Yes, many of our customers will notch their baseboards so that the mantel sits flush with the wall. Keep in mind that there are two 6’ ft power cords that come off of the right back side of the unit. Make sure that your outlet is offset to the right of the insert, in the empty volume between the edge of the insert and the edge of your mantel. You may then notch your baseboard on either side of the mantle so that it sits flush.
Please have the mantel on hand before you do anything custom to your baseboard so that there are no surprises.