All Polish property purchased after Jan 1, 2007 will be subject to a sort of Capital Gains Tax. I think the rate is 19%. This means, after subtracting all your costs - such as putting a new roof on the property, or tiles in the bathroom (providing these are all properly VAT receipted) - you will have to pay 19% on your profit when selling the property.
So hypothetically - You buy a house for 100,000PLN You spend 100,000PLN on redevelopment. You sell the property after 2 years for 300,000
So you have a capital gain of 100,000PLN (less inflation adjustment, which I think might apply, but not sure - so you can deduct about 14,000PLN (say 7% of 200,000 PLN?)
So you have 19% tax to pay on your profit of 86,000PLN = 16,340PLN
(Under the old system you would have paid 10% on your purchase price (if you bought within the past 5 years and didn't reinvest all the money in property immediately), so you would have paid 10,000PLN in tax.)
There you have it!
Of course every situation is different, and the prices in Poland are now getting so crazy that it's not at all sure you'll make any profit at all... So possibly 0 PLN tax - compared to the 10,000 PLN flat rate you'd have paid until recently.
Just to confirm, the tax rate on a capital gain will indeed be 19%.
An important point of information - the taxation on a property capital gain will be considered by the tax office completely independently of whatever you pay in income tax in Poland. So you can earn 1,000,000 zlots from your ordinary work and be taxed at 45% or whatever on that, but still only pay 19% tax on any capital gain on the property. (Unless your 'ordinary work' is property development, of course!)