Windowsill

The window of my bedroom is set inside the wall of the house giving me a concrete window sill 20 cms. wide. I have installed a ,11x 8 cms., key box after paying a 200euro bill after locking myself out. The president of my community has advised me that I am defacing the fabric of the building and the box must be removed by me or it will be remove by someone else. Is my window sill part of the fabric of the building or is it part of my apartment. In the uk I won a case against an upstsairs owner with a leaking door sill which was inset in the wall and not cinsidered to be part of the building but I dont know if spanish law will see it the same way.Thanks.