Slowly but inexorably, the lines are growing longer on the eighth floor of the building at 1201 E St. NW. That's where Washingtonians and suburbanites can pick up specialized federal income tax forms and instruction booklets to help them meet the April 15 deadline.

Last week, one clerk was helping anyone who walked in. Yesterday, four clerks were helping a short line. By next week, if experience is a guide, they--by then, far more harried--will be helping a long line of people cursing their fortune at having to wait.

There was one diversion the other day. One of Washington's sidewalk "bag ladies," having obtained her forms, was demanding a place to sit down and fill them out. Alas, no such space was available.

For the record, those who have waited until now to file returns are in the minority. An Internal Revenue Service spokesman for the D.C.-Maryland region said that 1.5 million returns already have been filed, leaving 700,000 still to come. Most of those, unfortunately, are the ones that may require the specialized forms available at 1201 E St. NW as well as at these outlying locations: in Maryland, at the south building of Wheaton Plaza and at the Penn Silver Building, 5408 Silver Hill Rd., Suitland, and in Virginia in the One Skyline Plaza building at 5205 Leesburg Pike, near Bailey's Crossroads.

For telephone aid in D.C. and suburban Maryland, call 488-3100; in the Baltimore area, including Columbia and other suburbs, 962-2509, and in the rest of Maryland, (800) 424-1040. In suburban Northern Virginia, call 557-9030; elsewhere in the state, call (800) 424-1040.

Incidentally: Although April 15 is a Friday, a spokesman said the IRS will not extend the filing deadline to the following Sunday. File by April 15, or else . . . .