How to Build an Interior Room

Determine the width and length of the additional interior room. Use a stud finder to locate the studs behind the existing walls where the new walls will adjoin.

Measure two sections of 2-by-4-inch lumber to the length of the additional room and two sections to the width, using a measuring tape. Cut the sections to length with a circular saw and miter one end of each section at a 45-degree angle with a miter saw. These sections form the top and bottom frame plates of the walls.

Cut a 4-inch-wide section of carpet and padding from the floor along the lines where the base plates will sit. To cut the carpet, use a utility knife and a metal straightedge as a guide.

Lay the bottom plates on the subfloor along the paths you cut from the carpet. The mitered ends should meet at the corner of the new walls. Using a drill equipped with a screwdriver bit, attach the bottom plates to the subfloor with 4-inch wood screws at 4-inch intervals.

Mark a vertical line from the end of each bottom plate to the ceiling on each wall. Use these marks to align the top plates on the ceiling with the bottom plates. Attach the top plates to the ceiling with 4-inch wood screws at 4-inch intervals.

Measure the interior distance between the plates. Cut three sections of 2-by-4-inch lumber to this length and stand one section against each adjoining wall between the top and bottom plates to form the end and corner studs. Check the studs with a carpenter's level to ensure they are vertical. Attach these studs to the adjoining wall studs with 4-inch wood screws. Stand the remaining stud at the corner of the new walls between the top and bottom plates. Drive 4-inch wood screws through the studs into the top and bottom plates at 45-degree angles.

Determine the location of the door. Cut out a 36-inch-wide section of the bottom plate at the door's location with a circular saw. Cut two sections of 2-by-4-inch lumber to the distance between the top and bottom plates to form the door studs. Stand one door stud on the bottom plate at each side of the doorway, and attach it to the top and bottom plate with 4-inch wood screws driven at 45-degree angles.

Cut enough 2-by-4-inch studs to place between the end studs and the corner stud at 16-inch intervals. Stand the studs at 16-inch intervals between the top and bottom plates and attach them with 4-inch wood screws driven at 45-degree angles.

Cut a section of 2-by-4-inch lumber equal to the width of the doorway. Install this section about 7 feet high between the door studs with 4-inch wood screws.

Install 4-by-8-foot sheets of drywall on both sides of the wall frame with 3-inch drywall screws. Using a straightedge as a guide, cut the drywall sections as necessary to fit the door frame and corners with a utility knife.

Tape the drywall seams and corners with drywall tape. Apply drywall compound to the tape edges and screw heads with a putty knife. Sand the drywall with a palm sander when the compound dries.