The first meeting of the Design Working Group for the green line extension project is Monday in Medford.

The first meeting of the Design Working Group for the green line extension project is Monday, project manager Katherine Fichter announced.

The schedule for the meeting — to be held from 6 to 7:30 p.m. at 51 Winthrop St., Medford — includes a project update on design status and scheduling, an outline of how station design workshops will go and a schedule for Design Working Group meetings.

There will be public comment at the end of the meeting, Fichter said Tuesday.

The green line extension project moves the Lechmere stop to the NorthPoint development side of Monsignor O’Brien Highway; adds a one-stop spur to Union Square in Somerville, roughly where Prospect Street and Webster Avenue meet; and lengthens the line through Somerville into Medford, including to Brickbottom, Gilman Square, Lowell Street, Ball Square, College Avenue and finally to Route 16, to a site near the Starbucks, Whole Foods Market and U-Haul depot. Officially, all but the outermost stop are to be built by October 2015.

Some design elements — correcting flaws in previous stations — are already set, Massachusetts Department of Transportation planners have said.

Enabling this was Friday’s granting of a state Environmental Protection Agency certificate to the extension by Secretary of Energy and Environmental Affairs Ian A. Bowles. It essentially gives approval to what’s now being described as a “nearly $1 billion” project. (A recent state cost estimate put the extension project at about $932 million without a Route 16 stop.)