Common areas and facilities. A minimum of 30% of the total lot area shall be devoted to common areas and facilities. No less than 50% of this area shall be maintained as open space usable for recreational and leisure-time activities of prospective residents of housing for the elderly. The remaining portion may be used for plazas, arcades, pedestrianways, and other facilities (excluding roads, access driveways, and parking areas) available for use by residents of the development and the general public. In addition, the developer of housing for the elderly shall be required to provide a community room, indoor recreation area, or similar facility within every principal residential building. All information regarding common areas and facilities shall be shown on the plan.