Should the applicant commence construction or alteration within said six-month period, but should he fail to complete such construction or alteration within said two-year period, the Zoning Officer will, upon 10 days' notice in writing, rescind or revoke the granted variance or special exception, or the issuance of the permit, or permits, or the other action authorized to the applicant, if the Zoning Officer finds that no good cause appears for the failure to complete within such two-year period, and if the Zoning Officer further finds that conditions have so altered or changed in the interval since the granting of the variance, permit or action, that revocation or rescission of the action is justified.