I was thinking to lengthen the cover I made for our armchair, because it was always falling down, so I quickly took some precuts from my scrapbin (it was me who cut them to 5"x5") an whipped up this small extension in no time.
The most difficult part was how to attach it to the already binded existing one, since it had no separate binding, but the backing was turned over. Finally I partially cut the seams at the top, where the new piece was attached, turned the existing backing under the original top and stitched it together. For the new part I simply binded as usual, with a scrappy binding, and for stitched a little overlap for the old part. I cannot say it is the most sophisticated solution, but it was easy, and since it is mostly behind the armchair, nobody will see it - except you in the photo... The greyish solid it the original border for the top.