Obviously many fake aspects to the set (and show)

Here's a few well know ones

Astro turf lawn
sliding glass door with no glass
occasional cars without windshield glass

pile on LOL

No toilet in the bathroom.

The soundstage back yard with the astroturf is probably the best-known fake set on BB. Some single camera shows use location shots for outdoor scenes. They probably decided early on that there were going to be too many scenes in the yard to do location shooting.

On the plus side the interior sets (other than the kids' bedroom setup) are really first rate.

Painted background of trees & houses