The outside bit of a heat pump is roughly 1m x 1m x 2m (or to help you visualise - the size of a moped) and an inside hot water tank, which is 2m x 1m x1m (the size of a modern fridge freezer), which you can place anywhere in your home like a cupboard or loft (although the closer to the outside the better). However, new space saving technology is being developed all the time, so expect this to come down considerably in future (more on this to follow!)