Hello Q. Well you are absolutely right about the overall height. Your layout does show 240cm. However, I used a length conversion program to get inches, which is what I use to draw. I had to manually insert the dimension, which is where the mistake is. I inserted "120cm" instead of "240cm". But the actual line I drew is CORRECT, which shows there is a difference between the overall dimension and the combined dimensions of the 154cm and steps. Not that it matters much, but I DO like to draw things as they are. It helps when it comes to fitting things like absorbers and such. So, if you can recheck the step heights, or what ever, I'd appreciate it.
Of course, there MAY be a slope in the floor or roof

....which would account for the difference. And thats a biggeee. I wouldn't doubt that one..or even both may be sloped, as this is a garage. Roofs, even being flat, usually are sloped in one direction to allow water to be directed towards a "scupper", which is actually a drain through a parrapet wall(a wall that extends above the roof). And garage floors are typically sloped here in the US. In fact, my last house had a garage floor that sloped 4" from back to front. You might take a level and check these for any slope. If they are, measure the difference in height within a range of say 150 cm.
The ceiling wouldn't matter too much, but if the floor is sloped...depending on the condition and slope of the floor, you may need to build a new floor. Either a wood membrane or concrete. That is if you want a level floor in your studio.
Either way, I'd check just so I know what you are dealing with.
One other thing. All your dimensions are taken on the INSIDE of the building, is that correct? Just for a comparison, can you take exterior dimensions. I understand that would be impossible at the stepped area, but at the front, on the alley end, can you give me an overall height from the street to the top of the building. That would help me understand some things. Thanks
fitZ
ps Can you take a closeup picture of the inside of the stepped wall? Heck, for that matter...every wall and ceiling too

Details help.