Be sure the septic system can handle additional waste and also make sure there is efficient layout to minimize the amount of plumbing needed.

Thanks Dytecture. Are there any specifications for septic tank size vs. no. of bathrooms provided some place?

Your local county health department is usually the one that gets involved first with additions of bedrooms and baths to homes that have a septic system. They will tell you what the requirements are for the size of the
tank(s) and they will also most likely require you to add additional drainage lines and increase the size of the repair field. The requirements can be different from state to state and even from county to county within a state. There will be requirements that the new addition remain X number of feet away from the septic tank, from the drain fields and from a well if you have one.