Sen. Charles McC. Mathias (R-Md.) yesterday joined Sen. Thomas Eagleton (D-Mo.) in demanding an explanation of why city and federal officials were not informed of Secret Service concerns that the new 12-story Metropolitan Square office complex poses a potential threat to White House security.

In letters to Secret Service director John R. Simpson and National Capital Planning Commission chairman Glen Urquhart, Mathias asked for written explanations of how the Secret Service communicated its concern to other government officials, and how those officials handled the matter.

The City Council and Mayor Marion Barry approved an amendment in 1979 giving developer Oliver T. Carr the authority to build a 130-foot office tower at 15th and G streets NW, where only 95 feet was formerly allowed. The council was unaware of the Secret Service fears, expressed that year, about the fact that the building overlooked the north portico of the White House.

Barry approved of the height increase, according to a spokesman, because he thought the security issue had been resolved in discussions with the Secret Service.

Mathias spokesman Randy Dove said yesterday the senator will await responses from Simpson and Urquhart before deciding whether to hold a hearing on the matter. Eagleton has already issued a similar demand for an explanation of how the height increase was granted.

Mathias also wants the officials to tell him what steps should be taken to deal with security on the planned second phase of Carr's project, which is nine feet taller than the first.