Most user manuals will indicate that using an extension cord for a large appliance such as a washing machine is not advisable. Generally speaking, using an extension cord for your washing machine should be avoided. If it is necessary, however, there are a few things you should look for in a cord to ensure your appliance does not become a fire hazard.
For your appliance to function properly, you need to find an extension cord that has a female end that matches the end of your washing machine cord. Some larger washing machines have unique cord prong orientations to prevent plugging them in to traditional outlets. Never modify a washing machine cord prong.
Ensure that your extension cord is at least as heavy gauge as your washing machine cord. This can be unscientifically tested by simply checking the heft of the extension cord. A thin utility cord, typically orange or yellow, is not suitable for a large appliance such as a washing machine.
You should use the shortest extension cord you can. The longer the cord, the more resistance in the wire and the more likely you are to have a problem with consistent current draw. Consider measuring the length you will need and having a cord made at an electrical supply store. Opt for the heaviest gauge wire they have available.
In your user manual, the manufacturer should indicate how many amps your washing machine draws. Purchase a cord or cable that is rated for this amperage, or more, but never less. A wire that is not rated to handle the load put on it by your unit can overheat and even catch fire.