Check on an appraisers forum to see if there would be any difference in value. Unless you are getting cash for the sale or a large down payment from the buyer the appraisal will be the determining factor if you actually get more at the sale. If you do this change, do it for yourself and not for resale. If you do this change, make sure you don't make the current floorplan dysfunctional as that will take value away. If you are using the exact same square footage and not adding any, then it is doubtful that your home would add value but you would need to get twin sales in the area to see how much, if anything, is added to the value. It may open up the buyer pool and sell more quickly though. As the others mentioned consulting with a couple of Realtors in your area would help you get a handle on the current market.

We remodeled a classic old two story 1777 center chimney colonial, pretty sure it is 40 X 30, 2,400 square feet, not counting small 3rd floor bedroom and a little attic space under the roof. Plain rectangular shape, no frills outside. Quite open downstairs with kitchen, pantry or front closet under the stairs, dining room, good sized TV room for DH’s ginormous TV, living room with view, fireplace, downstairs bath with shower, constructed not metal circular staircase to basement, mud room area with bench, shelves and good sized closet at side, most used door, three bedrooms and two full baths on second floor, including large master and master bath and a fourth bedroom and pull down attic space under the roof, vaulted ceiling over the stairs. It is mind boggling to imagine what you have the opportunity to do without the constraints of pre-existing infrastructure and a good architect.

(not a pro) When we did a big reno, we added a 2nd hall bath on the 2nd floor (so we have 2 halls baths and the master bath on that floor, serving 4 bedrooms.) I have one girl and one boy and when they were in middle and high school, it was very helpful that they each had their own bathroom, for expediency getting ready at the same time and for storage/privacy (they have different "stuff".) (They also learned how to clean their own bathrooms.) Also, I'm going on the market soon and having the second hall bath definitely helps differentiate my house from houses with only 1 hall bath upstairs. (I also have a full bath with guest room in the lower level for the occasional guest.)

Yes, septic systems are rated by the number of bedrooms, not number of bathrooms or number of people. That's how it is, you can't change that. The size of the septic tank is only one factor, the size of your leach field is the other factor and depending on your site and soil, it is the more expensive factor to upgrade, if they will allow you to do that. Your building site and size may play a role here. I can't see how you can get a permit for your new bedroom unless you make enough changes to one of your old bedrooms so it no longer meets your local definition of a bedroom, and I don't mean just moving in office furniture. I don't know where these people live who reply that it's not a problem, it would be a very big problem here on Cape Cod if you tried that. Talk to your building inspector for the most accurate information.