A Villa is the word used to describe houses which are single family homes and can be used year round. They are free-standing, i.e. they are on an own patch of land.
While "villa" used to describe a rather big building for affluent citizens (like in other languages), that is not the case anymore. Today, a villa can be a very small building.
It is distinguished from a row house (Radhus) not only by the architectural requirement to be separated from other building. There is also actual ownership of the complex, i.e. there is no cooperative and no monthly fee. However, you have to pay a fee to the commune based on the taxation value of the house.
It is also allowed to rent out the house.