"That will be your own room after all!"
Translation:Das wird doch dein eigenes Zimmer!
“wird sein” is grammatically correct. Germans just don’t use it because a) the use of future tense in German is rather optional, not required, and b) the logic of the situation. We know in the moment we are talking that at some point in the near future it will switch to being your room and from then on it is. While English looks into the future and states what is the case then, German stays in the present and states what is just now happening or being planned. I hope that makes it clearer.