I love the IKEA Expedit storage units that you can add on top of each other (or just get the big 5x5 one). but the photo with the wall shelves might be better for you to put siliding doors in front of it

looks like there'll be plenty of room for his special things but attempting to make a plan of the rooms to try different ideas i realised that the dimensions on the plans are inconsistent (has an entry hall wall been removed??) so you'll need to confirm the exact size of rooms to make specific plans for built ins but one way to create excellent, economical and flexible storage for some of my books we used IKEA Billy bookcases... 80cm wide by 202cm tall x 28cm deep bookcases with glass doors and a combination of wood and glass shelves ...(they come in different heights, widths and colours) we have black and white ones in different rooms for different storage and they make the rooms look spacious and organised..much neater and cleaner than open shelves and have the great advantage of being able to be tried in different locations until you're happy with the arrangement but must be screwed to the wall for security ..my plan shows possible locations for similar sized bookshelves but size of rooms need to be confirmed to be more accurate...

Thanks for your comment. There is concrete exposed aggregate around the pool.

You best invest in some proper design input for this one, it requires proper understanding around the regulations of safety barriers. It's not just the access doors from the house, it's anything that's open to the pool like climbing zones to boundary fences and side gates etc that need to be compliant. Safety is No. 1 priority on these things. There's no leniency, it's black & white. It's a pool owner's responsibility to have it fully compliant with latest standards regardless of how long ago it was built so best have a professional be dealing with all this, and best that it is negotiated and managed within the resolution of a design solution, so the outdoor space simultaneously looks great, functions well, adds appropriate value to the home and most importantly is safe. Best of luck PD www.pauldistefanodesign.com