Hola Slim

When we use estar with casa, if you're referring to your own house you normally drop the article: 

 

Estoy en casa.

The same way you would naturally say in English "I'm home". 

Even if it is not your house but it is the house of a person and you simple state that we still tend to drop it:

Estamos en casa de la abuela.

We are at granny's house.

Está en casa de Alfredo.

She is at Alfredo's house.

When we do need to use the article, talking about being at some house or going to a certain house, is when we are a bit more specific and give extra information about that house. For example:

Fuimos a la casa blanca que está a la salida del pueblo.

We went to the white house that is at the end of town. 

Saludos