On plan-view and elevation-view sketches of the wall, show the position of the nearest intersecting wall, and note clues to what lies behind the wall.

From outside the house, look for pipes coming through the roof directly above the proposed door opening. Pipes projecting above the roof indicate drainpipes in the wall directly below; you may find water pipes nearby, as well.

Check the rooms above and below the proposed opening, too. Is there a radiator against the wall? If so, there may be a hot-water or steam pipe below. A hot-air register in the wall under the proposed opening may mean there is a heating duct in the wall. Removing the register's grille and reaching into the duct can help you determine where it goes.

On your sketches, mark the locations of receptacles, switches, and wall lights, and then figure out the likely route for wires running between them.

If you have an attic, go into it with a light, and locate the top plate of the wall you propose to open. Measuring from the nearest side wall, mark on your sketches the locations of any wires, pipes, or ducts coming through the top plate. If your house has a basement or crawl space, repeat the process.