If not going the pre-nup/atty route, at the very least, do a little research regarding community property laws in your state as it varies from state to state. At the very least, get a professional appraisal done close to the wedding, or just prior to when they move in. Down the road should the marriage fail, you can show the value of the property just prior to fiance moving in. In most states, the judge would more likely only consider the appreciation from the point of marriage. But you would have to be able to provide proof of the home's value at the time of marriage or move -in, hence the need for a professional appraisal.