of general revenue bonds, currently paying about 7 percent interest.

The issue is calculated to resuit in $50 million for mortgage lending at rates between 8 and 8 1/4 percent, or about two full points below the going market rate. Loans will be restricted to persons earning up to 80 percent of the county's median income. That means the ceiling for a single person is $19,500. The maximum, for a household of five or more, is $27,500.

The upper limit on the house price is $59,000 for new and existing singlefamily dwellings, including condominiums and town houses. No rehabilitation or construction loans will be made. A 5 percent minimum down payment is required.

The commission's executive director, Bernard Tetreault, was asked why the property price ceiling was set so low in a county where the 1979 mediann sales price is around $72,500 and at a time when many households are stretching their housing budgets well beyond the 25 percent of income rule of thumb. He said that the price selected reflects the approximately 4,000 units -- or just over a third of all residential property sold in the county -- that are expected to sell this year in the county for $59,000 and under.

The $50 million will finance one out of four of those units, thus subsidizing 8.7 percent of the residential turnovers in one year. The ratio of income to cost is consistent with Department of Housing and Urban Development policy.

Tetreault said the restrictions will have the effect of limiting purchases to condominium units in the Takoma Park and Silver Spring area and to detached houses in the upper county around Gaithersburg and Germantown. However, he added, that another bond offering is tentatively planned for later this year and, depending on the outcome of the Series A issue, income and price limits may be raised.

Applicants may begin to apply by April 18 to the 21 financial institutions that are participating in the program. They are: DFS Inc., Virginia Mortgage & Investment Co., Equitable Savings & Loan Association, Unity Mortgage Corp., LM. GATTI Mortgage Co., Colonial Mortgage Service Co., Citizens Savings & Loan Assoc., The Savings Bank of Baltimore, Guardian Federal Savings & Loan Association and the B.F. Saul Co.

Also, Community Savings and Loan Inc., County Federal Savings and Loan Association, ICM Mortgage Corp., Suburban Coastal Corp., Fidelity Bond and Mortgage Co., Government Services Savings and Loan Inc., Steed Mortgage Co., Weaver Bros. Inc., and J. T. Barnes of Washington, D.C. Inc.