I am thinking of building a second storey extension to my existing single storey extension which was build 2 years ago.this ext. is on the side of the building the area is 18sq mts. Will i be allowed to build this ext. and whether will I need a planning permission.
Probably you will need PP but you won't know unless you ask the Planning Office.
I would be more worried, though, that your single storey extension was built as a single storey extension and not with a view to going up in the future. If so your foundations will probably be inadequate and the existring structure might not take the additional load. In this case you are looking at a "demolish and start again" scenario.