When the land to be taken for highway purposes is not for the widening of an existing road but is for an extension, cut-off or alteration which will afford to any owner of adjoining land a new frontage upon a county road so being laid out and opened, the commission shall consider in determining the amount of damages to be awarded to such owner for such taking, any increase in the market value of his entire tract from which the land is taken, by reason of the new road frontage given as part of the proceeding, but it shall not take into account any proposed future paving or improvement not a part of the proceedings to acquire land to open the road.