First you must have a plan drawn up by an architecture. Now have a legitimate company dig out, and pour a foundation for your new room additon. In some places and States where you can’t dig you may have a slab, concrete floor poured. Frame out the floor joists on your foundation. Apply a 3/4 in plywood flooring to framed floor joists. Using number 8 ringshank nails.
Frame walls on floor and make sure to set walls in enough to accommodate exterior siding or brick. Frame all walls with rough openings for windows and doors. Then either use prefabbed rafters to install roof, or you may be able to stick build the roof. Using 2×6″s for the rafters and a beam going down the center.
After attaching the roof on to the house. You then must sheet the roof and wall framing with half inch plywood. Next have the roof shingled by a roofing company or if you feel like you can tackle the project,procced onward,. Next Install all exterior doors and windows. Finish soffits and brick or side the addition with vinyl siding to match the rest of your house.
Now that the exterior of the house has been finished. You can begin wiring the walls. Then the walls will need to be insulated. Then hang the drywall. Have the drywall finished by a professional finisher. Install baseboard and trim molding throughout interior of the room. Paint or have a professional paint it if it’s to large for you to handle. Finish wiring electric outlets and switches. Carpet or tile floor. Enjoy your new living space.