It's a great idea to improve storage and access to crafting materials. SE has added pay for storage with the 2 additional retainers so they would unlikely be interested in a solution that undercuts that business case.

I'd suggest a Workshop. Make it similar to private rooms (those nifty Tardis-like extensions to your FC housing).
  • Accessible to anyone in the FC. Can assign access through member settings.
  • While in the Workshop, your crafting will automatically access all storage and draw in priority order from personal -> retainer -> FC chest (with option to turn access on and off through member settings).
  • To make material access complete, you can optionally place a (crafted) market board in the Workshop.
  • The "door" to the workshop should be placeable. Cottage houses suffer from a lack of wall space. Even the door to the private rooms creates furniture placement issues. While we're at it, make the door to the private rooms space placeable also.
The Workshop would be a great space to place the Summoning Bell, NPC vendors, FC chest, crafting workbenches, etc.