The Federal Home Loan Bank Board approved yesterday new proposed regulations to cover the controversial conversion of depositor-owned mutual savings and loan associations to stockholder corporations.

By a unanimous vote; the board decided to restrict further the amount of stock that could be owned by S&L officers and directors after such a corporate transaction and to prevent abuses by insiders at such thrift institutions.

Most of the nation's savings and loan associations are mutual thrift institutions, theoretically owned by all depositors. In recent years, a number of industry leaders have advocated a switch to the status of publicly traded stock corporations, arguing that S&L's could raise more capital and provide more home mortgage lending.

Since 1974, the FHLBB has approved 35 coversions to stock corporations as an experiment, including one large suburban Maryland institution, Metropolitan Federal of Bethesda. Seventy applications for similar conversions are pending.

However, several thrift institutions and a number of congressional critics, including Senate Banking Committee Chairman William Proxmire (D-Wis.), have been highly critical of alleged windfalls to existing officers at the expense of depositors who could not or did not buy up stock in the new firms.

Bank Board general counsel Anne Jones, recently nominated by President Carter to serve on the Federal Communications Commission, said last night she is hopeful the new restrictions will meet objections to the conversion process.

And Bank Board Chairman Robert McKinney emphasized at yesterday's meeting that his agency is not promoting conversions but is seeking rules that assure "fairness to all parties" if conversions take place.

The rules proposed yesterday will be published in the Federal Register early next week, and the board will take public comments for 30 days thereafter before enacting final regulations, which may then be challenged in court.

Probably the most significant action yesterday was a decision to avoid concentration of ownership at a stock S&L. Officers and directors would be limited to purchasing 25 percent of all shares sold in an offering compared with no limite previously. Jones said that in one case, about 70 percent of an S&L's shares went to insiders.

The board also voted to seek a wide sale of shares to the public. Each S&L depositor would be given rights to buy shares in the firm under a specified formula but individuals and their associates would be limited to 5 percent ownership. Officers and directors would not be able to sell their shares for three years.

Because of the new limit on stock shares, Jones said, the board decided to allow dividend payments of up to 50 percent of net income instead of 30 percent recommended previously. The counsel said there had been fears of S&Ls promising no dividends or low payments initially but then deciding on extensive dividends after conversion, with insiders in control and benefiting from the gain.