IC = inspection chamber. all the waste water pipes in your house will lead to an IC. the last IC in the house will be connected to the public sewer. if you have only 1 IC in your house, that will be the last IC as well.
you mentioned your yard is fully sheltered. I believe this could be an illegal addition done by the previous owner. accordingly to regulations, there should not be any structure built within the 2 m setback from the rear boundary. what is allowed is a roof eave or ledge which can be built up to 1m from the rear boundary. your concrete ceiling should be this roof eave/ledge. so anything after this roof eave/ledge is actually not allowed.
Officially, if you want to keep this illegal structure, you can write in to URA to seek permissions to retain this structure since it was erected by the previous owners. if not, you will either need to tear it down or just keep quite about it and hope no one complains about it. else if URA find out, you will be fined.