Cut five 4-foot lengths of untreated 4-by-4 lumber using a circular saw. Four will serve as the corner legs; the fifth will serve as the second of two vertical elements of the ladder.
Cut two 2-by-6 boards to equal the width of the loft floor and two 2-by-6 boards to equal the length of the loft floor.
Arrange the 2-by-6 boards on their narrow edges so that they form a rectangular or square shape. Drill two lag screws — one at the top and one at the bottom — through each of the corners of the frame to secure the pieces together.
Measure the distance across the inside of the frame and cut three 2-by-4 boards to that length. Set one 2-by-4 in the center of the frame and drill two lag screws through the sides of the 2-by-6 boards into the ends of the 2-by 4 boards — top and bottom.
Place the remaining 2-by-4 boards inside the frame so that they are evenly spaced on either side of the 2-by-4 in the center of the frame. Secure them in place with lag screws, following the instructions given in Step 5 for securing the 2-by-4 in the center of the frame.
Cut a piece of 3/4-inch-thick medium-density fiberboard to the length and width of the top of the frame. Place the cut MDF on top of the frame. Drill 2-inch wood screws through the edges of the MDF into the edges of the frame.
Set one 4-by-4 — on end — flush inside each of the corners of the frame. Drill two lag screws through the outside of each of the corners of the frame and into the 4-by-4 posts. Position the fifth 4-by-4 post parallel to any of the corner posts so that 18 inches of space separate them.
Determine, based on your child's size and agility, the number of rungs required in the ladder that will give him access to the loft.
Cut as many 26-inch sections of 2-by-4 lumber as necessary to provide the rungs for your child to safely and comfortably climb into the loft.
Secure the rungs across the outside of the two 4-by-4s that are spaced 18 inches apart, using two lag screws on each end of each rung.
Recruit the assistance of at least one other adult to lift the finished structure and turn it top-side-up and position it in the desired location. Choose a corner if space allows to provide two walls for the loft.
Personalize the loft to meet your child's interests. Paint the frame with non-toxic paint in colors that your child likes. Alternatively, paint cardboard to resemble a castle, outer space or anything else your child is interested in and hang them around the perimeter of the frame.
Place pillows and blanket inside the loft. Add a desk so your child can do homework. Set a small mattress, pillows or a sleeping bag underneath the loft to create an adventurous sleeping nook. Extend these efforts 2 or 3 feet beyond the perimeter of the loft to ensure a safe fall zone.