Where a district boundary line divides a lot in existence at the time such district boundary line is adopted, the regulations of the less restrictive district shall extend 30 feet into the more restricted portion of the lot. This provision shall not apply to the Historic Overzone district boundary. For purposes of this provision, the restrictiveness of districts, from most restrictive to least restrictive, shall be as follows: R, SR, RSC, RC, CI, C2, C3, I.