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In the series' pilot episode, the show was filmed on the set of Growing Pains.
Answer to above Goof: It was never meant as them being the ones who moved into the Growing Pains house; Mark explains at the beginning that they wanted to see how the series would go first, and did not want to spend money on a set yet, and to just use our imagination to pretend it's not the same house. Later on, the setting is changed without explanation, telling us the new setting was supposed to be the house they lived in the whole time.
Major Continuity Error: How could Mark and his rommmates move into the same house from Growing Pains? Didn't the Seavers live in New York before moving to Washington, D.C. while the setting for Hangin' with Mr. Cooper was set in Oakland, California?