Construction or alteration of the flat has to be made with the permission of the Society, and approval of modification of the flat by Municipal Authority. This modification has to be done with the help of a Municipality approved architect.

Therefore you may better approach Municipal authorities for the needful with a convincing application.

You can contact the Municipal authorities accompanied by your architect for getting necessary approval for construction of additional bathroom.

These cases are very common in various Courts of India. 
It is crucial here to refer to Section 187 of the Karnataka Municipalities Act, 1964-

Section 187 in Karnataka Municipalities Act, 1964
187. Notice of new buildings.
(1)Before beginning to construct any building, or to alter externally or add to any existing building, or to construct or reconstruct any projecting portion of a building in respect of which the municipal council is empowered by section 181 to enforce a removal or set back, or to construct or reconstruct which the municipal council is empowered by section 179 to give permission, the person intending so to construct, alter, add or reconstruct shall give to the municipal council notice thereof in writing and shall furnish to it at the same time, a plan showing the levels at which the foundation and lowest floor of such building are proposed to be laid, by reference to some level known to the municipal council, and all information required by the bye-laws or demanded by the municipal council regarding the limits, dimension, design, ventilation and materials of the proposed building, and the intended situation and construction of the drains, sewers, privies, water-closets and cesspools, if any, to be used in connection therewith, and the location of the building with reference to any existing or projected streets, and the purpose for which the building will be used.
(2)No construction or reconstruction referred to in sub-section (1) shall be begun unless and until permission for the execution of the work is granted under this section.

Therefore, detailed discussion is required in such cases with complete documents.  You may contact my secretary to connect with me for clarification. 

Prior consent of society / association is necessary to construct additional bathroom 

construction would not be allowed as it is not as per sanctioned plans 

You can approach muncipal corporation for permission based on architect plan 

but muncipal corporation would insist on society written consent before granting permission 

Thanks for your appreciation 

As mentioned earlier muncipal corporation would not grant approval without society / association written consent 

You are welcome for your appreciations.

Once you get an approval from Municipal authorities the society cannot restrict you from proceeding with the proposed construction, if they do so, then you can file a suit for permanent injunction against the society before civil court to restrain them from preventing you to construct the additional bath room.

Dear Client,

In the Present Scenario, being the owner of your 2BHK flat in Bangalore, you have the right to make interior modifications under the Karnataka Ownership Flats Act, 1972, provided they do not compromise the building's structural integrity or common areas. Constructing an additional toilet is permissible if it does not pose a safety risk, and you have already obtained an architect's approval confirming its feasibility. Herein, the Apartment Owners’ Association, under the Karnataka Apartment Ownership Act, 1972, cannot arbitrarily deny permission without substantiated technical reasons. Their concerns regarding load-bearing or water seepage must be validated by an expert, which they have so far failed to arrange. To proceed further, it is suggested that, you send a formal written request to the association, including your architect’s approval and assurances of safety, to appoint their own expert for verification within a reasonable deadline. If they remain unresponsive or deny the request without considering or providing any evidence, you may approach the Bangalore Municipal Corporation or any other independent structural engineer for approval. You may also consider filing a complaint with the Consumer Disputes Redressal Forum for undue inconvenience or financial loss.

Hope you find this answer beneficial for resolving the dispute.

You can have a additional bathroom if it’s not against the sanctioned plan or municipal corporation designs 

- As per law, a society/RWA has no legal authority, to issue moral, ethical and social diktats to its residents.
- Further, Society by law and resolutions must be limited to the mandate of governing the society’s affairs and nothing more.

- You can file an application before the management of the society with the medical certificates for allowing you to construct an extra toilet , after mentioning that it will not affect the construction of the building and will also no create any problem for other owners. 

- If no positive response, then file a suit before the Court. 

Dear Sir,

After getting municipal clearance then get issue a legal notice to the Secretary of the Association if negative reply is received or no reply is received then approach the Civil Court and submit that you are  ready to bear extra maintenance for such water seepage expenses. You will definitely get the orders in your favor.