All 16 contestants entering the “Big Brother” house Wednesday night for the Season 19 premiere will find a newly redecorated structure that’s been changed significantly from last season.

It’s also immaculately clean — which won’t last much longer, say “BB” executive producers Rich Meehan and Allison Grodner.

“I’d say by Week 2 the house starts becoming very ‘lived-in,’ ” says Meehan. “[The houseguests] get very comfortable very quickly and their clothes are piled on the floor, on their beds, on their dressers … There are times when we’ve walked into the house and quickly leave. It’s pretty dirty and doesn’t smell the best.”

“It smells like feet,” Grodner adds. “We have a discussion ahead of time about ‘production issues’ or ‘roommate issues’ and cleanliness is definitely a roommate issue, like if someone decides to be a complete slob and pile things up in the sink. We’ve had plenty of stories from people watching the show online about contestants coming out of the toilet and not washing their hands.

“We had a huge incident in Season 3 when Gerry was cooking and went into the bathroom — then came out of the bathroom [without washing his hands] and went back to cooking — and he was hand-tossing a salad. That was noticed by his fellow housemates and a huge fight ensued.”

Arguments are bound to happen on “Big Brother,” in which the contestants (or “houseguests,” in show parlance) live together in the two-story house — which sits in a CBS studio in LA — over a three-month period. They’re tracked by cameras 24/7 as they tackle various challenges, eliminating one (or more) houseguests each week. By show’s end (around mid-September), only three contestants remain; one of those will snare the $500,000 winning payoff.

And if you’ve ever wondered, the contestants clean the “Big Brother” house themselves (or not). “It’s up to the houseguests to do their own cleaning, and some casts are real slobs and some are really neat,” says Grodner. “Inevitably there’s a neatnik in the cast. You start to hear, ‘I’m the only one scrubbing and cleaning.’”

“But that’s interesting because they’re playing a game and no one wants that [complaining] to be the reason they end up being nominated [for eviction] or being evicted,” Meehan says. “Last year, someone tried putting up rules about cleaning — and that person got evicted first.”

“You have to be flexible,” Grodner adds. “The online audience loves to watch who’s a big slob, who leaves a dirty dish, who picks up after everyone else.”

After the finale, the “BB” house is taken apart (its furniture is sold elsewhere).

“It’s a set, so it gets demolished then rebuilt,” Grodner says. “Prior to the [new] houseguests entering, we go in with an industrial cleaning crew and make sure the filth from the past season is gone. It’s like a construction site — it has to be vacuumed out and hosed down.”

“Big Brother” Season premiere 8 p.m. Wednesday on CBS