Oh, not again... cat pee!
The smell of cat urine is horribly strong, as it serves to mark their territory in the wild and therefore contains substances with a scent that's almost impossible to eliminate. That's why there's nothing worse than opening the front door and discovering with your first breath that, yes, they've done it again somewhere between the welcome mat and your closet. And it's even worse when you go to scrub your floors and the water only brings out all those masked smells yet again...
We can all agree that this is a problem that needs a solution (and fast!), but in order to find a definitive and effective solution, we have to start by understanding why this is happening in the first place. Clearly, the fact that your cat has starting peeing all over the house is a sign that something is wrong, and if we want to change their behavior, we have to seek out the cause: they could be sick, stressed, or uncomfortable with their litter box, for example.
Let's take a look at the most likely causes and some solutions for each of them.