It really depends on your house state, I don't see anything that will stop you from making an additional door and wall, but in any case, I suggest you consult with your local construction company about that. They know the local laws pretty well and will provide a good range of options you can use to build a wall. Also, if you plan to set the eavestrough in this room, you should make the upper wall heavier to ensure that your eavestrough doesn't fall. If you are living in Oregon, I can suggest a professional construction company that usually helps me with anything