Once your child comes of age and enters the teenage phase of their lives, they will have grown out of the things that they liked as a child. Therefore, they might not like their bedroom anymore, especially if it is designed with a younger child in mind.
If this is the case, they might start asking you if they can change their bedroom. This could be an expensive task but it is expected if you want them to feel more like an adult and at home in the house.
If you have said that they can change their bedroom, then you should include them in the design process and if they are at the age where they have a job, you should make them pay for half.
This may sound harsh but if they have paid for elements of their bedroom, they are more likely to look after it. Most of the time, a teens bedroom can look like a bomb has hit it and when you ask them to tidy it, they act like you have just asked them to do this massive task.
If you want them to keep their room tidy and you want them to respect the space, then they will have to learn to value the furniture. The only way to do this really is to make them pay for every bits and piece.
It is important that you do not take over the design process because if you do this, they will feel like you have taken their individuality away. If you let them do the designing themselves and you do not like something, than make a suggestion for an alternative and they might be willing to meet you half way.
They will need an adult's bed now. They might even want a double bed in their room. If they have space, you should let them have a larger bed because it will last them for a long time. After all, at this age, they might still be growing. So, you should get a bed that will last them a long time.
You should not suggest a bunk bed to save space, even if the top bunk is a double. They will associate bunk beds with being a kid and they will want something more grown up.
They only time that you should suggest a bunk bed is if there is a double on the top and a desk area underneath. This is because they will have somewhere to study and they will like having extra space. However, you should plan for a regular double or single bed.
Some parent's do not like their teenagers having televisions in their rooms because they could be distracted from sleep or their studies. However, if your teenager is a gamer, or they like different shows to the rest of the family, you should allow it so they are free to do what they like without disabling the rest of the family. You could have it installed on the wall, to save space.
You should plan to have some form of seating in the room, if there is space. This is because they might want their friends over, or they could need somewhere to sit when they are studying. You can not expect them to sit on their bed all the time.
You could purchase a specific bedroom chair that is similar to a dressing table chair, or you could buy an office chair. If there is space in the room then you could even buy a living room chair.