1 - Generally, we use " à l'étage" for the place which are just two levels " ground floor" and "second floor". If the house or building has more than two levels, we add " au 1er étage" not just "à l'étage". But for your understanding, "à l'étage" means one floor up.
2 - "du dessous" refers generally a place. Example : Où se trouve son cabinet ? A l'étage du dessous. Where is his company ? Just downstairs.
" en-dessous" is used when something is below another thing. Example : Où sont mes clés? En-dessous du cousin. Where is my key? Below of pillow.
3 -"du dessus" refers generally a place. Example : Où se trouve son cabinet ? A l'étage du dessus. Where is his company ? Just upstairs.
" au-dessus" is used when something is above another thing. Example : Où sont mes clés? au-dessus du cousin. Where is my key? above of pillow.
I am going upstairs : je monte à l'étage.
I am going downstairs : je descends à l'étage. Grammatically it's correct. But for me, in this case you have to be more accurate. Like, "Je descends (vais) au premier" can be "I am going downstairs" (for this case it's like you : I'm going to ground floor). As a result, you listener knows where you're going.
Sorry for my english, i'm beginning.