The board making the order for the assessment of any betterment upon land which is not built upon may extend the time of payment of the assessment until it is built upon or for a fixed time; but interest at the rate of four per cent per annum shall be paid annually upon the assessment from the time it was made, and the assessment shall be paid within three months after such land is built upon or at the expiration of such fixed time.
Mass. Gen. Laws ch. 80, § 13A