Our installer found us a tall thin hot water tank that fitted in quite a small footprint with the expansion vessels above it, another alternative is a horizontal tank that can go in the loft space, worth researching. We have only a small tank as there are usually only two of us and we have a shower room instead of a bath, and it has been perfectly adequate. It does look rather like something out of a 50’s sci-fi film in our airing cupboard!