Translation guide
Used to add another point, often one that intensifies or emphasizes the previous statement. In Japanese, this is expressed with various additive phrases, depending on formality and nuance.
To add another fact or reason, similar to 'plus' or 'also'.
A common and versatile phrase meaning 'on top of that' or 'in addition'. Can be used in both spoken and written Japanese.
He was late. On top of that, he forgot his homework.
Casual and conversational. Often implies that the added point is negative or surprising, like 'to make matters worse' or 'and what's more'.
It started raining. On top of that, the wind is strong.
Adds emphasis, often translated as 'and moreover' or 'and what's more'. Used in both casual and formal contexts.
This restaurant is cheap. On top of that, it's delicious.
To add another point in a more formal or written context.
To emphasize that the additional point is negative or problematic.
その上 is neutral and can be used in most situations. おまけに is casual and often carries a negative nuance. しかも adds emphasis and can be used in both positive and negative contexts, but is slightly more conversational than その上.
Do not translate 'on top of that' literally as その頂上に (sono choujou ni) or similar. Use the phrases listed above.
売上が増加した。加えて、利益率も改善した。
Sales increased. On top of that, profit margins improved.
The train was delayed. On top of that, it was packed.