Re courses after receiving notice to remove fixtures/attachments

We are resident on the second floor of an apartment and we constructed a washroom in the setback area and we covered the open are provided for ventilation with iron bars and fiber sheet so that rodents couldn't enter.when we constructed no legal or written objection was done by the neighbours or builder as there was no building behind our apartment. Now that there is construction of a building going on behind and they have not left the desired amount of gap between our and their building. The neighbours living below us have given us a notice to remove the iron structure,fiber sheet and the washroom because their ventilation is blocking. I don't want to remove the said things and they are threatening to initiate legal proceedings against us. We have not received any notice from the builder. What should we do?