zusätzlich
additional
As a supplement to something already existing; extra or on top of that.
Für die Reise müssen wir zusätzliche Kosten für das Gepäck einplanen.
For the trip, we need to budget for additional costs for the luggage.

additional
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additional
As a supplement to something already existing; extra or on top of that.
Für die Reise müssen wir zusätzliche Kosten für das Gepäck einplanen.
For the trip, we need to budget for additional costs for the luggage.

'Zusätzlich' (additional/additionally) can stand before a noun as an adjective and is then declined. But it can also be used as an adverb, often at the beginning of a sentence, which triggers inversion (verb in position 2).
Wir benötigen eine zusätzliche Decke, weil es kalt ist.
We need an additional blanket because it is cold.
The word is derived from 'der Zusatz' (addition/supplement), which means 'die Ergänzung' (supplement) or 'die Addition' (addition). So 'zusätzlich' (additional/additionally) describes something that is added to something else.
Das Hotel bietet Frühstück als Zusatz an.
The hotel offers breakfast as an add-on.
'Zusätzlich' (additional/additionally) is often combined with nouns that describe costs, information, or tasks. Typical examples are: 'zusätzliche Kosten' (additional costs), 'zusätzliche Informationen' (additional information), or 'eine zusätzliche Aufgabe' (an additional task).
Bitte lesen Sie die zusätzlichen Informationen auf unserer Webseite.
Please read the additional information on our website.
also / even
'auch' and 'zusätzlich' both say that something is added. 'auch' is a small word in a sentence and shows that a person, thing, or action is included as well. 'zusätzlich' puts more emphasis on something extra being added to something already there. It can stand as an adjective before a noun or as an adverb in a sentence. If you simply mean 'also' or 'as well', 'auch' is more natural. If you mean something extra, for example additional costs, help, or tasks, 'zusätzlich' fits better.

Für den Kurs brauchst du zusätzlich ein Wörterbuch.
For the course, you also need a dictionary.
In der Stadt hilft der Chef auch den neuen Mitarbeitern.
In the city, the boss also helps the new employees.