The mosque has four middle-sized corner rooms, similar to the Yıldırım Beyazid Mosque built two decades earlier. This means that the Yeşil Cami too was a multi-functional building, combining prayer house and ‘zaviye’ (dervish lodge, sometimes referred to as: ‘misafirhane’/guest house or also ‘tabhane’/hospice).

On the picture: A wall with rays of niches, and a fireplace in the middle, in one of the northern corner rooms, which can be entered by narrow doors at the foot of the arch that separates the central space from the mihrab area.
The niches were used to store implements and books.

Regarding early-Ottoman ‘multi-functional mosques’ .
The T-form ground plan of the congregation hall of early-Ottoman mosques (the so-called ‘Bursa-type’ mosques) is derived from the late-Seljuk medrese-with-covered-courtyard, with three large iwans (half-open rooms) opening upon a central space. In the older Bursa mosques, these four spaces are still distinct from each other: their floors are not on the same level.
As the first Ottoman rulers (at least up to their conquest of Constantinople) had strong ties with various Sufi Orders, it is clear that the first Bursa-type mosques were designed as multi-functional buildings to meet the needs of these brotherhoods: a place to worship (the ‘mihrab’-room), but also to teach and debate (the ‘iwan’-rooms on the left and right) and even to stay overnight (in general two or four rooms, in the corners of the building). The central space of the prayer hall acts as a ‘public square’, connecting all areas with each other.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler’ - Vakıflar Genel Müdürlüğü III, Ankara 1983
& ‘Encyclopedia of the Ottoman Empire’ - Gábor Ágoston & ‎Bruce Alan Masters, 2009.