How to Add a Door to an Under-Stair Closet

Using a stud finder, identify the location of wall studs and any overhead joists that might obstruct or be affected by the new doorway. Carefully measure and mark the precise dimensions of the planned opening on the wall surface. Ensure the marked lines are perfectly vertical (plumb) and horizontal (level) using a spirit level. The width should accommodate the door slab plus a small margin for the frame and shims.

Using a drywall saw or a reciprocating saw equipped with a fine-tooth blade, carefully cut through the wall material along the pencil lines previously marked for the door opening. Make precise cuts to avoid overcutting or damaging surrounding wall areas. If the wall is plaster or contains materials other than drywall, use the appropriate saw blade for a clean cut.

Cut two full-height studs (king studs) to frame the sides of the opening. Then, cut two shorter studs (trimmer or jack studs) to support the header. The header, typically a piece of 2x lumber or a doubled-up 2x, will span the width of the opening. Nail the trimmer studs to the king studs, ensuring they are flush with the inside edge of the king studs, and then attach the header across the top of the trimmer studs. Add a sill plate at the bottom if needed. For increased structural integrity, consider adding cripple studs above the header if the opening is significantly shorter than the wall height.

Assemble the door jamb (sides and head) according to the pre-hung door manufacturer's instructions. Insert the assembled jamb into the framed opening, ensuring it is plumb and square. Use shims between the jamb and the framing to achieve a perfect fit and to hold the jamb in place while you secure it. Screw or nail through the jamb and shims into the king studs. Once the jamb is securely installed, hang the door slab onto the jamb hinges.

Measure and cut casing trim pieces to fit around the exterior of the door jamb. Miter the corners for a clean, professional look. Apply a small bead of construction adhesive to the back of the casing if desired, then nail the casing into place along the jamb and the wall studs using finishing nails. Ensure the nails are set slightly below the surface for easy filling.

Follow the manufacturer's instructions to install the doorknob, strike plate, and any other desired hardware on the door. Once the hardware is installed, use wood filler or putty to fill all nail holes in the trim and jamb. Sand any rough spots smooth. Apply primer, followed by your chosen paint or stain, to the door and trim. Allow adequate drying time between coats as per the product instructions.