I recently have expanded my gym to include a yoga studio that can offer fitness classes, personal training or just an area to stretch.
There was a large demand in my city to offer those amenities, then unluckyly that meant I had to take space from somewhere, but i ended up stealing square footage from the locker rooms in order to make the space.
I distraught about how I was going to find the space for lockers, showers and toilets. I had the water hookups right there, I just needed to build closeby. The concern was getting a builder to add onto my building was so much money. That is more of a several year plan, however for right now I am sticking with a modular locker room and lavatory solution. There is a supplier that uses shipping containers and creates anything out of them; You can have a bin that just holds individual lockers and resembles a real locker room. You can then add onto this bin to make it bigger to hold many stalls for toilets and individual sections for showers. It looks prefer a real lavatory when you see the inside. I have made it so the shipping containers are attached right to my gym. You hardly notice walking from the actual building in the shipping containers. The transition is smooth and classy. The locker rooms are big and have everything a gym goer could need too. There are mirrors, sinks, soap dispensers, hand dryers and locations to put your stuff. It was the best solution for me.