CONGRESS COULD DOUBLE the size of its facilities in the next 30 years without doing violence to the traditional geometry of the Capitol complex or encroaching much on the Capitol Hill residential community. That is the central theme of the interim report by the Architect of the Capitol and the planning group now shaping the long-overdue Capitol area master plan.

The report offers some interesting idea for public debate. If environmental damage could be avoided, some office space might be carved out beneath parts of the Capitol grounds and the East Front parking lot. The Senate could expand primarily toward Union Station in a way that might imporve that rather be-draggled area. On the House side, where working conditions are most cramped and less space for expansion is readily available, new offices should be built first on the Congressional Hotel site and perhaps in the courtyards on the Longworth and Cannon buildings. Beyond that, the report offers four alternatives, some of which manage to avoid - barely - uprooting the residents of New Jersey Avenue SE.

Aside from its emphasis on orderly planning, the strongest point in the report is its call for new transportation systems - a shuttle bus now, and eventually an underground "people-mover" - to serve the thousands who work on the Hill and the millions who visit it each year. Unfortunately, such regard for basic services and amenities does not show up enough in other parts of the report. There is little mention of restaurants and shops, or of the need to keep future buildings and plazas from being too grandiose.

But such points, while worth discussing, are really details. Our basic concern involves not the Architect's proposals but the assumption from which he began: that the congressional establishment is going to keep on growing in the current manner for the next 25 or 30 years. We find that staggering. If Congress does continue to sprawl, piling on more staff and projects haphazardly, it could theoretically have over 33,000 employees and need twice as many buildings 30 years from now - but long before that, either the taxpayers will have risen up or the legislative branch itself will have collapsed from overweight.

What Congress should be concentrating on is not how to arrange more space, but how to use staff and space much more effectively. Various Senate and House panels have recommended a raft of management improvements in the past few years.The House Administration Review Commission, led by Rep. David Obey (D-Wis.), will be bringing a large package of generally constructive proposals before the House next month. So far, however, few of these efforts have gotten past the study stage. For instance, House members badly need more working room in the Capitol, but pulling teeth is much easier than uprooting any employees - including the Architect's - who are now ensconced in prime offices.

Congress's refusal to come to grips with its monstrous parking problems is even more discouraging. As long as Hill parking is relatively ample and free, of course, about two-thirds of Senate and House employees are going to keep driving to work - alone. But the other day, for the second time, a plan that involved Hill parking fees came up on the Senate floor and was resoundingly voted down.

Until the lawmakers have really tackled such problems, building more offices would only foster more inefficiency. Thus while we do support development of a master plan, it should be treated as a conceptual framework for long-term growth - not as an invitation to fill every nearby square with buildings in the next 25 years.