what does "indulgent " here mean? how could a house be "indulgent?"
Through the chilly curtain of sleet, in the inter-mittent wash of the great light on the jutting cliff to the south, the massive silhouette of Bluff House loomed over Whiskey Beach. It faced the cold, turbulent Atlantic like a challenge.
I will last as long as you.
Standing three sturdy and indulgent stories above the rough and rugged coast, it watched the roll and slap of waves through the dark eyes of windows, as it had—in one incarnation or another—for more than three centuries.

Question

what does "indulgent " here mean? how could a house be "indulgent?"
Through the chilly curtain of sleet, in the inter-mittent wash of the great light on the jutting cliff to the south, the massive silhouette of Bluff House loomed over Whiskey Beach. It faced the cold, turbulent Atlantic like a challenge. 
I will last as long as you. 
Standing three sturdy and indulgent stories above the rough and rugged coast, it watched the roll and slap of waves through the dark eyes of windows, as it had—in one incarnation or another—for more than three centuries.

Answer

If someone is over-indulged we mean he is spoiled and allowed to get away with whatever he wants to do. We know that Bluff House is massive. The "three sturdy and indulgent stories: suggest not just a strong house but one that has a lot more more room or money spent on it's buiiding than usual.

Answer

You can use it to describe a thing that is extravagant or possibly excessive - an indulgent meal, indulgent holiday, etc.