i am having a dormer built on my semi house, the builder has put 18mm plywood on the bedrooms floors, will this cause me a problem in the long run or do i need to demand to have floorboards put in?

It all depends on the spacing of the joists, if they are at 400mm centres then it will not cause a proplem. If the joists are set at 600mm centres it would be a good idea that there are noggings in place at the edge of the boards (inbetween the joists) to stop downward movement when walked on. Also as Bailey mentioned, make sure there are access traps in place for future repairs.

Short answer is no. I would recommend that you ask for the ply to be cut ready for access points such as light roses possibly sockets as well in the event that something did go wrong the it is highly unlikly. hope this helps

I would be tempted to double board it if it’s plyd unless it’s tongue and groove used and much like the answer above the centres of your joist will determine how solid your floor is , we usually only use tongue and groove as they fix into each other locking the joint in , and they’re not as wide so much less likely to flex