Norwalk Hospital has announced the facility’s largest expansion ever — worth more than $200 million — but has not yet submitted its plans to the city’s zoning department, officials said. Last summer, Nuvance Health, which owns and operates Norwalk Hospital, announced plans for a $220 million expansion of the hospital, with an emphasis on the facility’s maternity services. This month, a New Canaan couple donated an additional $20 million to the project. The total estimated cost of the project is about $250 million, according to a hospital statement. With hundreds of millions invested and plans for the new facility to open within four years, Nuvance has yet to apply with the city’s Planning and Zoning Commission to acquire permission for construction to commence. The new building is anticipated to open in winter 2025, according to the project announcement.