Marriott Corporation's plan to buy back much of its stock will increase the share of the company owned by Marriott family members by 50 percent and will make it virtually impossible for any outsider to wrest control of the company away from the founding family. f

Consolidation of the Marriott family's control over the $1.5 billion a year business is disclosed in the formal stock purchase offer issued yesterday.

The corporation is offering to pay $23.50 per share for up to 10.6 million shares of its common stock. Shareholders have until Feb. 22 to decide whether to accept or reject the offer.

Marriott is buying back its stock in part because the company is making money faster than it can spend it, the offering statement discloses. "Cash flow from operations has been increasing in an amount beyond the existing and anticipated future capital needs of the company," it notes.

The 10.6 million shares is 33 percent of the slightly more than 32 million Marriott shares outstanding.

According to the offering statement members of the Marriott family now own 20.1 percent of the corporation's stock. Company founder J. Willard Marriott and his wife, Alice, have 6 percent. Their son, J. W. Marriott Jr., the company president, owns 3.8 percent and his brother Richard Marriott, group vice president owns 4.5 percent. Other family members own additional shares.

Neither those four nor their offspring plan to sell any stock, the statement says and there is "no reason to believe" that distant relatives will sell any significant amount.

If the company buys back one-third of the stock, the family members' share of the remaining stock will increase to 30.1 percent.

"The increase in the percentage ownership of common stock by members of the Marriott family resulting from consummation of the offer may render more difficult, and therefore discourage, any attempted acquisition of control by any other entity of group . . . without the acquiescence of the Marriott family," the offering statement notes.

How difficult it would be to take over the company against the wishes of the family is made clear by the corporation's charter, which requires approval of 66 2/3 percent of the shareholders for any merger.

With 30.1 percent of the stock in the family's hands, another 3.5 percent in an employe pension plan controled by the management and a small amount owned by other officers, the insiders have 35 percent, enought to block any take-over attempt.

The stock purchases will cost about $251.6 million, including an estimated $2.5 million in expenses and slightly more than $243 million for the shares.

To encourage stockbrokers to persuade their customers to accept the offer, Marriott will pay brokers 25 cents for every share purchased through them, up to a maximum of $1,000 for any one stockholders' shares.

Marriott will get part of the money to buy the stock from the sale of six hotels and will borrow the rest from an insurance company and banks.

The Equitable Life Assurance Society of the U.S. is buying the hotels for about $159 million, less some construction costs that have not been determined. Because the stock will be purchased before the hotels are sold, Equitable has agreed to lend Marriott up to $146 million to buy the stock; the loan will be repaid with money from the hotel sale.

Another $105.6 million will be borrowed from banks. The company's banks have agreed to increase Marriott's credit line from the present $129 million to $448 million, but the company says it does not plan to use more than $250 million of its borrowing capacity.

Marriott also disclosed that it may need more than $100 million to buy another company. "A limited number of acquisitions involving businesses within the company's existing lines of business are currently under consideration," the statement says.