Same as making houses bigger from the inside or making them the exact same size from the outside. Though there's actual functional reasons for picking one or another.
In leaving outside walls visible, for example, your maps will become slightly taller, but you might like the extra detail.
You can also make other stuff like imagine the dissection and remove the front walls completely (leaving the player 100% un obscured), or instead leaving the top obscuring part when you get close to bottom, as if the viewer is like looking from the top by removing the roof from a dollhouse.
Ok thanks for your advices, i think i will finally use the "Without wall" method.
As @Rugman said..." I would add a door and try to make the outside wall look like it does outside the house as much as you can "
-> and that could be more complicated : some elements from outside tiles are not available for inside tiles and the number of layer is limited.