grandfather clause
Noun
Meaning
an exemption based on circumstances existing prior to the adoption of some policy; used to enfranchise illiterate whites in south after the American Civil War
Source: WordNetHypernyms
Examples
Butch Huskey wore the number throughout the rest of his Mets career because of a grandfather clause placed on the retired number by MLB. citation Mo Vaughn also wore 42 during his stint with the Mets, because of the same clause.
The owner also gave up a portion of his parking lot, but the building itself would be protected by a grandfather clause requiring a certain length between a road and a building since it was a pre-existing building.