Perpetual American Savings and Loan of Washington and Coast Federal Savings and Loan of Sarasota, Fla., announced plans yesterday to form a nationwide brokerage subsidiary.

The plan must be approved by state and federal regulators.

Because the brokerage services would be offered under the same roof, S&L customers would be able to buy and sell securities or invest in money market funds at the same place they have their savings accounts and obtain their mortgage loans. For the S&Ls, the brokerage fees would mean a boost to earnings.

In their application to the Federal Home Loan Bank Board, the chief executives of the two associations referred to the need for S&Ls to "move with the changing needs of our customers" and to "meet the challenge of money market mutual funds head on."

The bank board immediately issued a statement through its general counsel Thomas P. Vartanian questioning the legal ramifications of such a venture, but promising to "take a fresh look at these questions in today's world."

The Securities Industry Association called it "a startling development and a most interesting one that will have to be examined in the context of changes in the securities, banking and financial industry."

The concept was originated by Coast, which also conceived the shared appreciation mortgage. Coast, which has assets of $720 million, then sought out as partners large, prosperous, aggressively managed S&Ls located in strategic geographic areas. Perpetual is the first to meet those criteria; three or four others are expected to be named soon. The proposed amount of capitalization was not revealed.

Besides the equity participants, other S&Ls will be subscribe to the financial research and counseling services offered by the brokerage firm, which is expected to have headquarters in Tampa. It is negotiating with one of the major securities firms to execute its trades. But the brokers and advisers will have their offices in the participating S&Ls.

Regulatory aproval will be required not only from the Bank Board, but also from the Securities and Exchange Commission, the National Association of Securities Dealers, and state securities commissions. The proposed venture faces serious legal obstacles, the biggest of which is the federal Glass Steagall Act which separated the banking industry from the securities industry back in the 1930s.

Congress is expected to review that statute in the next few years, given the increased competition between banks and other financial institutions such as brokerage firms that offer banklike services through check-writing privileges on money market fund accounts.

Stephen Ege, a Washington attorney for the originators, said yesterday that whereas Glass Steagall prohibits banks from underwriting securities, he does not believe it would prohibit S&Ls from acting as customers agents in trading securities. He has worked out a three-tiered corporate arrangement in an effort to satisfy the law regarding interstate operations and broker-licensing requirements.

The brokerage firm, known as Savings Association Investment Securities, would be wholly owned by the Savings Association Financial Corp., which in turn is owned by the service corporations of the various S&Ls. Dan McConnell will be chief executive officer of both SAIS and its parent corporation. McConnell is an investment banker who specialized in commercial paper when he worked for Greenshield Inc., a Toronto brokerage firm.

The bank board has had previous requests from S&Ls desiring to set up money market mutual funds through service corporations, but has delayed making a decision, primarily because of legal difficulties. McConnell said yesterday he would prefer that federal regulators allow S&Ls to offer customers more competitive rates. Failing that, he said SAIS would consider starting its own in-house money market fund.