Older buildings are on "mixed" lease, where it is legal to use the same "flat" or "shop" for BOTH commercial and residential. If this building is in the Hong Kong Island side, from Central to Kennedy Town, then yes, many landlords in the Western district have legally subdivided their buildings by doing the full paperwork of submitting plans , hiring engineers, etc.
The government stopped approving these since 2018; but the older ones before 2017 that were already approved can still be in use.