The landlord or the tenant? You can offer a short rent free or reduced rent period to a new tenant to allow them to make the necessary changes/upgrades to your commercial unit. Its quite normal. The amount of discount given largely depends on how desirable the location is. Providing relief for a current tenant is also possible if the tenant is making significant upgrades, but not recommended otherwise. Its a slippery slope reducing rent and your yield is what provides the bulk of the return in the long run, so any cuts in the short term will multiply out into the long term and significantly hamper the return.
I think for warehouses there's a reduced rate, or at least there was a few years ago. But you needed to apply for it/declare .As for other commercial property, you are going to have to pay the full rate. But to be honest, its pretty much the same rate across Poland and at 23zl/m its not a great deal to pay especially if you are focused in any of the big cities - your property is going to be earning many multiples of that rate. More of a problem in lesser developed areas. And thats one of the big turns offs from investing outside the big cities - you are paying pretty much the same taxes but have much lower income potential.