relocating washing machine opposite water supply/drainage

Hi should not leak if run it under floor boards would depend on which the joists run assuming it’s a second floor flat

It is quite common for pipe work to be running under floorboards and as long as the work is completed to a good standard you shouldn't have any issues with leaks. Just a side note, you are quite limited by the depth of a joist with regards to angles to be achieved on waste pipes to have enough fall to allow water to flow correctly. I hope this makes sense. Many thanks Chris

It’s difficult to completely answer the question correctly, simply because we cannot see a plan. For instance, if the under stairs cupboard backs on to the kitchen and the units are running around the wall, then it’s possible to run both the water feed and waste around the wall. If not, and you do have to run the waste beneath the floorboards, then what is the distance between the cupboard and the waste outlet? As this will determine whether you have enough fall from the stand pipe to the waste outlet. The other thing that could be done, it the waste can be run on the outside of the building in a pipe lagging, so long as there is an external wall on both the under stairs cupboard and kitchen. If you use a good reputable plumber, the worry of a leak should not come to mind 👍

Practically speak,it will be easier to respond accurately if you have a complete floor plan.