Proposed changes to Somerville’s zoning ordinance to allow a supermarket to be built in Assembly Square will be considered by aldermen next week. Federal Realty is looking to develop a 400,000-square-foot building on a 12-acre parcel that would include a 100,000-square-foot supermarket. But zoning regulations for Assembly Square, written in an effort to curb “big-box” development, limit retail space to 50,000 square feet. In January, the city’s planning board approved a proposal to recommend an amendment to city zoning laws that would allow the supermarket. It’s now up to the Board of Aldermen to decide whether to allow such a change. The aldermen’s Land Use Committee is expected to discuss the amendment in a meeting on July 18 at 7 p.m. in City Hall.