"Matti, you have a beautiful home."
Translation:Matti, sinulla on kaunis koti.
I guess the suggestion is that it is spoken directly to Matti, who is one person, so it is singular you. But I'd say plural is equally valid and reported it myself. For example if you mean Matti's family, or Matti and his spouse, or any other people he might be sharing the home with.
Koti is a home and talo is a house. A source of confusion may the fact that some languages treat the concept of a house and a home as basically the same thing. Correct me if I'm wrong, but it seems that for example in Spanish, "la casa" refers to both a home and a house. While these are obviously not mutually exclusive things, they are also not the same thing in Finnish nor English. A house refers to a specific type of building, whereas a home is the place where someone lives. The latter doesn't even have to be any kind of building, since it could also be an RV, for instance.