declaration prealable

[bigears]

hi

I would appreciate some advice.  I have a house that has a cave at the back which is underground and accessible from the kitchen,  Above the cave there is a storage room/area with a door to it from the garden, it is not accessible from within the house.  This room (I cannot think of a better description) had nothing in it when we bought the house, presumably it has been used for storage or even a workshop.  Some smallish window openings still exist but have been blocked up for security.  I have now  tanked out the cave and put a new floor in above.  I want to further develop the room above to be used as part of the house, with a door to it accessible from within the house, actually from the first floor landing.  I also want to replace the existing door (the one from the garden) with a window, this is visible from the road outside our house.  I also want to put in a french window in another wall (not visible from the road, or any public access).  The new room will be less than 20 sq m and the total floor space of the house will be well short of 170 sq m. 

I would like to gain permission to convert this room to a habital room, with the two windows mentioned and of course would be delighted to pay the extra taxe d'habitation.  I have been advised by some just to do it and not the seek permission.   My concerm is that it may be problematic and time consuming if there are problems or it may require a permis de construire as the room may be regarded as once having an agricultural use.  It is a country property and is situated in a small hamlet, it is nevertheless just a house.  Any comments from knowledgable people would be appreciated.