Westminster Investing Corp., a B. F. Saul company, now holds about 90 percent of the outstanding shares of Derwood Investment Trust after purchasing 282,073 shares by Jan. 29, the final date of its tender offer, the company said last week.

Westminster, based in Chevy Chase, purchased the shares of Derwood, which was formerly Realty Income Trust, for $3.1 million -- or $11 a share. Westminster and its subsidiary, Foxwood Investors, currently own $1.1 million in shares, or 90.3 percent of the outstanding common stock. There are approximately 335 holders of the trust's common stock.

B. F. Saul, Westminster and Derwood Investment Trust officials refused to comment yesterday on the transaction or on the properties that Westminster and Derwood own.

In September, Realty Income Trust changed its name to Derwood Investment Trust, and its real estate investment trust status was dropped. While the trust specializes in venture capital and preferred equity positions in real estate -- such as subordinated land-purchase leasebacks -- it also provides a full range of equity and mortgage financing.

Realty Income-Derwood, a 24-year-old firm listed on the American Stock Exchange, was trading at $10.50 a share yesterday. In March 1983, in a change of management, its headquarters was moved from Providence, R.I., to Chevy Chase.

When Chevy Chase Savings and Loan Association needed money in the midst of the savings and loan crisis last summer, Realty Income-Derwood was one of the sources B. F. Saul tapped. The trust provided $8 million in return for 1,600 shares -- or 16 percent -- of a new issue of privately held stock issued by Chevy Chase, the largest savings and loan in Maryland with $2.2 billion in assets.

In November, a tender offer for Realty Income-Derwood stock was made by Westminster Investing Corp., which offered $11 a share for the 404,000 shares owned by about 978 investors. Westminster and its subsidiary, Foxwood Investors Inc., both private firms, already held 850,600 shares of Realty Income-Derwood stock, or 67.8 percent of the shares.

If Westminster succeeds in buying the rest of the shares, Realty Income-Derwood would cease to be a publicly traded company. Realty Income-Derwood specializes in retail and shopping-center developments in the Washington area.

One property owned by Westminster is the project under construction at 601 Pennsylvania Ave., which will include 225,000 square feet of office and retail space, a 240-room luxury hotel and 196 condominium apartments, according to Pennsylvania Avenue Development Corp.

For its 1985 fiscal year, Realty Income-Derwood said it earned $1.6 million ($1.05) on revenue of $3.4 million, compared with earnings of $1.5 million (94 cents) on revenue of $3.6 million in fiscal 1984.

B. F. Saul Real Estate Investment Trust also announced at its annual meeting in Gaithersburg last week that it plans to sell its Vantage Point property in northeastern Atlanta for more than $15 million, or $65,000 per acre. The trust said that the sale of the approximately 430-acre undeveloped property is subject to a number of contingencies.

B. F. Saul executives would not disclose the buyer of the property or elaborate further on the transaction.

The sale of the Atlanta property is part of B. F. Saul's plan to sell several of its land parcels and newly completed buildings during fiscal 1986, said William A. Wildhack Jr., spokesman of B. F. Saul Real Estate Investment Trust.

The trust said it also expects to sell its Fort Lauderdale Commerce Center Phase I property, and its Peachtree Industrial Park and Winters Woods in Atlanta, during the June quarter.