Chihuahua training

Question
I have a one year old chihuaha that I got at 6mo old.  He was potty trained on paper/pee pads.  I initially kept him in a baby pen with pee pads when I was not at home and then switched to pee pads on the floor.  He knows to go on the pee pads when I'm not home but will also go outside when I take him.  He sleeps through the night without pottying and I take him out first thing in the morning.

I recently moved into a house with a roommate who has a westie.  Hershey has now learned how to get up and down the stairs and for the last two days has poopied on my roommates hallway rug.  Despite having a daily dogwalker (they say he rarely goes at all with them).

Other than now putting a pad on the rug, is there anything else I should do.  I don't like the idea of putting him in his crate from 7:45 am until the dog walkers come at 2-3 but my roommate is obviously not happy with him pooping on her rug.  I've also noticed that if I've taken the pad up and haven't replaced it quick enough, he will go where the pad was, so why all of a sudden he's going on this rug I do not know.

He does like his crate and will sleep in there and bring toys in there but he sleeps with me at night.

Answer
I think crating him is the way to go right now,he has to learn he can not make accidents in the house,and many dogs are crated for up to 12 hours,so I think he will be fine-and try that for about three weeks then you can let him stay in a room,and if the accidents start again,back in the crate he goes full time

You can just crate him in the day if he is fine at night