Hello Gym,
whether you are allowed to change your property in the intended form without approval depends on the development plan. If there are no specifications regarding the height, the approved building application applies. The backfilling or the final/planned terrain height should be correctly represented there; otherwise, there will be problems during the building inspection later on, because such (locally limited) backfillings are considered as attachments under building law and may trigger setback areas and thus possibly encumbrances on neighboring properties.
Complete raising or lowering of the building plot is legally viewed differently. However, this should also be noted in the building application so that the approval authority can comment on it before construction begins. Be sure to discuss this with your architect, because you obviously want to end up significantly higher than the natural terrain surface permits.
Best regards
Dirk Grafe