I have a case here we are planning to construct an additional bedroom with toilet in the second floor.The real problem is that my contractor is not sure whether the current drainage pipe size can handly one more toilet.We currently have a 110mm dia pipe laid below ground and the new toilet we are planning for a bath tub,wash basin and WC.

I would suggest consulting a plumbing engineer in regards to the proper pipe sizing for your plumbing situation. If you do not have access to an engineer, I would atleast consult the local plumber.

Hi after checking with the local experts here i got a reply telling that i should definetly increase the size of the pipe below ground for safetly reason.As well as if there is a problem in changing the existing pipe then we can have other solutions.What ever the case it is very much preferable to connect the second floor's drainage line seperatly to the septic tank or the municipal drainage line by which you can avoid creating more problem to the existing system:)

I too feel that you connect the additional toilet exclusively to the septic tank or the muncipal drainage main which ever is there.But one important thing should be taken care that you dont forget a individual vent and rodding point for this as if in case this is blocked you have to have the choice of cleaing

From my point of view it would be better option that you should connect the second floor's drainage pipe to the main municipal drainage line or septic tank rather than increasing size of the existing pipe.That will also be better in monitoring of cleansing and proper flow of garbages.