Section 14. If the right to assess betterments has been established, the board, within six months after the completion of the improvement, shall assess a proportionate share of the whole or part of the cost thereof upon each parcel of land with respect to which a benefit or advantage has been determined as herein provided and in the amount so determined, unless the betterments so assessed shall exceed the cost of the improvement, in which case they shall be proportionately reduced; and such assessments shall not thereafter be abated. The lien for such assessments shall take effect upon the recording of the order of intention to take under section three.