It's a good plan, buy a house when the price of bitcoin goes high. So when the bitcoins price goes down you don't care about that and your nominal price still same. Like example on 4 days ago, when the price of bitcoin at the top price around $1050+ you use all our coins buy a house and when the price goes down like 3-4 days ago, you still fine, because you already use the bitcoin for buy a house.
One you buy a house, you already made your decision to cash out your bitcoins into fiat, that means your money will not grow anymore. It's actually depend on you plan, if you need a house then go ahead, it's your profit anyway and as an investor we also need to reward ourselves.
On the other hand, if buying a house is due to investment, then always consider which one would bring good return in the future, is it investing in bitcoin or your house?