Normally the room should be above ground unless in a split entry, it should have a closet and its own door so that you dont have to walk through one room to get to another and if you have a septic the septic is based on bedrooms. so it is possible to have a 3br septic design and only 2 bedrooms were built but it would be less ok if you had 4 bedrooms and a 3br septic design. if you have city sewer than of course the septic size doesnt have an affect. Where as you are replacing your leach field it also goes by how many people live in the house. a 2 br design is really for 3 people where a 3 br would be for 4/5 people. The size of your leach field will affect your future value. So trying to skimp now may hurt you later. The person doing the septic design should be able to tell you exactly what you need and assist you with it. Good luck with you project.