I live outside the balcony on the second floor. Is it legal to install billboards on the sidewalk on the first floor so that I can dismantle the air conditioner?

It must be illegal. He is unreasonable. First of all, the external wall area belongs to the owner of the whole building, not one. For example, the air conditioner hanging upstairs, hanging on your wall, is actually owned by the owner of the whole building. It is equivalent to the default that everyone supports hanging in their own homes. However, people on the first floor are not qualified to say that the owners on the second floor. Although that place is located outside the balcony on the second floor, it does not belong to the owner on the second floor, but it is also the pool of the owner of the whole building. If you want to hang billboards, you need to get the consent of the owner of the whole building. On the second floor of your house, you can also go door to door to find all the owners and see whose position they occupy. If you don't stand with them, let them take down the air conditioner. After all, it's all male.