In Residence Railroad Station Districts, no dwelling authorized hereby shall be erected or altered to accommodate or make provision for more than 12 units of housing per acre or more than a proportionate number of dwelling units on any fractional part of an acre or such other amount as fixed by the Board of Trustees of the Village of Babylon. For the purpose of this section, a dwelling unit shall be such combination of rooms, with provisions for living, cooking, sanitary and sleeping facilities, arranged for the use of one family, which shall consist of not more than four separate rooms exclusive of bathrooms and hallways.