It depends on the building and the weights being used. We trained in a warehouse in Baltimore for two years and then cracks were found in the basement columns and we had to stop. We trained on platforms made of 2x4s, 2x6s, plywood, and rubber.
Of course, if your clients aren't going to use more than 50-70 kilos, I think it would be alright.
In New York, we dropped off a platform at a fitness place and when the weights were dropped on the seventh floor, you could feel vibrations down on the second. Honestly, it depends on the building and the weights being used.
However, sound dampening can work. use carpet under the platform, some indoor outdoor stuff. And use composite rubber bumpers on composite rubber, like what Werksan has (they sell ti separately) or at www.rubbercal.com
. Rubbercal has these 2.5 inch thick playground tiles. I use them for stone lifting; I drop my 330 pound stones on them and they bounce with no bad effects.