A White House spokesman in California said officially for the first time yesterday that the Reagan administration was not necessarily committed to increasing defense spending by as much as 7 percent a year.

In Washington, other senior administration officials said privately they now expected some cuts would be made in previously planned military budget requests and expenditures. Particularly affected would be projects whose bills come due in 1983 and 1984, to reduce the strain anticipated on those budgets and to provide a better chance of meeting President Reagan's stated goal of a balanced federal budget by 1984.

White House officials have also become increasingly sensitive to criticism of proposed cuts in Social Security and other social programs and have indicated they are looking for ways to make at least some offsetting cuts in defense spending.

In Santa Barbara, Calif., deputy press secretary Larry Speakes made it clear that defense spending will continue to increase as the Reagan administration pursues its efforts to rebuild American defenses across the board. But he said that the increase could fall under the 7 percent real annual boost that the president has talked of as a formula for the next five years.

"The 7 percent figure, I think, there has been some misunderstanding," Speakes told reporters. The spokesman said he probably, in the past, has conveyed the wrong impression, "that we were committed to a full 7 percent." As he now understands it, he explained, the 7 percent "was a cap," meaning the top target figure rather a firm, inflexible commitment.

Reagan's chief economic advisers reportedly have asked him to cut the planned $1.5 trillion, five-year military budget by $20 billion to $40 billion over the next three years to keep hopes of a balanced 1984 budget alive and to avoid slashing to the bone social programs that have already been cut back. The currently predicted budget deficit for next year is $42.5 billion.

Today, Defense Secretary Caspar W. Weinberger and White House budget director David A. Stockman are scheduled to fly to Santa Barbara to meet with presidential counselor Edwin Meese III and others to discuss the 1983-84 budget situation. Speakes said that Weinberger would also go to the Reagan ranch for a short meeting with the president.

The budget deliberations come as the president is also in the final stages of making decisions on huge multibillion-dollar strategic nuclear weapons programs, including the MX missile and B1 bomber.

Budget officials say money is already set aside to cover the start of whatever the president decides, but there are continuing signs of uncertainty in the defense community about what is the best course to follow for the highly controversial MX program.

In Washington yesterday, the Pentagon's top scientist, Undersecretary of Defense for Research and Engineering Richard DeLauer, said Weinberger "wanted to delay making a decision on the final basing" for the new land-based missile until the Pentagon "had done more work" on developing antiballistic missile (ABM) systems to defend it against attack by Soviet missiles.

In a breakfast meeting with reporters, DeLauer provided the most extensive public explanation thus far about the doubts and uncertainties in the Pentagon concerning where and how to base the MX.

DeLauer said he did not know what the presidential decision would be on the MX and indicated it was still being considered. He said it is "reasonably accurate" to say that "certain people" within the administration are advocating some form of a "deceptive basing" scheme for the MX that would be a scaled-down version of the shell game proposed by the Air Force in the Carter administration. Unofficial reports have indicated this would involve 100 missiles shuttling between 1,000 shelters to confuse Soviet attackers.

But DeLauer made clear that he and his boss, Weinberger, had serious doubts about such a plan until more was known about antimissile defense. DeLauer said several times that "it is just tough to get an undefended system that is survivable" because the Soviets can add warheads to their missiles faster than we can build shelters to hide the MX in.

A deceptive basing system such as the shell game that is defended by an ABM "has great potential," DeLauer said, "and we ought to go to work on that," meaning the ABM development work. He said after about four more years of work, the Pentagon ought to have a pretty good idea of whether an ABM would work as a protector of missile bases. In any case, the United States is now restrained from building an ABM by a treaty with the Soviet Union.

So why does it make sense to build a scaled-down deceptive basing system, as some are advocating, without a missile defense, DeLauer was asked? "That's a good question," the Pentagon's research chief answered. "The last thing anybody wants to do is put a lot of money into something that does not lend credibility to the deterrent force" of U.S. missiles.

DeLauer said the whole package of strategic nuclear force improvements that will ultimately be revealed needs to be a coherent, well laid-out plan. But the question of what to do about land-based missiles has not been decided and the uncertainty about antimissile defense is one big reason "why we are having trouble coming down on exactly what we are doing."

DeLauer said the scaled-down system of 100 missiles and 1,000 shelters would cost about $20 billion. Those who support it argue that it forces the Soviets into expensive new programs, buys time while the United States tries to develop ABM defenses, and adds a missile to the U.S. force able to attack Soviet missiles effectively.

But DeLauer said he did not necessarily support that view. The ability to survive an attack, in his view, was the crucial quality to be sought in any new land-based missile.

DeLauer and other senior defense officials yesterday also suggested that there were ways other than an immediate start on MX production and deployment to eventually solve the problems caused by the alleged vulnerability to Soviet missile attack of the existing 1,000 U.S. Minuteman land-based intercontinental ballistic missiles.

DeLauer said that the idea of building a common missile for use by the Navy and Air Force was still very much a live issue. He said "common missile" was a bad name but that something other than MX that used combinations of common and specialized motors, guidance systems and other equipment --presumably from the MX and the Navy's new submarine-launched D5 missile -- could be developed, though he gave no clue as to how the land version would be based.

Research on an airborne version of MX will also continue.

Other Pentagon officials said it was necessary to understand that nothing now being contemplated as a new ICBM, including the MX, would do any good before the late 1980s in terms of providing assured survivability for land-based missiles. Therefore, it was not a bad idea, as DeLauer put it, "to take some time and a little more effort" to figure out how to solve the problem by either defending land-based missiles with ABMs or carrying the missiles aloft in new aircraft.

In the meantime, about 200 new bombers, the first 100 or so of which will be variations of the once-canceled B1 bomber, still more air-launched cruise missiles and better submarine-launched missiles, will be used to beef up the nation's strategic strike force.