Despite recurring reports about possible over-building of new office space in the Tysons Corner area, leasing activity is continuing strong in downtown Washington and in some suburban areas.

A recent survey made by Howard Ecker & Co., a Chicago-based firm dealing in office leasing in major cities, shows that more than 3 million square feet of new space is currently being built here and another 10 million square feet is planned to meet demand.

While the Ecker report is regarded by one Washington leasing professional as underestimating the new space under construction, the average of $15 a square foot price for new area-wide space is seen as being almost on target.

However, it was pointed out that there is a big disparity between new downtown leases -- which are mostly in the range of $20 a square foot -- and those being made in some areas of Virginia and Maryland in the $11-12 range. Leases in Crystal City and Rosslyn, both close to downtown, are now calling for about $16 a foot.

The Ecker report showed base rents in new office buildings across the nation was averaging $16 a foot, from a low of $10 in Kansas City to a high of $30 in New York City. Other cities where office rents are above average include Boston, Chicago, Denver, Los Angeles and San Francisco.

Elsewhere on the office leasing front:

American Management Systems Inc., a computer systems and services firm, has leased 186,000 square feet of office space in a new building at 1777 N. Kent St. in Rosslyn. The space will house the company's corporate headquarters. The long-term lease, with a rental of more than $40 million, is one of the largest single leases in Northern Virginia history. The building is owned by Geneva Asociates. It was designed by Vlastimil Koubek Associates and built by the Charles E. Smith Building Corp. It will be dedicated as the Ams bUilding early next year.

The International City Management Association has contracted to take 40,000 square feet on four floors of the building started at 1120 G St. NW, where occupancy is scheduled in 1982. ICMA is a professional and educational association for city administrators. The Braedon Companies negotiated the $6.1 million lease with Bender Associates.

Centec Corp. has moved into the fourth and fifth floors of a new building at 11260 Roger Bacon Dr., Reston. Founded five years ago, Centec provides conservation and environmental services to private and federal groups. Other tenants in Centec building are SCS Engineers, 1st American Bank, Bailey, Banks & Biddle, Wellborn Management and Bogley, Harting & Betts.

Julien J. Studley Inc. announced that Datatronix, a light assembly electronics firm, has leased 11,500 square feet at 2100 Reston Ave., Reston. Studley also handled the lease of 6,500 square feet to the Mental Health Law Project at 2021 L St. NW.

Boston-based Spaulding & Slye Corp., which has an office in Bethesda, has started development of a 40-acre office park at I-270 and Shady Grove Road in Montgomery County. Nine low-rise buildings are planned on the site that was obtained from Western Development Corp. Initial occupancy is scheduled late in 1981. Spaulding & Slye also negotiated a lease for University Research Corp. to take 5,400 square feet at 6110 Executive Blvd., Rockville Weaver Bros. was a co-broker in the deal.

Maxima Corp., an information technology and transfer firm, is moving from Silver Spring into 11,000 square feet in the Bethesda Air Rights Building. The Braedon firm negotiated the lease.