329. Building a Shul

Tefillah u’Birkas Kohanim 10:16

One who is davening with a congregation may not daven before the congregation. If one enters shul and finds the congregation davening the silent Amidah, then if he can start davening and complete before the shaliach tzibbur reaches kedushah, he should daven Shemoneh Esrei. If not, he should wait until the shaliach tzibbur starts reciting the repetition of Shemoneh Esrei, then daven along with him word for word until kedushah. He responds to kedushah with the rest of the congregation and then recites the rest of Shemoneh Esrei on his own.

If one began davening Shemoneh Esreh before the shaliach tzibbur, but the shaliach tzibbur reached kedushah before he finished, he does not interrupt his Shemoneh Esrei to answer kedushah. Similarly, one does not respond “amen, yehei shemei rabba...” while in the middle of Shemoneh Esreh, and one certainly does not answer "amen" to any other brachos.

Tefillah u’Birkas Kohanim 11:1

Any place that has ten Jews must establish a place for them to gather for congregational prayer. This place is called a Beis HaKnesses (synagogue; shul). The residents of a city can compel one another to build a shul and to purchase scrolls of the Torah, the Prophets and the Writings.