My one year old dog that is house trained is all of a sudden peeing on the floor. She has doggy doors to get outside whenever she wants. She had not done this in over 8 months and seems to do it every day for the last week now. Someone is home with her almost always. What could be causing this?

My adult dog wishes she could pee in the house simply because she doesn't like the cold outside. I have to keep an eye on her (she gives me a funny look). Is it winter where you are?

Have you taken her to a vet to rule out a bladder infection or other medical concerns that may be causing this?

Joypulv does make a good point, I have a male who refuses to go outside in winter. He waits until I am not looking and pees in a corner.
IF that is the situation and it gets extremely cold where you live you might want to look into puppy pads. They are the plastic-backed cloth that many people train their dogs to pee on inside.
A warning with those though... they can backfire and make the problem worse, some dogs do get confused and after using a pad inside think it's a free pass to pee anywhere inside. Most are smart enough to figure out they are only to go on the pad though.

And of course, Altenweg makes a good point as well, any sudden changes in behaviour should always be seen to by a vet first.