There are a number of designs that have to half-height dishwasher stacked atop each other, so only takes the space of a normal dishwasher, that accomplish exactly this. The only problem is they tend not be as good quality as conventional dishwashers, and you only have half of the usual capacity (which can be worked around in many cases when necessary)
Some half height dishwashers are decent. The real trick seems to be to very carefully measure dinner plates - the dishwashers only just fit most plates, and if you put just one plate in that's slightly too large, it blocks the washer jets, and then the whole load comes out dirty and people curse the poor performance of the machine.
