I think there have been suggestions that the alcove under the stairway on the ground floor terrace could be boxed in and inside the 2 bed the alcove under the stairs there could also have doors fitted. There's also the option of lockable storage boxes such as you can purchase in a B&Q equivalant. I think Emma from Snagit Murcia was offering the understairs cupboard service amongst others.
A long time back there was a suggestion on one of the threads about having storage/seating built along the rear wall of the roof terrace but I dont know if anyone went any further with that idea or got prices.