How can one describe the narrator's home in "The Great Gatsby?"

Your question made me laugh a bit.  Nick, the narrator of The Great Gatsby, has a VERY modest home situated (squished?) between two mansions of West Egg.  West Egg, of course, is the part of Manhassett Neck that houses the "new rich" and considered the "least fashionable of the two" (as opposed to East Egg that houses the "old rich").

Therefore, Nick's house can be best described as both "modest" and "plain."

I lived at West Egg, the – well, the least fashionable of the two, though this is a most superficial tag to express the bizarre and not a little sinister contrast between them. My house was at the very tip of the egg, only fifty yards from the Sound, and squeezed between two huge places that rented for twelve or fifteen thousand a season. The one on my right was a colossal affair by any standard. My own house was an eyesore, but it was a small eyesore, and it had been overlooked, so I had a view of the water, a partial view of my neighbor's lawn, and the consoling proximity of millionaires - all for eighty dollars a month.

These qualities of being both modest and plain are not unlike Nick himself.  And it is here, in Nick's house, that Gatsby decides to meet Daisy again, for the first time, after many many years.  Why?  He wants to show that he has been able to achieve her very dream:  wealth.  Gatsby wants to show that he has overtaken people like Nick, "the bond man," and become a millionaire.  He does this by the exact juxtaposition of his grand mansion next to Nick's modest home.

Why does Gatsby do this?  Because for years he has had parties at his mansion only in hopes that Daisy would randomly show up from East Egg.  She never did.  Therefore, when Daisy's cousin, Nick, buys the little modest home beside Gatsby, he finds the perfect opportunity to woo Gatsby.

In our society, supposedly it is never good to have the most expensive or the least expensive house on the block.  Unfortunately for Nick (and fortunately for Gatsby), Nick has the least expensive house right next to Gatsby's most expensive one.