Start with the loft. Almost every NCR flat has 600 to 900 mm of dead height above the wardrobe. Boxed loft units there add more storage than any other single move.
Hydraulic storage beds hold a full season of bedding and suitcases. Choose a gas lift rated for your mattress weight, not the cheapest one.
Use the space under the staircase or the passage wall for a 300 mm deep shoe and utility unit. Shallow depth keeps the passage usable.
In the kitchen, replace one base cabinet with a tandem pull-out. It holds roughly double what a shelf of the same size holds, because you reach the back.
A window seat with a lift-up top gives seating and storage in the same footprint.
On the balcony, a WPC storage bench survives the weather where a ply one will not.
Keep the top 300 mm of any wardrobe for things you touch twice a year, and plan a step stool into the design rather than pretending you will not need one.