Yard requirements for each main structure shall not be required; however, wherever a building is built upon a lot adjacent to a residential district boundary, there shall be provided a side yard of 10 feet or more on the side of the building adjacent to the district boundary line. Landscaping in the form of evergreen trees shall be maintained on property lines joining any residential district boundary. On corner lots, the side yard which faces on a street shall be 20 feet or more.