is it still legal to put up building on farm land without planning permission as long as it for agricultural?

Not necessarily.

Planning permission is required for change of use and for development. If the current use is agricultural then you will not be changing the use. However, building a chicken house is development, so planning permission would be required.

In some areas some minor developments are permitted under Permitted Development rules. However, exactly what is and is not permitted under these rules varies depending on where you are, so you will need to contact your local planning department or a local solicitor for a definite answer.

Btw, just because land is agricultural does not necessarily mean that you can use it for any agricultural purpose without people complaining, and having those complaints upheld.