The lien herein provided for shall be subject to prior mortgage liens, unless the holder thereof has received notice of the intention of the furnishing of said labor, money, material or supplies and consents thereto in writing after which the holder of such mortgage lien shall become jointly liable with the owner of said property for the expense of producing, altering or repairing of same.
Laws 1917, c. 187, p. 351, § 3.