Dekalb county ga 75' R75 zoning

[Kevin Polite]

Have a lead on a house on a 40' lot in Dekalb. Currently zoning requires a 75' lot for a new build otherwise expensive rezoning request. Does anyone know how much reno you can do to a current standing home or is it grandfathered in? I'll see the inside of home this weekend, but I can tell it need a complete rehab, but dont know if this eill trigger having to go through zoning. Anyone who's worked DeKalb County? Note: this is unincorporated DeKalb, not city of Decatur.