Very nice design! Just one thought, will you need all the walls?
When redecorating a small house many years ago, an architect proposed several walls and different rooms. After a lot of thought, I ended up doing the opposite: removing walls instead, so it is now basically one large room, but with two levels. Which means there is a natural divide between house functions, but lots of air and feeling of space. So even if the house is small, the inside feels big. BTW, these two staircase models are both made for tiny houses (future project, same house):
Staircase ladder and
Staircase samba
Both based on actual, off the shelf products.
). I think that there are some who are looking at this and wondering "Why in the world would someone need that much storage space for just some sewing stuff?" I have a simple answer - I personally have a rather extensive collection of vintage, old and modern sewing patterns (currently at a count of about 700) as well as a not-very-small fabric stash. 
).