Small & narrow teen room makeover

The goal is to transform a narrow child bedroom into a calm, modern retreat that still feels age-appropriate for a teenage boy (at the moment 10 yo, but it should of course look nice in the next 5+ years). The palette should stay in neutral tones—think warm greys, soft whites, and subtle wood accents. Since the room is narrow, the window on the north-north-eastern side and on the ground floor it can be pretty dark, so we'd like light colours here and some interesting lignting ides.

The bed (Ikea SLÄKT) should stay (it can of course be moved) as it is pretty new and practical. I'd like to keep the wardrobe (Ikea SMÅSTAD / PLATSA) too if possible to save money, but it's not necessary, if it's not working with the concept. However we need A wardrobe definitely. The shelves can be reused, but don't need to. The desk must soon be replaced anyway.

One important note - nothing should be placed approx. 20-30 cm from the outer wall. We had some problems with mold there and the outer walls must reman free to "breathe". A bed on that wall will not work (especially not the one we already have) as it would limit the air flow through the radiator in winter.

Also note - the right wall (looking at the window) is concrete, the left one is a 10cm gipsum wall, so the right one can carry much more shelving etc. than the left one.

Not exactly sure how this will work as this is the first time we'd engage with a professional for the interior design help. Feel free to make a proposal of what makes most sense from your point of view and comment our requirements - we're not just going to simply compare the prices and go with the cheepest one.

What we'd like to see is:
• A scaled floor plan(s) showing optimal furniture placement layout(s)
• 3D renders or realistic visualizations to communicate the final look
• A shopping list: most of our furniture is from Ikea, so that what we would prefer. We like the price/quality relation and this somewhat sets the limit on the final hardware price (which is very limited). We're located in Germany, so if something else is being used, please make sure it's available here and for a reasonable price (similar to Ikea's). Also we're of course ready put effort in some custom made details.

Attached: photos and floor plan of the room. Use the "screenshot" to locate the room on the floor plan.
We will of course provide any and all additional measures that you might need or check the ones in the drawing.