For the bathroom you can put the sink in the corner, and use one faucet to control the sink and the shower. Doing that will let you cut the size of the bathroom down substantially. 3′ x 4′ could probably work. You just need a container to keep the paper dry.

At 144 sf this tiny house would be smaller than my popup camper, although it only has 84 sf of floor (with two king beds cantilevered off the ends). There’s plenty of space for just me on a weekend trip. For two of us for a week it still works. It’s a little more challenging for us, perhaps, due to musical instruments. With 4 people we’re in each other’s way and stuff is piled up.

People who spend a lot of time in campers have figured out how to reduce things to a minimum, although 100 things seems a bit too hard for me. I see he counts pens, and I’m sure I’ve got 100 of them Anyway, you might find it useful to meet some people at campgrounds and see how they organize their interiors. I can’t live without a coffee maker and my wife needs a tea kettle on the stove and a crock pot.

It is amazing though how much more efficient built in furniture and storage is. I made built-ins for our home-office and they work really well.

Here’s an idea you may wish to consider:

On one wall you could have a built in bench. The bench can be padded and lift up for storage (a quilt for winter perhaps). The table could drop down even with the front edge of the bench. With this arrangement you have provided seating for the table and seating for the room in one unit. Since it’s in the corner, you can wrap the back around the corner and it’s a comfy place to read a book. You can place a shallow cabinet or some shelves above the bench, or a window.

I like the sleeping loft concept. With a 6/12 roof pitch a 12′ room only has 6′ of headroom at the peak, less the depth of the loft joists, but if you build the house with 10′ studs and put the loft at 8′ you can get enough headroom to stand. For me, a tiny house would have to have a porch with a roof over it. Using the shotgun style you could extend the loft over the porch, creating much more usable space without appearing to.

I would make the square 14×14. The extra two feet makes a huge difference, and still stays under 200 sq ft.

Excellent suggestion on figuring out the pile of stuff needed. It reminded me of the 100 things challenge. http://www.guynameddave.com/100-thing-challenge.html

where are the inhabitants going to put:
clothes
shoes
towels
food (or do they eat out/go to grocery store everyday?)
cleaning supplies (vacuum/bucket/mop/soap)
garbage
pots, pans etc

I would suggest making a pile or two of all the things you use in a week or month (or even better a year) and then figuring out how to fit them into a tiny house.

I have lived in small/tiny houses and have used rented storage or other space for off season/seldom used items.