If your child is growing fast, or you're a tall adult, you may find a regular-sized twin bed too short for comfort. Most adults, even tall adults, find queen or California king mattresses just fine. However, they want a lot of floor footprint in exchange for a little extra foot room. Fortunately, you can build an extension for your twin bed in an afternoon. These plans will extend your twin bed to longer than a California King. If that's not enough, you can always change the lumber dimensions to suit your needs.
Measure the height of your bed from the top of your mattress to the floor. Press the end of your measuring tape into any carpet to account for its depth.
Measure the height of your cushion or pad.
Cut your 4-inch by 4-inch post into four segments. Each segment should be as long as the height of your bed minus the height of your pad. For example, if you had a 6-inch thick pad and a 30-inch high bed, each segment would be 24 inches long.
Sand all faces of your lumber. They don't have to be polished smooth, but they should be free of snags and splinters.
Arrange the four beams into a rectangle with the 36-inch beams inside the 12-inch beams, all beams laying on their narrowest face. Screw them together with two wood screws per joint, driven through the face of the 12-inch beam and into the end of the 36-inch beam.
Lay the plywood sheet on top of the rectangular frame. Screw it in place with one wood screw per corner. Drive the screws in deep enough that the head is flush with the surface of the plywood to prevent snags.
Set one post on end in the corner formed by two beams on the underside of your frame. Screw it in place by driving two screws through each beam and into the side of the post.
Repeat step seven three times, placing a support post in each of the three remaining corners of your frame.
Set the finished extension frame--which by now should resemble and end table--at the foot of your bed. Lay your pad or cushion on top. Because of the different texture and softness between your mattress and the new pad, it's best to put the extension at the foot of the bed.