My friend had to move in to care for her 93 year old mum who caught Covid during lockdown (presumably from a carer, and not tested because they weren't testing outside hospital then, but all the classic symptoms). Her mum recovered after about 10 days enough to be left alone in her flat again with carer visits, and my friend was absolutely fine throughout. (Sadly, today, over 6 months later, my friend and her husband have tested positive so are self isolating. They were contacted by track and trace having spent time outdoors with friends).
Down my parents' road, two men were hospitalised during the first week of lockdown. One was in for 3 days, the other for a week, but in both cases they have fully recovered and noone else in either house was ill. Although, knowing what we now know, they might have caught it but been asymptomatic, I suppose.
Several cases I know where a parent has had it but their partner and children, all in close physical contact before the diagnosis and unable to distance properly in the house after (only one toilet and bathroom, one living space and no spare room) all stayed apparently fit and well.
It is absolutely not a foregone conclusion that everyone in the house will get it, OP. In fact, I think that may be the exception.