Spiral staircases were extensively used in European old castles to reach the top of towers. They were made of local stone and their building under stairs storage ideas required great knowledge of mathematics in order to be done right. Craftsmen who staircase ideas knew how to build them were in great demand.
A room created in the loft space will always be under stairs storage solutions unconventional in shape and slanted ceilings mean that there will be some areas of the room that are less accessible. This low space under the eaves is the perfect place to build in some storage such as shelving, cupboards or cabinets as the area under the stairs storage would not otherwise be used. Apart from giving you extra space to put things in, a cabinet built in this space would not encroach on the room as a free standing unit would. Using this space effectively minimises the need for other furniture stair storage pieces in the room making the room feel larger and less cluttered.
You can also use the space under the stair treads as a pull-out drawer for things which you don't use that often, but those you don't want to fully rest in your basement. Maybe you can put in there some shoes you only use during winter, or older gadgets which you storage under stairs don't get to use as often as before.
For storage purposes and in the case of a small kitchen you need to consider what you want and need in this room. Are you a keen cook? Most seasoned cooks prefer a smaller kitchen where everything is conveniently to under stairs shelves hand. Take a look at the gadgets you have and what you are actually using. Those that storage stairs aren't in regular use should be given a stair shelves new home.
As soon as you have thought of converting the place to under stairs shelving an extra storage room, call your architect immediately. They must see the actual place so they can come up with a blue print of the design for the attic. If possible be involved in every step so under stairs storage unit that you can monitor the things that are done.