Within 1/8th of an inch over 4' is acceptible. Your LifeSpace Garden or SIP garden will function out of level, but you want to ensure the OverFlow of your garden is always positioned at the low point. This way the reservoir will never encroach about the SIPs into the soil zone - which is key! - however, if your garden is significantly out of level, you will lose reservoir capacity making reservoir fills more frequent.