What is the legal description for building a mosque, and are the minaret and dome essential given their high costs? What is the Sharia ruling on extravagance in mosque furnishings, such as various types of marble, doors, carpets, and lighting?

Building mosques is an act of piety and goodness that accrues immense reward; it is an ongoing charity (Sadaqah Jariyah), and many texts have been narrated regarding its virtue. It is permissible to build a dome on the mosque for lighting and ventilation, and a minaret to broadcast the muezzin's call and to identify the mosque, provided that it is limited to what fulfills the purpose without extravagance or excessive ornamentation. Umar ibn al-Khattab commanded that the mosque be roofed and forbade its ornamentation so as not to cause temptation among the people.