"All in one" housing exteriors can't have any sub-part added to them.
In-order to make a multi-piece exterior, you have to remove the "all in one" exterior completely and then add all the required sub piece exteriors.
In other words, you can't take a Weapon Outfitter house and change out the roof or walls, you have to remove the Weapon Outfitter exterior completely, and then build up the exterior with roof, walls, doors, windows, etc.. Of which you can mostly mix/match within restrictions of the particular housing district and plot.