Each situation varies, all depends on the positioning of the sinks waste pipe and the other waste pipes going away from your WC. As per other post it needs to be looked at to tell which route to take to join the existing wastes.
Generally in older houses wastes tended to go outside asap before going below ground, but in more modern houses they may well 'join up' and travel within the house and or go underground before leaving the premises.
If you type the key words of your question into our search box to the left of the site you may find the answer is already posted or is in the DIY projects section of the website. Every post goes through a monitoring process and using the search box may speed up your answer.