I had a similar thing happen to me-though no part of my property was not 45cm below ground ( which would not be grounds to object anyway).
I objected based on scale, height, impact etc. Permission was denied. Happy days. They reapplied with a slightly smaller size (area) but not height. The new plan was exempt on scale grounds and I could not object even though the impact on me was the same.
The planners said they agreed with me that the height of the proposed development was too big but as long as it was exempt on scale (remaining garden area) there was nothing I could do. In essence, they said the planning laws were an ass in this regard!