For buildings with non-complying floor area, the maximum amount of floor area in the altered, enlarged, relocated or reconstructed building shall not exceed the amount of pre-existing floor area, nor shall it exceed the maximum floor area permitted by the applicable district regulations by more than 20 percent. In addition, subsequent to such alteration, enlargement, relocation or reconstruction, no extension or change of use may create a new non-compliance or increase the degree of existing non-compliance with floor area.