I particularly like this house.

I definitely have some lusting over it.
You know, I don't know.
I'm currently in the family stage, and we were really happy at 480 sq ft. We currently have 650 sq ft or so -- which is really more than we need. We have one completely unused room!
We currently use the kitchen, dining, lounge, bathroom, and back room -- just like in our old 480 sq ft cottage. The backroom is playing "closet/everthing" room again. I have a freecycle dresser in there for our clothing. The closet holds the hanging clothes, suitcases (with out of season stuff), pillows and puffy quilts, and vacuum cleaner. my ILs are bringing us some book cases (i've already gotten rid of several books that I'm surprised that I kept). DS's toy trunks are in there (and his books will also go on the book cases). We keep our thai mats in there during the day when not in use for sleeping.
The lounge is the everything room. We've been working in there on our computers, DS brings out his toys. I bring the bed out at night, and during the day, I put it away. Bedding goes into the dresser in the dining room (freecycle) -- sheets and towels in one drawer; blankets in another; kitchen linens in a third, and then some kitchen implements (like my mandolin slicer) in another. I have two empty drawers. We tidy it before we go to bed -- toys away, computers away, clothes set out for the next day, dishes are washed and put away, etc. Basically, it's all sorted.
I'm line drying clothes in the back room -- everything dries so quickly (overnight!). I can just go from rack to dresser/closet very easily this way.
We're looking at making the extra room into DH's office space, or a space where we can do yoga, meditation, work, etc. Not sure yet how that space will work. So far, we're happy not using it. We'll see what happens as we expand outward.