If you have a clean aerated concrete surface on the edge of your too-short wall, it's relatively easy to fix. Buy aerated concrete adhesive (thin joint mortar) and suitable aerated concrete blocks. The easiest method is to buy blocks in the same thickness as the wall and then cut them to 50mm or whatever measurement you need to extend. If the wall is going to be plastered by 50mm, perhaps make them a bit shorter. Then it's mostly just a matter of mixing the adhesive and putting up the blocks. Of course, after checking that it becomes straight and neat. The nice thing about good adhesive for aerated concrete is that the joints become so thin and the adhesion is so good that you can lay quite a bit without the structure moving before the adhesive/mortar has set.
You can easily cut aerated concrete with, for example, a semi-dull handsaw. To get straight cuts, you can run the blade along a board that you fasten with clamps.
To help with the angles, you might optionally mount a board flush with the wall you are extending, so the blocks end up in the right position immediately. If you need to plaster afterwards, gypsum plaster is easy to work with, but make sure the type you buy is suitable for wet rooms.
