Congregants standing ahead of the imam during prayer
Question
Answer
According to the majority of scholars, it is impermissible for the congregants to stand ahead of the imam. There is no harm if the congregants pray on a higher level in relation to the imam or vice-versa. The important thing for the validity of congregational prayer, is that the congregants do not stand in front of the imam; they have to be in the same place as him to be able to follow his motions by seeing or hearing him or hearing the muballigh [the person who repeats the words of the imam loudly so that everyone in a large congregation can hear him]. However, there are some differences of opinions held by the jurists on this issue.
Based on this and in reference to the question, expanding the mosque in this manner is recommended and considered a pious deed. There is no harm if the ground level of the extension is lower than that of the mosque. Those praying in the extension must be careful not to stand ahead of the imam until the integration is complete and the place becomes a single unit surrounded by a single wall when it will then become possible to move the imam's place ahead of the entire congregation.
Allah Almighty knows best.