The officer or board shall also ascertain, so far as practicable, the names of the owners of the real estate to be taken and of the property to be benefited by the proposed improvement, the interest, in the real estate so to be taken, of each owner thereof, and when the names or estates are not known, they shall so report. They shall further appraise the value of the interest of each known owner of real estate to be taken and the damage which will be done to such owner by the taking. Where the estates in any plot of land are unknown, they shall appraise the value of or the damage done to the fee simple. They shall estimate all other expenses likely, in their judgment, to attend the completion of the improvement, shall estimate the amount likely to be realized from the sale of any buildings, or parts of buildings, required to be taken on account of the improvement, and shall so determine the probable net cost of making the improvement.