The Montgomery County Housing Opportunities Commission (HOC) has agreed to buy the Hewitt Gardens apartments in Silver Spring, using a new method for preventing rental apartments in the county from being converted into condominiums.

The commission plans resell the complex immediately, with a total of 142 units, to a private developer who would have to maintain them as rental apartments for at least 15 years. In addition, 20 percent of the apartments would have to be rented to low-income families and 30 percent to moderate-income households for at least 20 years, under HOC's requirements.

This is the first time the purchase mechanism has been used to prevent a conversion, but commission officials said they are negotiating on other properties in the county, as well.

The number of rental units in Montgomery County has been diminishing over the past few years, with 7,200 units having been converted to condo ownership since the beginning of 1979. All of the new rental units built in Montgomery County in the past several years--2,500 from the start of 1979 through mid-1981--used federal government financing and rental assistance, which is being ended or severely curtailed.

Without this help it will be difficult for construction of new rental units to be economically feasible for a developer, the commission's rental housing experts say.

The purchase at Hewitt Gardens and Hewitt Gardens North was negotiated over the past year after the current owners asked HOC if it would want to buy the apartments to prevent their sale to a developer who wanted to turn them into condominiums. One of HOC's primary functions is to insure the availability of rental housing for low- and moderate-income families in Montgomery County.

"This is one of the few ways HOC has of maintaining rental housing in Montgomery County," said Irving Hurwitz, HOC manager of housing construction programs.

The $3.349 million purchase involves an assumption of a first trust by HOC and a second trust held by the current owner. Because the commission is a government entity, the interest it pays is tax-free to the seller. Therefore, the seller is willing to take a low-interest rate on the second trust.

This in turn enables the commission to offer the properties to a private buyer at a favorable price with good financing.

HOC is advertising for sealed bids on the properties and hopes to be able to turn them over to a private developer the same day it settles on them, expected to be in July. A minimum bid will insure that the commission will cover all of its costs in the transaction.

The housing agency will retain ownership of the land, but will give the new purchaser an option to buy. This has the immediate advantage of enabling the buyer to take 15-year tax depreciation on the entire sales price because the buildings are depreciable but the land is not.