Once again, I've had another bad situation with landlord and landlady trying to scam more money out of me, rip me off and not give me my deposit back after agreeing to leave the flat on their terms. It's clear they are not paying taxes for the flat, and they are also applying for large credit sums.
There are two things you need to remember about renting in Warsaw:
i) Landlords do not pay tax on the money you give them. If you want any paperwork (so, for example, you can claim part of the cost as a business expense because you run your one person company from the flat), the price will rise by at least 20%.
ii) You do not get your deposit back. In my time in Poland I have rented nine flats. I got my deposit back once.
I have a copy of flat contract (although - written in English - maybe not submissable as evidence in a Polish court), with landlord / landlady signatures, dates ... the works.
That will most certainly be admissible in court.
BTW: always pay your rent by bank transfer, never in cash.