The Dutch door did actually originate in Holland. It is an old design, dating back to the 1600s, according to lovetoknow.com. It consists of a bottom and top half that can swing independently. Because the Dutch knew it's not always a good idea to have two independently swinging sections, they added a latch that secures the two halves together, allowing it to swing as one unit. The design came to the United States with Dutch immigrants, and the doors are used as exterior and interior doors in houses as well as in stables. Although it is possible to buy a Dutch door unit, save some expense by making a Dutch door from a regular one.
Use a screwdriver or cordless drill to take the existing door from the hinges. Lay the door on a pair of saw horses and remove all of the hardware. Fill in any holes with wood putty, allow the wood putty to dry completely, then sand.
Place the door in the doorway and mark the position of the hinges with a pencil. Because you essentially will have two doors, you will need to add marks for two other hinges.
Secure the hinges to the door, then install the door to ensure it still swings easily. Adjust the hinges if necessary before continuing. Remove the door from the doorway and place it back on the sawhorses.
Use a ruler or yardstick to draw a line across the door where you wish to cut it in two. If your door has glass in the upper portion, draw the line below the glass. If the door has no glass, draw the line in the middle of the door. Draw another line 1/4 inch below it. Use a circular saw to cut along both of the lines.
Use a router to cut a rabbet (a groove) on the top of the bottom door. Cut a 3/8-inch rabbet that is about 1/4 inch from the top of the bottom door. Create a level of waterproofing by cutting a 1/8-inch aluminum bar so it extends beyond the door on both sides. Secure it in the top of the bottom door with stainless steel screws. Add some caulk beneath the point where the bar extends beyond the door.
Secure a door stop that's 3/8- by-3/8-inch to the bottom of the top door with screws. This will prevent the door from swinging the wrong way. Sand the edges. Apply sealer to the bottom of the top door, then secure weatherstripping to the doors with copper nails.
Hang the doors and close them. Cut a strip of wood that's 1/4- by-1-inch, and 1/4 shorter than the width of the door. Use a router to bevel the top and bottom edges by 10 degrees. Set the flat edge of the strip to the exterior bottom of the top door. Mark its position carefully. The strip of wood will look like a lip at the bottom of the top door, but it should not be too high or too low so it interferes with the bottom door. Center the strip on the door. Once you have marked the position, secure the strip to the door with finish nails and add caulk to any gaps.
Finish the door by painting or staining it. Add a new knob or handle or keep the existing one. Add a latch at the back of the doors to lock them together when you want the door to function as one unit.