WHAT WAS thought to be an agreement to build a $300 million annex for the National Air and Space Museum near Dulles International Airport is suddenly an undone deal. Thanks to heavy congressional pressure from a member with sway over the Smithsonian Institution's finances -- Rep. Sidney R. Yates (D-Ill.) -- officials are now reconsidering that plan. Mr. Yates has begun a congressional inquiry into the Smithsonian's proposal. At his urging, Smithsonian officials traveled to Denver, where a group is working hard to snatch the facility. But with the main Air and Space Museum right here in Washington, isn't Denver a bit of a distance to go for an annex?

Why Rep. Yates is so interested in undoing the Virginia plan in favor of Colorado is uncertain, other than his statement that the proposed Denver location might cost the federal government less. But even that isn't clear. Virginia officials say it would be prohibitively expensive to move the Smithsonian's collection of aircraft to Denver. Besides, when the Smithsonian's Board of Regents decided on the Virginia site last January, the desire was to build the facility where it would be accessible to the main Air and Space Museum. At that time, Maryland was trying to locate the annex at Baltimore Washington International Airport -- another site that would make more sense than Denver. That bid may be renewed now, according to a spokesman for Gov. William Donald Schaefer, even though the state appeared to have backed off after the decision last January.

Both Maryland and Virginia are much more logical sites, but a decision did appear to have been made some time ago, and it was in favor of Virginia. Glenn K. Davidson, Washington liaison for Virginia Gov. L. Douglas Wilder, summed up the feeling in his state: "We are dismayed by what has been occurring. At what point does the decision become final?" The Smithsonian should stick to its original idea and keep the main museum and its annex at least in the same time zone, if not closer.