Greeting and welcome!
I once lived for a summer and autumn on a 19 feet single hull
Triss Magnum (goolge it if you want to see the interior and what type of boat
it is, so you get a picture..)
Well, it is small.. But it worked really well for me to live on alone 24/7 and also for shorter trips with friends. The boat
model is flush decked but quite wide for its' lenght, and therefore good space inside.
I did not have a toilet or a shower
. I just agreed to the comfort of visiting toilets and showers on marinas
and gyms when I was in port. In that way the interior saved much space for other compartment such as bigger sleeping area, kitchen, storage
Things I did to improve the living space:
1) In order to get more space in general and get rid of some clothing
from the wardrobes, was to tighten up a net in the ceiling. Between the net and the ceiling I could put jackets, pants, towels etc.
2) In order to get more storage
volume for items and equipment
of different kind, I lashed waterproof barrels on deck
to the mast
3) I rebuilt the ladder in a way so it opened up much more space inside
4) I made a good shelter for the cockpit
so I could use it as a "living room" even these days with rain and shitty weather
(when I lived on it in port)
In such small boat
, if it is equipped with an inboard motor
, one idea could be to remove it, make some bigger interior space from the motor
room and instead get a outboard