Just like any other place you might choose to live there are rules and regulations that must be followed when making changes or additions to a home in Portugal. Specifically, this is a country that is very concerned with maintaining the land, whether it be for environmental reasons, appearance sake, or agricultural issues. They go to great lengths to identify sections of the country which are to be labeled special for one reason or another, and if you own land, or a home in that area you are subject to stricter rules and regulations.
With that said, you can rather easily find out what is and is not acceptable by visiting the local Camara. They will most likely handle your inquiries one at a time, as opposed to trying to educate you on all of the guidelines. If you are in anyway in doubt of whether you need permission to make improvements, or what you view as an improvement, it is strongly advised that you pop on over to the Camara to find out for sure.
In addition, the laws do change from time to time, and so if you do not complete the work you have proposed within a reasonable time after receiving approval, take the time to venture back and confirm all is still well to move forward.