Living in a 400 square foot house would have its advantages and disadvantages. I believe that it would be a challenge. I am so used to having space to do things and have room to spread out. In a 400 square foot living quarter I would not have the wide-open area that I am used to. I would feel confined in a jail cell in that small of a home. However it does have its advantages.
The advantage of having that small of a house is the space and time one takes to clean. Having that small of a house would ensure less time allotted to cleaning. Being that small the house would only have the absolute essentials the bills would be less than an average persons bills. This is due to the fact that the electric and heating would be less. One could only fit a small amount of food in the kitchen. In all being in a tiny home would be less expensive.
However a disadvantage of being in that small of a home one could not have a large gathering. They would only be able to have a couple of people over. The space could seem claustrophobic and would get to be too small at some point. It also would be hard to put furniture in. One would truly have to reconsider what they wanted in there house and the size needed to fit the space given. It would be a challenge to find things to fit comfortably in the home without making the furniture look cluttered.