Yes, they install, but sometimes there isn't enough pressure because of a weak pump. My mom on the 12th floor had that problem too. She went to a good plumber https://allstarplumbing.co/, and they installed a booster pump. I didn't hear of it being illegal, although it probably depends on the country you live in. Either way, your best bet is to talk to a professional, and if there is a problem with the installation, he'll let you know. And he may also suggest an alternative to solve the pressure problem.