why no "nuestra?"

Hola William, Marcos and Goo,

This is a very good question. So, to see exactly which sentence William was referring to, it is this one:

As you can see, and this is why I believe William was confused, we use "la casa" but translate it in English as "our house" (not the house). Possession in Spanish is sometimes "obvious" in some sentences and while in English you still use the possessive adjectives "my, your, his, our..." we don't need to use them in Spanish when it is clear who the possessor is. This is why we use the definite article instead: "la casa" (clearly referring to our house, as it is implicit that we re-decorate our own houses, not other people's houses).

This is something that you constantly find when talking about body parts; here is a kwiziq lesson about this topic; but it is somehow extended to other contexts where there is obviousness, like this one. 

It's not wrong to still use the possessive adjective. We can also say: Nosotros redecoramos nuestra casa hace unos meses. It simply sounds a bit redundant , but it is still correct.

I hope this clarified it.

Saludos cordiales a todos y felices fiestas.

Inma