Dog Has Started to Pee in the House?

I have a 16 month old Staffy male dog and I've had him for about 5 month now. He is a great dog and behaved, but he has all of a sudden started peeing in the house. He never had done it before, but now he's peed on my bed, against doors, on floors, rugs etc.

I will take him out to do the toilet, but he then will come in and still pee in the house. My mum has her dog in the house, as well, who is a bitch. My dog has never peed in her bed, but just started to pee in his own! Needing answers please.

By rabsmith