En casa or en la casa?

Question

Previously, I have only come across en casa without the defnite article.

Answer

Hola Emanuel
Both "en la casa" and "en casa" are correct. Depending on what you want to say more specifically we will use one or the other. For example:

Estoy en casa. 
(I am home)

When you want to say you are home (at your own house) we don't normally use the article "la".

Estoy en (la) casa de María.
 (I am at María's house)

When you want to say you are at someone's house the article is optional.

Estamos en la casa blanca, al final del pueblo.
(We are in the white house, at the edge of the village.)

Here, when we are more specific about the house, so we are talking about a house with a certain quality, (la casa blanca...) we need the article. We cannot omit it.
I hope these different examples help,
Saludos
Inma