Add a room as in add an addition to your house? Or change the interior configuration? If the former, it is no different in the US if the zoning does not already

Add a room as in add an addition to your house? Or change the interior configuration? If the former, it is no different in the US if the zoning does not already allow for the expansion (I.e. as in the case of most suburban houses, the house is already built to the maximum size on the lot and the home owner needs a variance)