A 20-year lease has been negotiated for all of the 181,000 square feet in a new office building under way at 1722 I St. NW. Representing a major law firm, Arthur Rubloff & Co. arranged the multimillion-dollar lease, reported to be one of the biggest ever made here, with 1722 I Street Associates, the developer. The tenant will be Sidley & Austin, a national law firm now at 1730 Pennsylvania Ave. NW. The law firm will occupy three floors of the building and lease five floors of office space and three levels of retail space. Completion is scheduled in mid 1982.

Construction has been started on the first building of what is planned to be the new Flint Hill commercial office park at routes I-66 and 123 in Fairfax County. The 48-acre site is planned ultimately to have 800,000 square feet of space on a 48-acre site being developed by Jerome A. Kaplan, Merrick C. Marshall and the Lerner Companies. The first building, which will have 127,000 square feet expected to be leased in the range of $14.50 a square foot, is scheduled for completion this fall. Brian W. Ball of Shannon & Luchs is handling leasing.

Newly organized, a real estate management, development and leasing organization called Gates, Hudson and Associates Inc., has offices at 6928 Little River Turnpike, Annandale. The principals in the firm are Robert T. Gates, Charles E. Hudson, Jeffrey G. Weaver, Patricia Blackburn and Richard Mazzucchelli. All formerly were with Mark Winkler Management Co., a major realty firm in Northern Virginia. The new firm announced that its management portfolio already includes 2,250 apartments, 25 office buildings and 3 shopping centers. For development, the firm has a construction arrangement with Eugene Simpson and Bro. Inc., Alexandria.

Weaver Bros. Inc. recently arranged permanent financing of $16 million with Metropolitan Life Insurance Co. for the 12-story building completed at 1333 New Hampshire Ave. NW. Major tenants in the 315,000-square-foot building are the law firms of Cadwalader, Wickersham & Taft and Akin, gump, Hauer & Feld. The building was developed by a limited partnership headed by Theodore B. Gould.