My question is in regards to najasat. Is it true that in western countries you have to assume that all the floors of the house are najis since someone else was living there before?
So you can not have wet bare feet to walk on such surface as it gets najis, is that a true concept?

Also are you to assume that the whole bathroom floor is also najis? Some people do not use slippers for the bathroom as they have the mats and they step bare feet on other parts of bathroom floor (tiles). Is that considered najis?

Everything is tahir until you know that it has become najis.
This rule is fundamental