§ 45b. In the case of Bills containing power to impose on any lands or houses, or to render any lands or houses liable to the imposition of any charge in respect of any improvement the plan small define the improvement, and also the improvement area (being the limits within which the charge may be imposed), and in the case of a street improvement the plans shall show in figures the breadth of the street as intended to be improved, and the centre line of the street, and shall define the improvement area, comprising therein all properties which or any part of which, are within a uniform distance from any part of the centre line of the street, and such uniform distance shall not exceed three times the breadth of the street as shown on the plans, or where the breadth of the street is not uniform, three times the breadth of the narrowest part.