Conditions for Building a Mosque

What are conditions of building a mosque within the Zaydi tradition? Are there any rulings in the financial aspect? 

as-salamu alaikum,

The rulings on building a Masjid are generally discussed under the rubric of Waqf in classical Fiqh books, as a mosque is inherently a dedicated endowment.
Consequently, the legal rules regarding its construction, maintenance, and reconstruction are largely tied to the principles of Waqf.
Every Masjid is considered a Waqf; it is not the personal property of any individual.
Its construction, repair, and use are governed by the rights and obligations of the Waqf, including the stipulations of the endower, the responsibility of those who rebuild it, and the management of its resources.
This is reflected in the sources like this:

• Chapter on repairing the mosque: any Muslim may rebuild a collapsed mosque from their own wealth, but if using mosque funds, official authority (wilāyah) is required (“فإن كان من مال المسجد فلا بد من الولاية”).
• Chapter on actions that make a Waqf valid: a mosque becomes a Waqf when it is dedicated, even without explicit wording, and its endowments and benefits are tied to the mosque, not to the individual (“عاد لكل من الواقفين للمسجد وأوقافه… عليهم لا ملكاً”).

Thus, the construction, dedication, and administration of a mosque are inseparable from the rules of Waqf.

In the translation of mukhtasar al-mufid:

Endowment (Al-Waqf)Al-Waqf is one of the most virtuous means of drawing closer to Allah. The pious predecessors, may Allah have mercy on them, dedicated the best of their wealth to seek nearness to Allah — particularly through the construction and maintenance of mosques. As the Prophet Muhammad (peace and blessings be upon him and his progeny) said:"Whoever builds a mosque, even if it is as small as the nest of a small bird, Allah will build for him a house in Paradise."The principal of the endowment, along with its branches, may not be used for commercial purposes. It is the property of Allah, the Exalted, and therefore cannot be sold, transferred, or traded. Whoever is responsible for managing the endowment must spend its profits and revenues according to the legal purposes specified by the endower.The manager is obligated to act justly and wisely in handling the endowment. For instance, if the market price of crops or rental property rises due to increased demand, it is impermissible to sell or rent them at the previous price. Doing so without accounting for current market conditions constitutes mismanagement and is considered invalid and forbidden, as it violates the principles of proper stewardship of the Waqf.

wa salam