As every property is different, we are unable to offer specific advice on drainage however since the Addroom does not require extensive groundworks, you are unlikely to disrupt existing drains. You may still require access to your drainage system, however. Please consult a local builder should you have any concerns about drainage. Here is an article which may also be of help to you checkatrade article
Please note that we are unable to offer guidance on how to install/make recommendations on modifications of our product beyond what we supply as standard.