You don't own the house until it's paid off.
Here's an example:
Let's say today you have $100,000 cash in your bank account and no other assets to your name.
Your net worth is $100,000.
--
Now let's say that you buy a house tomorrow for $500,000. You put 20% down ($100,000) and secure a loan from the bank for the final $400,000. Here's your new net worth:
Asset: House $500,000
Debt: Loan ($400,000)
--
Total: $100,000