Shulchan Arukh, Orach Chayim 151/שולחן ערוך·אורח חיים קנ״א

1The Laws of the Holiness of a Synagogue (containing 12 S'ifim): In a synagogue or house of study, one may not behave frivolously; for example: laughter, sarcasm, or idle conversation. One should not eat or drink in them, nor should one groom oneself in them or strolling around in them. One should not enter them in summer due to [seeking shelter from] the sun, nor in the rainy season due to [seeking shelter from] the rain. Torah scholars and their students are permitted to eat and drink in them out of necessity. (And there are those who say that in a house of study, it is permissible even when it is not out of necessity) (Ran, [on] chapter "B'nai Ha-ir" [i.e. Talmud Megillah, Chapter 4]). No monetary calculations should be made there unless they are for the purpose of a mitzvah; for example, collecting money for tz'dakah, or for the redemption of captives. And we do not deliver eulogies in them unless would be for a eulogy for one of the prominent members of the city, for all the people assemble and come to eulogize [such a person]. If one must enter there for one's own needs; for example, to call someone - one should enter and read a little or say a word of [Torah], and after that, call [the person] so that it should not appear as if one has entered for one's own needs. And if one doesn't know how to read or repeat [e.g., a mishnah], one should say to one of the children: "recite a verse that you are learning for me", or one should delay there for a bit, and [only] after that, leave, because sitting among them is a mitzvah, as it says "praiseworthy are those who dwell in Your house". (And the amount of sitting time corresponds to [the time is takes for] walking [the width of] two doorways.) (According to the opinion of "those that explain" in siman 90 at the end of s'if 20) (Mordechai at the beginning of chapter "HaShut'fin" [Bava Batra, Chapter 1]).

2There are some who say that what we taught about the sanctity of houses of study is intended to speak about public ones which are similar to a synagogue, but an individual who establishes [a room for] Torah study in one's home for one's [own] needs, it doesn't have so much sanctity.

3We don't sleep in a synagogue, even for impromptu sleep, but in a house of study, it is permitted.

4For the needs of the synagogue, it is permitted to eat and sleep inside it. For this reason, they sleep in a synagogue the night of Yom Kippur. And even for the needs of another mitzvah; for example, when they gather to intercalate the year in the synagogue, it is permitted to eat there.

5If the synagogue has two entrances, one should not enter through this [i.e., the first] entrance to make a path to leave via the second entrance to shorten one's route. If there was a path passing there before they built the synagogue, it is permitted. Similarly, if one didn't enter it initially in order to shorten one's route, it is permitted to make it a path. And when one enters it to pray, it is permitted for the one who entered by this [i.e., the first] entrance to leave via another entrance.

6It is permitted to enter the synagogue with one's staff, traveling bag, or afunda (meaning, kinds of money-pouches/bags that are translated as yalkut and tarmil (specific types of bag)). There are some who forbid entering it with a long knife or with an uncovered head.

7One may spit in it [i.e., the synagogue] as long as one rubs it out with one’s feet or if there are reeds/straw there [on the floor] that if one spits into them, it will not be visible.

8Mud that is on one's feet, it is appropriate to wipe it off when one enters to pray. And it is appropriate that there not be any dirt on one or on one’s clothing.

9We practice treating them [i.e., synagogues] with honor: to "chab'dan" and to "rav'tzan". (the meaning of "chibood" is to clean/sweep it. "Ribootz" is to sprinkle water on the ground). And we practice lighting candles in them to honor them.

10Even after they [i.e., synagogues] have been destroyed, they remain in their [state of] holiness. And just as you practice honoring them when they were inhabited, so we practice when they are destroyed, except for [the aforementioned] sweeping and sprinkling water. If weeds grow in them, we pull them out and leave them in their place, for the sake of weigh down the soul - so that the people will see, and it will awaken their soul and they will try to rebuild it.

11If at the time of building the synagogue, they made a condition to be able to use it [for mundane purposes], it is permitted to use it after it is destroyed, but while it is in use, the condition is ineffective. Even when it is destroyed, to use it for a disgraceful purpose, like planting or financial calculations for the people, the condition is ineffective. What are we dealing with? With synagogues outside of Israel; but with synagogues in Israel, no condition is effective.

12One should be careful not to use the floor above the synagogue for a regular use that is disrespectful. For instance, sleeping there. It is unclear if other uses are permissible or not. Rem"a: All of this is specifically regarding a permanent synagogue that was built originally like that, but a house that was designated to be a synagogue after it was built, it is permitted to sleep above it (Piskei Mahar"i).