[On the Zakat of Income-Generating Assets]

Answer—and Allah grants success: The middle opinion regarding the zakat of income-generating assets—such as mills, cars, tractors, houses, shops, and the like—is that the nisāb is reckoned from the yield, which is the rent. If their annual hire and rent reaches two hundred dirhams in the year, then one quarter of one-tenth (2.5%) is due. This is what is mentioned in the marginalia of Sharḥ al-Azhār from the author of al-Ḥāṣir ʿalā Madhhab al-Nāṣir. This view is a position between two positions, for the scholars hold two opinions in this matter:
The first: According to the jurists of the madhhab—following Imām al-Hādī may Allah be pleased with him—they said zakat is obligatory on income-generating assets. Thus, in a rented house and the like, one quarter of one-tenth (2.5%) of its value is, according to them, due when the year completes; the house is appraised at the end of the year, then its owner pays one quarter of one-tenth of its value.
And others outside the madhhab said: There is no zakat at all on income-generating assets.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1