Is this legal?

I am guessing that it is a 12/12 roof pitch and that your usable space for your bathroom would be 4 ft by 16 ft? If correct, this means that you have a long triangular shape aprox 5 feet high tapering down to nothing to squeeze some storage into.

Are you looking for usable every day storage or stash away one a year stuff type storage? And do you really want to store stuff in a bathroom with the moisture and hygiene related challenges that may present?

We have a similar situation in our upstairs bedroom and had deep custom built drawers installed ...but...we had way more room to work with. If you have drawers, there has to be room to pull them out and room to stand in front of them without bonking your head.

If you install doors-either sliding or hinged it will require crawling on your knees to access the stored items. The other option would be to purchase attractive lidded storage containers and tuck them into the available space without drawers or doors.

Not sure I would have gone down this road, but you have and good luck with your challenge! Just imagine where your wall would fit into the picture and here's a few inspiration shots:

Greenwich Views, Greenwich, London · More Info

Contemporary Home Office · More Info

Mews House refurbishment in London Fitzrovia · More Info

Graces Road · More Info

Warren Road Master bedroom · More Info