If it is there on the day you fill out and file the FAFSA, it is going to be an asset. You could ask for an override as a ‘one time thing’ and a school might grant it if, for example, you are closing on a home soon. However, the easiest thing to do would be to buy the new house before you file FAFSA.
Will this matter? Are your income and other as sets low enough that your children would qualify for FA like Pell grants?