You don't need a timer, save the date (try timestamp [date in seconds]) on the player's file. Then, when they login and it's been over 7 days compared to what is saved on their file, reset their house.
Or if you're using MySQL then it's easy to check with a timer (preferably repeat set to every hour). You just have to again save the LastLogin, and make a query checking if any players haven't logged in for 7 days or more and own a house (also you should add this method when they login to be safe).
Method for MySQL can be used as well if your file saving system is SQLite.