Oh, I get it !
Hypo's point on wording is right. Rather depends what you want. If you have no intention of leaving anything to your parents, fine, but if you put "I leave my parent's house 23 Green Street to my son Jack" but they sold the before they had died or didn't leave it to you, Jack wouldn't get it or the proceeds.
Best, if you want to do it cheaply, is to use a good online service, or a will form from a law stationer or W H Smith, that comes with full notes of possible things you might wish, and things to avoid ,as well as pointing out matters which you might not have thought of about.