Learn Portuguese (Portugal): A vossa casa é bonita.

In European Portuguese, we almost always put a definite article (o or a, meaning "the") before a possessive word. Because casa (house) is a feminine word, we use A before vossa. Literally, it is like saying "The your house".

Does vossa mean there is more than one house, or more than one owner?

It means there is more than one owner. The words vosso and vossa mean "your" when you are talking to two or more people. It ends in -a here simply to match the feminine singular word casa (house). If they owned multiple houses, it would be As vossas casas.

Why do we use é instead of está for "is"?

We use é (from the verb ser) for permanent or inherent characteristics. Describing a house as beautiful is treated as a lasting quality of the house. If you used está (from estar), it would mean the house looks nice temporarily, perhaps because it was just cleaned.

Why is the word for beautiful bonita instead of bonito?