Whenever a permit or variance is granted by the Board of Appeals, such permit or variance shall be acted upon and, in the case of structural permit or variance, completed, within two years after the granting of same, unless an extension is granted by the Board of Appeals, and, if not so acted upon and completed within two years or such extended period, such permit or variance shall expire, provided, further, that in granting area or yard variances, the Board of Appeals may provide, in a specific case and for good cause shown, that such variance shall not be so limited.