You discuss it very much whereas I have heard Allah's Messenger (ﷺ) as saying: He who builds a mosque-- and the narrator Bukair said: I think he also said: (for) seeking the pleasure of Allah- Allah would build (a similar house for him in Paradise). and in the narration of Harun (the words are):" A house for him in Paradise."
Hadith Commentary: The Virtue of Building Mosques
This noble hadith from Sahih Muslim (Book of Zuhd and Softening of Hearts, Hadith 533c) reveals the magnificent reward for those who contribute to building houses of worship for Allah's sake.
The Intention: Foundation of Acceptance
The phrase "seeking the pleasure of Allah" indicates that the primary condition for receiving this reward is sincere intention. The act must be purely for Allah's countenance, not for fame, recognition, or worldly benefit.
Scholars emphasize that intention transforms mere construction into an act of worship, making the builder's effort a continuous charity (sadaqah jariyah).
The Reward: Divine Construction in Paradise
Allah's promise to "build a similar house for him in Paradise" demonstrates divine justice and generosity. The reward corresponds to the deed but surpasses it in perfection and eternity.
Imam An-Nawawi comments that this house in Paradise will be of unimaginable beauty, free from imperfections, and filled with eternal blessings that no eye has seen nor ear heard.
Scholarly Insights
Classical scholars note this includes both complete mosque construction and contributing to existing mosques through expansion, renovation, or provision of necessary facilities.
Ibn Rajab al-Hanbali explains that just as the mosque serves as a place for spiritual purification in this world, the heavenly house represents the ultimate purification and nearness to Allah in the hereafter.
The variation in narrations ("a similar house" vs "a house") shows the comprehensiveness of the reward - either matching the earthly mosque's quality or granting a magnificent dwelling regardless.