Fearing a takeover by a New York City real estate company, UNC Resources Inc. of Falls Church moved yesterday to make an unfriendly acquisition extremely expensive.

The UNC board of directors adopted a complicated plan that, under certain conditions, would require a corporate raider to pay $50 a share to redeem millions of shares of preferred stock. The effect, a UNC spokesman said, would be to almost double the cost of an acquisition from about $220 million to almost $400 million.

A UNC spokesman said the plan was intended to force anyone trying to acquire the company to negotiate with the board of directors on a takeover price.

Large blocks of UNC stock have been purchased recently by Maxxam Group, a New York Stock Exchange-listed real estate company. As of Sept. 26, Maxxam Group told the Securities and Exchange Commission it had acquired 19 percent of the UNC stock. It later won federal approval to increase its stake to as much as 50 percent.

Maxxam is controlled by Houston investor Charles E. Hurwitz. A request by Hurwitz for a seat on the UNC board of directors was rejected by the company.

A takeover by Maxxam, UNC suggested, "would have the adverse consequences to shareholders the plan seeks to prevent."

UNC President Dan A. Colussy spoke to Hurwitz yesterday to tell him of the board's action, a UNC spokesman said. Hurwitz' reaction was reported to be non-committal. Hurwitz was not immediately available for comment.

UNC said its anti-takeover plan would protect stockholders from partial tender offers.