Unlike later entries, the original Animal Crossing on GameCube has no fixed "final" upgrade. You can keep expanding as long as you have Bells—and the mortgage payments are substantial.
Each step requires paying off the current mortgage, then speaking to Tom Nook to start the next. The final mortgage totals ~700,000–800,000 Bells across all upgrades.
Once you reach the maximum layout, you can still:
But no further rooms are added. The journey to the largest house is a marathon of fishing, bug catching, fossil hunting, and turnip trading—but the satisfaction of that final payment is uniquely Animal Crossing.