It really is not a very spacious bathroom so the best solution in terms of distribution is that you place the bathtub, in front of the access door. As it will be what you see most, choose a different coating for the front or, if you put, screen, choose one of very minimalist and decorative design. Having no natural light, opt for white tiles for the walls and play with tiles in a darker shade on the floor. How about a coating that mimics wood? It will give warmth. Next to the bathtub, locate the toilet and in front of it a low-bottom countertop with a large square or rectangular basin that stands out. Take advantage of the space below with a drawer module, but leave a space between the top of the module and the countertop.