A new development proposal for Somerville’s Assembly Square would target biotech tenants with an 18-story, 480,000-square-foot office and lab tower as part of a 1.9-million-square-foot mixed-use project.

Somerville Office Assoc. LP, an affiliate of New York-based R.D. Management, hopes to kick off The Office and Research Center + The Residences at Assembly next spring with a 188-room hotel and 147-unit apartment tower.

The 9-acre site on Mystic Avenue, Middlesex Avenue and Revolution Drive is occupied by a 195,000-square-foot office building and surface parking.

R.D. Management said it’s contracted with an unnamed hotel developer that is ready to begin design and construction of the 5-story hotel, according to an environmental notification form filed with the state Executive Office of Energy and Environmental Affairs. The first phase, called “Block 21,” also would include a 15-story, 159,000-square-foot apartment building.

The following phase “will dramatically increase the site’s density” with a 240-foot-tall, 774,000-square-foot tower containing
ground floor retail, a 4-story parking garage and 13 floors of office and biotech space fronting on Middlesex Avenue.

Phase three would include a 250-foot-tall, 1-million-square foot building with ground-floor retail, four levels of parking and offices on floors 6 through 19. The final phase is a 5-story, 72-unit apartment building at the corner of Mystic Avenue and Revolution Drive.

The commercial segment of the development could be positioned to siphon off spillover life science demand from industry hub Cambridge.

Direct lab vacancies in Cambridge held steady at 0.7 percent during the third quarter, according to Colliers International’s Market Viewpoint report. Lab users are in the market for more than 850,000 square feet of space, according to tenant representation brokerage Cresa.

RD Management owns retail properties in 22 states including the Shops at Billerica, Meadow Brook Center in Lowell and Milford Crossing.

Michael Ades, a partner with RD Development, has met with Somerville land-use officials and Mayor Joseph Curtatone, Somerville Director of Economic Development Tom Galligani said. The company has not submitted its plans for local approval yet.

Ades could not be reached for comment. The filing states that Curtatone “and other elected officials have encouraged the proponent to begin construction of phase one.”

Boston-based Spagnolo Gisness & Assoc. is the architect.