Let \( A \) represent the area of the house in square feet.
According to the information given, the area of the yard is 4,200 ft². The total area of the house plus the yard can be represented as:
\[ A + 4200 \]
The problem states that the total area of the house plus the area of the yard equals 8 times the area of the house alone. This can be expressed mathematically as:
\[ A + 4200 = 8A \]
To summarize, the equation that models the situation is:
\[ A + 4200 = 8A \]