The special benefits conferred upon the lands involved in the general improvement district by any such improvement or by the acquisition of any property or rights therein shall not be deemed to accrue at any one time but shall be deemed to be benefits continuing throughout the period of the life of the project, which render said lands subject to assessment, from year to year as herein provided, to pay for and carry out the object for which such improvement was made or property or rights therein acquired.
RCW 89.30.475