He should have have a bathroom...
There's Willie, working all day, and Julia, she's at your house quite alot. Come on Barnabas, give them a break.
I second TERRY308's practical (yet, extremely all-important) suggestion. You know, it's one thing if Barnabas is currently one of the "living dead," schnoozing comfortably (and "undisturbed") in that coffin all day. However, during the not infrequent times that Mr. B. is "mortal," well... he must hate like hell to have to get up in the middle of the night to observe nature's call, schlepping out to that outhouse behind the Old House.
And, let's face it, neither the mortal Barnabas nor Willie Loomis are getting any younger. So, their nocturnal "trips" are undoubtedly going to become more and more frequent with the passage of the years, IMHO.
Bao the Bartender, who says, that you can never have enough "johns" in the morning (or, for that matter, in the middle of the night!).