You cant unless you already have a private house.

An Fc home cant be turned to a private house, youd have to relinquish it and move from an private house to it, as the random timer would prevent you from just buying it.

You got 2 options, which are

-Give up the plot, move quickily from your private house to there.

-Stay in an 1 person FC (or start a new one with it)