In the pictures given, the lower windows will only have two inches or so clearance above them after the floor is added. This will look odd, compared to the others. The upper windows will be within 18" of the new floor and required to be changed to safety glass, as Framer53 brought out. The return air duct vent will need to be re-located. Heat and cooling will need to be supplied to the new room. You also need light and ventilation, which may or may not be met.
The existing interior walls may or may not be able to carry the new load. Unlike Dave's (new construction) someone could size your new joists, thinking they are helping you, yet don't know the rating of your existing walls to carry the added loads. A Structural Engineer would know, not a lumberyard employee, or a DIY'er on a forum. The professional Contractors on this forum will refer you to a S.E., the other DIY'ers will say sure, 2x? at 16"o.c. You may need a concrete foundation wall and footing underneath, rather than the doubled joists you may/not have now. You may need special blocking to make the new floor a diaphram for shear flow as it is seperate from the rest upstairs.
If certain requirements are not met and OK'd by your local building Department, you could be required to tear it out at sale time. Number of bedrooms, square footage, etc. are all recorded on your house's record with the County, for public viewing.
A S.E. will know the bolt size, spacing, length, and placement to safely hold up your new floor at the attachment to the walls. A Contractor would hire a S.E. because no one else can figure it as all houses are different. He also takes responsibility and liability for the addition, not the L.Y. employee. We just had another post similar with many telling you what size joists, but that is the tip of the iceburg. Attachment to the existing framing is just as important, in my opinion.
In many States, the work you do on your house without a permit makes you personally liable for an accident or casuallty years later.
At the very least, notify your Homeowner's Insurance company, see if they will cover the addition without a permit. You will be one-up on your neighbors, and insured if something should happen down the road. Be safe, G