The Council may have plans of your original house, and may include a drainage layout. It may be possible for the original plans to be extracted from the archives, dependant largely on how old the house is. It cannot be guaranteed that what is shown on any plan is what you will find on site. You will be charged an administration charge for searching for old records.
A better way to determine the drainage layout is to either employ a surveyor to investigate or lift manholes in your garden and do your own survey.
Remember there may be surface water as well as foul drains on your property, you must not connect foul water to a surface water system or vice versa. You will also need to remember that other people may have rights of drainage and therefore use of the sewers passing through your land.
You have a right to see plans deposited for any planning application for your house and these may contain drainage plans.