Section 18. If, after examination, inspection and a hearing, they find that the improvements prayed for are required by the public convenience and necessity they shall so lay out and establish the same as to do as little injury as practicable, and if it is necessary to take private property for such improvements may take it on behalf of such county under chapter seventy-nine. Any person sustaining damage in his property by the laying out or establishment of such improvements shall be entitled to recover the same under said chapter.