I want aircon in a part of the house which is half stairway and half room on the second floor. The terrace door looks out on a mountain range and gets great light in the afternoon, perfect cosy corner to sit in the rainy season. The half room has about 8 sqm.

Because cold air drifts down it is easy to waste a lot of electricity if too much of the current flows down the staircase.

My thought was to place the aircon on the left wall over the stair and let it blow straight over the stair into to the half room on the left. Thereby most of the cold stream would reach the left before it has a chance to drop down the stair. Does that make sense?

I would put the unit on the inside wall of your loft with the idea of cooling your downstairs as well. I had a similar setup in the past and that worked great for what I wanted.

Just put it so it blows directly where you would like to sit.

Tks, both recommendations make sense, placing the aircon on the right wall blows the air to the sitting area plus it will drift down the staircase, potentially also adding to the cooling of the dining room

Just put it so it blows directly where you would like to sit.

yep, anything else is just wasting cooling power