Learn Russian: Вы идёте в офис два?

The number two has different forms depending on gender in Russian. Because офис is a masculine noun, we use два. If we were talking about a feminine noun, like room (комната), we would use две.

Why is the number два placed after офис instead of before it?

Placing the number after the noun treats it as a name or label, just like in English when we say office two instead of two offices. If you put it before the noun, you would be counting them, which would also force the noun to change its ending.

Does the word офис change its ending because of the preposition в (to)?

It is actually in the Accusative case here because it shows the destination of your movement. However, for non-living masculine nouns like офис, the Accusative form looks exactly like the dictionary form, so no ending is added.

Why does the sentence use идёте rather than ходите?

Идёте is used for a single, one-way trip happening right now. The question is asking if you are on your way to that specific office at this moment, rather than asking if you go there regularly.