In this case, we have a work that concerns common elements of the Community. So, to perform this kind of Works, the owner have to have the joint owners´ permission. This permission would have to be unanimously by the owners. In other case, the installation would be ilegal.
If the installation is ilegal, it is neccesary to ask the President of the Community to require the neighbour in order that as soon as posible he withdraws the installation of air conditioner of the front of the building. If the neighbour ignores the requirement of the President, the only solution is go to the courts.
It is necessary to do it rapidly, as soon as the installation is detected. In other case, this installation already might not be prohibited. In any case this requirement must be done before fifteen years from the installation, since from the above mentioned term it prescribes the right of the Community to prohibit it. Definitively, the Community must act as soon as possible, since the situation will be irreversible if the Community allows to pass time without saying anything.