Permission for basement flat

Builder should contact local municipal authority for said permission. 

According to the NBC (National Building Code), basements are primarily intended for non-residential purposes such as storage, darkrooms, bank cellars, parking areas, or for housing equipment that requires constant air conditioning. 

Using a basement for residential purposes, including building a flat, is generally illegal and unsafe due to factors like improper ventilation and emergency exit issues. 

Securing permits is mandatory before completing any basement construction, especially if the work involves altering the structure or using the space differently than originally approved in the building plans. 

The builder has to get permission from concerned authorities.

What is your concern / locus standi for construction of basement ?