NEW YORK CITY-Tenant-rep firm Studley is planning to expand within its new headquarters space, signing a 10-year lease for an entire floor of 61,000 square feet at Boston Properties’ 399 Park Ave. The space is 50% larger than its current offices at 300 Park Ave., where it’s been headquartered since 1987.

Mitchell Steir, Studley CEO and chairman, negotiated on behalf of his firm along with company president Michael Colacino and executive managing director Michael Goldman. John Powers, chairman of the New York tri-state region at CB Richard Ellis, represented Boston Properties.

A spokesman for Studley confirms a report in the New York Post that the 399 Park deal came together quickly, with Powers presenting an offer to Studley as recently as Oct. 2. The Post, report also says that Studley cancelled negotiations for space at 601 Lexington Ave., also owned by Boston Properties and formerly known as Citigroup Center., in favor of the 399 Park space.

Terms were not disclosed. Asking rents along the Park Avenue corridor average $81.08 per square foot, down 33% from a year ago, according to Cushman & Wakefield.

Studley expects to move into its new home sometime during the first quarter of 2010, according to a release. The firm has more than 140 professionals and support staff housed at 300 Park, and will have the capacity for more than 200 employees at its new space.

Boston Properties has owned 399 Park since 2002, when it bought the 1.7-million-square-foot, 40-story office tower for $1.06 billion or $630 per square foot, a record sum at the time. Among the other tenants at 399 Park is Citigroup, which has its world headquarters there.