^That is my understanding also.
The problem is that some parish layouts are such as to make it hard to tell if a particular "space" adjacent to the main altar is, even in the absence of a consecrated altar, an extension of the floor of the main altar or, in your terms, a "side room".
At one parish I attend, a tangible distinction can be made between the floor of the main altar and the floor spaces adjacent to it in that the latter are divided from the former by a step and small shelves. At Communion time, the Priest takes the Eucharist to the boundaries marked by the steps and shelves and the congregation commune from there.
At another parish, while the adjacent floor spaces on either side of the main altar extend far and wide, there is no tangible marker of any boundary line and so the Eucharist is taken outside.