It may not be written in will, if income from genuine source has been used to build such house then it will get reflected in your mothers account. If it can be proved that your mother used her money to build such house then it wont be necessary to prove that it has been built by you. Further if you have any other documentary evidence say electricity connection in that house then it would be pretty clear that house existed prior to to transfer also.
In short it is not a matter of worry, there are many ways in which existence of that house prior to transfer through will can be established.