What is the difference in the usage of "so big a house" and "a so big house"?

Both of them I have seen often. Can I use them interchangeably?

"So big a house" might be used in several contexts:

Why do you need so big a house?

It was so big a house that you needed a map to find your way through it.

"A so-big house" is wording which kind of diminishes the "bigness":

It was a so-big house, large enough to move around in but certainly not big enough for a party.

(These examples are admittedly a little strained, but better ones would come in a larger context.)

(I will add that "a so big" often appears on the web as an error, where "so big a" would be more appropriate. These uses are apparently written carelessly or by someone not skilled in English. The "correct" uses are fairly rare.)