My daughter decided she wanted a trundle to store things for her doll (and in case her doll wants to have a sleepover). The regular plan doesn't leave enough space under the bed to allow for a trundle, so I modified it by making two headboards and making the bed a little longer. The Trundle is a simple box with ball bearing casters on the bottom.
2×2 (4 feet total)
2 – 1x3s 8 ft. long
2 – 1×2 8 ft. long
1 -- 1x4 6 ft. long
scrap 1/4″ plywood/beadboard/cardboard
5/8" Roller Ball Bearings ($.98 each at Menards)
4 -- 2x2 @ 11" (Legs)
10 – 1×3 @ 6″ (Panels)
2 – 1×3 @ 16 1/2″ (Top of Headboard and Footboard)
2 -- 1x3 @ 21 1/2" (Siderails)
2 – 1×2 @ 12 1/2″ (Bottom of Panels)
2 – 1×2 @ 15 1/2″ (Top of Panels)
2 – 1×2 @ 14″ (Bed Frame)
2 -- 1x4 @ 12 1/2" (Trundle Top and Bottom)
2 -- 1x4 @ 20 1/2" (Trundle Sides)
1 -- 21 1/2" x 14"
1 -- 20 1/2" x 14"
Please read through the entire plan and all comments before beginning this project. It is also advisable to review the Getting Started Section. Take all necessary precautions to build safely and smartly. Work on a clean level surface, free of imperfections or debris. Always use straight boards. Check for square after each step. Always predrill holes before attaching with screws. Use glue with finish nails for a stronger hold. Wipe excess glue off bare wood for stained projects, as dried glue will not take stain. Be safe, have fun, and ask for help if you need it. Good luck!
Build the bed with the instructions from the original bed plan (http://ana-white.com/2010/11/another-farmhouse-bed.html)except you will build TWO HEADBOARDS and the bed frame will be a little longer. Attach the bed frame ~4 3/4" up from the bottom of each leg (measure to double check as this measurement may vary if you use something different than 5/8" roller ball bearings and 1/4" plywood--you may even want to build the trundle before doing this step to make sure of your fit).
Then build the Trundle by making a box similar to the way you made the bed frame only with (4) 1x4 pieces. Nail the 1/4" plywood on the bottom of the box all the way around. Then nail the ball bearings on each corner.
This plan makes a trundle that hides under the bed slightly if centered. You could make the top/bottom trundle pieces 14" and the 1/4" plywood 20 1/2" x 15 1/2" if you want the trundle to be the same size as the bed.
It is always recommended to apply a test coat on a hidden area or scrap piece to ensure color evenness and adhesion. Use primer or wood conditioner as needed.