Is it permissible to perform تحية المسجد (two rak'ahs upon entering a mosque) in a small room belonging to it before it opens, and is this prayer sufficient as تحية المسجد after entering it?

We congratulate the questioner on his early arrival at the mosque. If the room he sits in is part of the mosque and designated for prayer, then performing the two rak'ahs of greeting the mosque therein suffices him. However, if it is not designated for prayer, such as being for placing mosque lamps and the like, then it does not take the ruling of a mosque, and greeting the mosque therein is not sufficient.