You could borrow against your BTC to pay for the house, or at least a down payment. You can also do the same for the mortgage payments. But ideally if you have mortgage payments, you have income you can use to pay. And ideally, the price of BTC goes up enough over time that paying back the loan is "free" (it could also go the other way and you get liquidated if you cannot maintain your collateral ratio, so be careful with that). There is also the option of selling BTC for fiat to buy the house / make the down payment, which likely has tax implications and of course you lose exposure to BTC price movements. Good luck and happy house hunting!