In our last place we extended a the tiny box second bedroom into the hallway (we didn't need such a big hallway in a flat) and then got a joiner to build a full-height wardrobe into over a bit left sticking out of the wall. Thank God we did that as we ended up in that flat for about twice as long as we expected and with two small kids, so the storage in that small room really made that so much more manageable.
In our current house we're really glad we moved the bathroom door and created a laundry cupboard upstairs so that we don't have to have a machine in the kitchen/be dragging laundry down there. Also unreasonably pleased with the door we put in for downstairs loo - we got the house with the old downstairs loo still opening to the garden and it was really hard to work out where to move it. It went by hallway in the end, which is really narrow, but builder suggested a door that slides into a gap in the wall and while it was pricey, it's so nice that we don't have an awkward door in that space.