The assessor, in order to prepare the valuation list, may whenever he thinks fit, by notice require the owners or occupiers of all holdings to furnish him, within fifteen days with returns of the rent or annual value thereof and a description of the holdings in such detail as the municipality may direct; and the assessor, at any time between sunrise and sunset may enter, inspect and measure any such holdings:
     Provided that at least forty-eight hours, previous notice of the intention to enter, inspect and measure any holding shall be given to the owner or occupier thereof, unless he waives his right to such notice.