ASBI Pocket Beit Midrash: Aruch HaShulchan,Laws of Synagogue 150:3-4

The Talmud (Shabbat 11a) teaches "any city whose roofs are taller than it's synagogue will eventually be destroyed as it says 'to raise the house of the Lord our God and to stand up its destruction." And the Tosefta (Megillah, chapter 3) teaches "a synagogue should only be built at the highest point of a city. As it says "at the head of the busy streets she calls." Meaning that the synagogue should be built where the ground level is highest and in that way, even if the building itself isn't very tall, nonetheless, since it is located at a high place, automatically, it is taller than any other building in the city.

The Tur and Shulhan Aruch write that it is only the inhabited portions of buildings that should not be higher than the synagogue but that towers and turrets for decorative purposes can be taller than the synagogue. And this too is in the Talmud there.

And this indicates that the reason for this prohibition is not about the honor of the synagogue alone, that the synagogue should be the tallest structure in the city. For if that were the basis, why would it be permissible for towers or turrets to be taller? Rather, the reason is that someone using the roof of his home should not be above the roof of the synagogue. And regarding this they said "any city whose roofs are taller than the synagogue..." because they had flat roofs and they used them. So, since our roofs are sloped, the Tur and Shulhan Aruch write that sloped roofs that cannot be used we measure from the tallest place that is usable. So if there is an attic under the roof, it should not be taller than the synagogue - this is the language of the Tur and Shulhan Arukh. For this reason, one should not use the attic of the synagogue itself.