foundation on rocky terrain

Thanks a lot for the comments.
Yes, the building (which it is already standing but incomplete) will house a tractor and equipment and have shelves to dry nuts and others. The ground next to it is very stable and flat (And it has not ever moved) and the existing walls have their own foundations (When they were filled to the top with rouble they were holding the ground from going over, so they can be deemed strong enough). The flat roof over it is intended to be used as a walk on terrace as it will be levelled with the terrain. The space in front of the garages is planed to have a water deposit to collect water from roofs.
I take your comments about planning. I just wanted to know if to place a foundation digging further on the rock is a must. It doesn't seem easy without going into the rock with an hydraulic rock breaker. I can lift the floor from the rock up but it looks very costly and difficult to dig a trench on the rock bed.
As for planning I have already asked and I can finish the building. It rests to see what is the best value for money way. Thanks again.