Building and furnishing mosques for people to worship in is a good and righteous deed for which the Lawgiver grants a great reward. It is a kind of sadaqah jaariyah which continues to bring reward after one dies.
Allaah says (interpretation of the meaning):
“The mosques of Allaah shall be maintained only by those who believe in Allaah and the Last Day; perform As-Salaah (Iqaamat-as-Salaah), and give Zakaah and fear none but Allaah. It is they who are on true guidance”
And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever builds a mosque, Allaah will build for him something like it in Paradise.” Narrated by al-Bukhaari (450) and Muslim (533) from the hadeeth of ‘Uthmaan (may Allaah be pleased with him).
And Ibn Majaah (738) narrated from Jaabir ibn ‘Abd-Allaah that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever builds a mosque for the sake of Allaah, like a sparrow’s nest or even smaller, Allaah will build for him a house in Paradise.” Classed as saheeh by al-Albaani.
The qataah (translated here as sparrow) is a small bird, and its nest is the place where it settles and lays its eggs. The qataah is singled out for mention here because it does not lay its eggs in a tree or at the top of a mountain, rather it makes its nest on the ground, unlike other birds; hence it is likened to a mosque. See: Hayaat al-Hayawaan by al-Dumayri.
The scholars said: This is mentioned by way of exaggeration, i.e., even if the mosque is as small as that.
There is nothing wrong with building a dome on the mosque if it is for the purpose of letting in light and air, or building minarets so that the muezzin’s voice may reach as far as possible, or so that the mosque may be recognized from a distance and worshippers may come to it, because the means come under the same rulings as the ends. But that should be limited to whatever will serve the purpose, without being extravagant. With regard to minarets, it is sufficient to build them straight and tall, without any decoration or adornment, but if the mosque is built without them, there is nothing wrong with that.
The scholars of the Standing Committee for Issuing Fatwas were asked: Is it permissible to build domes on mosques if that is for the purpose of letting in light and air?
They replied: We do not know of there being anything wrong with that, if the matter is as mentioned in the question. End quote.
It also says (6/254): Some people object to the building of minarets altogether and regard that as contrary to the Sunnah and as a waste of money. Others respond by saying that the minaret has become a symbol by which a mosque is recognized among a mass of other tall buildings which block the view from a distance. The mosque with its towering minarets makes many people feel that the Muslims are still doing well in the face of the many challenges that they are confronted with.
Answer: There is nothing wrong with building minarets on the mosque, rather that is mustahabb because it allows the voice of the muezzin to reach those who are being called to prayer. This is indicated by the fact that Bilaal gave the call to prayer at the time of the Prophet (peace and blessings of Allaah be upon him) from the rooftops of some of the houses that neighboured the mosque, and there is consensus among Muslim scholars on this point. End quote.
You should avoid extravagance in the furnishings, doors and so on, for Allaah does not love those who are extravagant. Al-Bukhaari entitled a chapter in his Saheeh: “Chapter on the building of mosques; Abu Sa’eed said: The roof of the mosque – i.e., the mosque of the Prophet (peace and blessings of Allaah be upon him) – was made of palm branches. ‘Umar ordered that the mosque be rebuilt and he said: Protect the people from rain, but beware of using red or yellow (for adornment) and distracting the people. Anas said: They build mosques about boast about that, but they do not use them for worship except rarely. Ibn ‘Abbaas said: You are going toadorn (mosques) as the Jews and Christians adorn (their places of worship). End quote.
Abu Dawood (448) narrated that Ibn ‘Abbaas said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “I have not been commanded to build lofty mosques.” Ibn ‘Abbaas said: You are going to adorn (mosques) as the Jews and Christians adorn (their places of worship). This hadeeth was classed as saheeh by al-Albaani in Saheeh Abi Dawood.
And Allaah knows best.