This appears to be a change in the structure and deviation from the approved plan. 

A structural alteration means making changes to any part of the supporting structure of the building including changes to external walls, bearing walls, the foundations or the roof.

Additions or alterations in flat without consent of Appropriate Authorities or Managing Committee are illegal.

If you changing the 'bones' of your house [structural modifications], you may need to consult an engineer depending on your jurisdiction. The engineer can assist you in determining permitting requirements. Most home renovations that involve removing walls do require a permit.

If it’s a common area you need society permission for the same otherwise you can do the same 

You cannot make any changes in outer wall of your apartment 

you cannot remove outer wall of your flat 

You need muncipal corporation consent  for making any changes 

You were explained about the law involved in it.

Now it is upto you to accept the law or not.

Dear Client,

Making any alterations to the outer wall of your apartment is strictly prohibited. The removal of the outer wall of your flat is not permitted under any circumstances. In order to make any changes that involve the outer wall, you must obtain the consent of the municipal corporation.

- Since, there is no society or association , then the norm of building by laws will be applied , and there may be requirement for taking permission of the same from the competent authority like municipal corporation 

- You can submit an application for getting permission before starting construction. 

- However, if the construction of the building will not effected due to said alteration and neighbors having no objection then you can get permission .

Then you can do the repairs unless you don’t violate structural designs