TWO DEVELOPERS scheduled to take their requests for major rezonings in the Tysons Corner area to the Fairfax Planning Commission next Wednesday are asking the commission to defer public hearings on their proposals.

The Hadid Investment Group's application for a 365-foot-tall building on the site now occupied by Koons Pontiac on the Leesburg Pike is being deferred until a date to be set later this month. Modifications of the development plan may be under way, sources said.

A controversial application by Tysons-Dulles Limited Partnership for the rezoning of a 15-acre site at Spring Hill Road and the main toll booths of the Dulles toll road is also expected to be granted a delay by the planning board. An official request for that delay was expected to be delivered to the county by the developer late this week.

Although neither project is connected, the reasons for both requests for delayed action may well be that the individual developers are waiting to see if the Fairfax Board of Supervisors rejects or approves the 107-acre Tysons II project, which is scheduled to be voted on Monday night.

THE POPULARITY of adjustable-rate mortgages "could diminish considerably" as thrift institutions become more prudent in originating them, according to Joseph Hu, a housing economist at Salomon Brothers in New York. Indeed, "Depending on the specific interest rate environment, fixed-rate mortgages could again become the leading mortgage finance instrument," he said.

The shift has come about as thrifts confront the reality that ARMs without rate caps threaten borrowers with big increases and potential "payment shock," but the presence of payment caps prevents them "from becoming truly-interest-rate-sensitive assets" that will protect the lenders from runups in the rates they must pay depositors, he said.

Hu suggested that a prudent ARM for a thrift should carry an initial rate 1 percentage point above the institution's marginal cost of funds -- a rate that in current market conditions would put the loan at 12.5 percent. The loan should be indexed to one-year government obligations (adjusted to a constant maturity), carry a two-point margin -- that is, the rate charged the homeowner should be two percentage points above what the government pays -- and have a two-point annual cap and a five-point lifetime cap.

But he noted that, as thrifts move closer to these criteria, ARMs' popularity with home buyers will decrease. Standardization will make it easier to package ARMs into mortgage-backed securities, however, thus possibly holding down their rates somewhat.

"No longer can it be automatically concluded that, as long-term mortgage rates raise, single-family housing starts will fall," he said. "However, it is still true that, if an increase in long-term mortgage rates is accompanied by a similar movement in the initial rate for ARMs, as has occurred in recent months, housing activity will undoubtedly decline."

TAX ASSESSMENTS on the brand-new National Place and recently refurbished Regent Hotel are accurate, the D.C. Board of Equalization and Review has ruled, even though an attorney for the owners argued that the properties are worth millions of dollars less than what they have been assessed.

Gilbert Hahn, the attorney representing both properties, argued before the board on Oct. 1 that National Place, recently assessed at $145.4 million, is actually worth $78.7 million, because the complex, consisting of office space, retail shops, a hotel and a theater, is not expected to earn much money in the next few years.

George Altoft, from the District's Standards and Review department, said the city assessors came up with the $145.4 million assessment for National Place after looking at the cost of the Pennsylvania Avenue land ($57 million) and the cost of construction ($123 million).

"When a property is new, assessors go with what it cost," he said. "Assessments are not made on expected income. That is hardly reliable."

Hahn also argued before the board that the Regent Hotel, assessed at $32 million, is actually worth $3.5 million, a difference he again attributed to projected revenue, which is expected to be low.

PERSONNEL FILE . . . The Washington Building Congress was to install its 1984-85 officers this week. They are: Stephen J. LaScola of Fischbach & Moore Inc. of Landover, president; John M. Derrick Jr. of Potomac Electric Power Co. of Washington, first vice president and president-elect; William I. Magruder of McCarthy Construction Co. of Arlington, second vice president; William H. Doggett of William H. Doggett, AIA, Architect & Planner of Bethesda, third vice president; James A. Hooff of Omni Construction Inc. of Rockville, secretary, and Mark C. Garrett of Skinker & Garrett Inc. of Washington, treasurer. . . . The Manufactured Housing Institute has elected Joseph J. Harris, president of Champion Home Builders of Dryden, Mich., chairman of the board for the coming year. . . . Philip G. Padgett Jr. has been named president of Oxford Securities Corp., the equity placement subsidiary of Oxford Mortgage and Investment Corp. . . . William R. Palmer has been promoted to vice president of Spaulding & Slye Construction Co. . . . Althea Susan Davis will manage Epic Realty Services' new service center in Fairfax.

ON THE MOVE . . . Hewlett-Packard has leased 18,000 square feet in Danac Office Park in Rockville. The space will be used for a computer training facility. Danac Associates Inc. was the leasing agent. . . . Federal Express has taken 30,278 square feet in Rivers Technology Park in Columbia. Rouse & Associates, the developer, says the park is now two-thirds leased.