Any change in sanctioned building plan of permanent nature requires further permission from the Municipal Authority.

Since making internal staircase would be of permanent nature causing structural changes in both the flats you invariably need permission from BMC in default of which you can be criminally prosecuted along with imposition of penalty.

You can combine the flats only after getting permission from Housing society and BMC.

Please check building plans sanctioned by Municipal Corporation. Under section 347 (c) of the BMC Act, no alteration can be made to a residential building, plans for which have been approved by the corporation. Once a plan is sanctioned and flats built, any change—be it setting up a partition, demolishing walls, merging flats or using a residential apartment flat as a warehouse—require the permission of the civic chief,

Since flats have been constructed as per sanctioned plans no changes can be made without BMC consent.

Yes. It can be done.

You need the consent of cooperative society and BMC for combining the 2 flats

2) you cannot carry on structural alterations in the flats

You will have to seek permission from the Building Proposal Department of the Brihanmumbai Municipal Corporation (BMC) to do this.

This is mandatory in light of section 347C of the BMC Act.

Any change in structure can only be done after taking permission from the BMC and the local authorities.

Since the change in structure will be considered a substantial change therefore you are required to take permission from the authorities.

This is mandatory as per the BMC statute.

Regards

Yes. U may put the building in danger by such act. It must be a shared building. If only you are the building owner then you may consider alterations under architect consultations.

First you need to take the permission from BMC for combining the two adjoining flats. Otherwise Section 347-C will attract towards the matter and you may face the legal consequences.

What you are suggesting is a crime in the eyes of the Law. Any modification sought to be carried out has to be approved by the Planning Authority. As per section 52 of the Maharashtra Regional and Town Planning Act, if such a modification is made without prior approval of the Authority, one can be liable for imprisonment upto 3 years, along with fine, on conviction.