ASBI Pocket Beit Midrash: Aruch HaShulhan, Laws of Synagogue 151:7

There are those who say that which we taught concerning the sanctity of batei midrash is only when they are communal, akin to a synagogue. However, if an individual sets aside a space for his own need to learn there, this space is not endowed with the sanctity of a beit midrash. And there is no doubt about this for the beginning of its building was for a generic home. And even now that a room has been dedicated for study, there was no intention to sanctify the room with the sanctity of a beit midrash. And there is no need for an explicit condition to have been articulated at the time of building for in the absence of an alternative the general assumption prevails.