Construction in chawl

BMC shall issue notice, inspect the premises (if necessary) and hear both the parties before taking any action.

You can move to civil court for restoration of easement rights, if you have evidence that the construction is illegal and lacking proper permission of authorities concerned including BMC.

It will be better to consult a local prudent lawyer for analyses of facts/ documents, professional advise and necessary proceeding.

yes you can ascertain the facts by RTI and obtain the records and act further by all appropriate law

You can pay Rs.10/- as RTI fee and for Rs.2/- per A4 certified copy from BMC and can obtain the approvals and sanctions of any building, and you can also get a copy of the complaint against you by the other party from BMC.  Depending on the facts, wait for a month for BMC action on your complaint.  File another RTI seeking action taken on your complaint/grievance.  Consult a local advocate for proper guidance showing him all the papers.  If the other party's construction is as per approvals/sanctions of BMC, you can not do anything further.  If there are violations, then there may be a scope to file another grievance petition enclosing actual photographs and copy of approvals and pointing out specific construction deviations from the plan or violations of other laws.

Admittedly no notice has been issued. Any suggestion depends upon the Notice, if at all issued.

You are right that you first obtain information about the permission granted to him or not by sending an application seeking this information through RTI act.

Then after confirmation of his illegal construction, you may send complaint about this to the Municipal commissioner or to the local civic authority.

Besides you can file a suit for mandatory injunction  seeking easement rights for free flow air which is now obstructed due to the structure erected by him hindering the same, for the  right to the free passage of light or air.