Do we need to waterproof cement floor in the garden to prevent water from accumulating under the soil?
IMO it is never a great idea to pile soil against the walls of your house. It may lead to damage to walls as you mention, and/or water intrusion at the foundation. 
Buildings have rain gutters to direct water away from the foundation, you are now going to do the opposite.

I strongly suggest that you use a nice decorative pot or urn for your planting(s), so you will always be able to assess the conditions of the wall and foundation in the future.