first you need to visit the 'gminia' (gemeinde) that the land is situated in - you have to check if they have something called 'plan zagospodarowania przestrzennego' which is sort of developement plan - if they have such a plan then if the land in question is devoted to some particular use in the future (like housing developement, transport infrastructure, landfill etc) you won't be able to change the current use of it (like planting a park)
if the land in question does not have any particular purpose in their plans and the soil is low class they will let you do quite a lot with the land including building a house (but there must be a public road that connects to it - also you need to officially become a farmer in Poland - a farmer in Poland has an official status and it's not enough to buy land to become a farmer (for a Polish person - if you have some agriculture background from Germany including horticulture, even raising animals you can eventually become a farmer in Poland) - aha - when we're at it - AFAIK farming land will not be freely accessible to foreigners until 2016 - you need to obtain an official consent from Ministry of Agriculture at the moment to buy farming land (and you should expect that they will expect that you create some workplaces in your newly bought farm) - anyway expect a lot of beaurocracy
- IS IT POSSIBLE TO TRANSFORM/CHANGE/CONVERT A BIG AGRICULTURALLY USED AREA (ABOUT 5 HA)
TO A FREE TIME AREA (LIKE A BIG PARK WITH MANY TREES A.S.O. IN POLAND)?
- WHICH ADMINISTRATION/OFFICE IS THE RIGHT ONE FOR THIS INTENTION?
and afterthought - if the thing will be supposed to be a commercial enterprise I think you actually would have to eventually 'de-farm' the land i.e. change it's official use/status - I'm not really familiar with the procedure but if the soil on this land is low quality (low class 5 and 6 are really low class 4 is borderline) then there will be much less problem with 'de-farming' it - I said eventually 'de-farm' because I think it can be plausible to start out without 'de-farming' and planting an official 'tree plantation' on the land (this does not require you to change the official status of the land i.e. 'de-farming') - some minor landscaping on such a tree plantation is plausible but if you planned something bigger (a pond, some bigger artificial hills) you would need some official consents to it - a lot would depend on your relations with the decidents in the area (you would need to let yourself known, attend and throw some parties - you know socialize with the right people)