Open plan is where you live in one huge room. The only room that might be separated by walls would be the bathroom.

Advantages: Space, light, you can see where everything is.

Disadvantages: Living with someone in an open plan room and having no privacy. Too bright in the summer you wake up earlier, too hot as well. In winter it takes a lot to heat up.

The other problem with open plan is that your kitchen is in your living room. If you cook smelly food that will seep into your furniture.

I don't go for it myself, that said I do like people in my kitchen when I cook and that is not possible in a small kitchen.