Raising floor level

I'm adding a room onto my house and the carpenter has just finished installing the bearers and joists. The problem is the joists are about 10mm lower than I need them to be in order to marry in with the adjacent room's floorboards. I had planned to use 19mm cypress pine for the new room to match the existing. He actually notched the bearers to achieve this level (Don't ask me why) and reckons it will be alright in the end but I've lost confidence in him. I thought I might insert some packing all round to raise the level but i'm not sure if this is sound or possibly install a plywood underlay when laying the floor. Can anyone advise?

Nice trick. Wait and see what you end up with once the room is complete. I assume you'll be waiting until very last to put the floor in? If not you should.

What you actually do to 'fix' the problem (apart from battering your chippie) depends on which way the new floorboards will run with respect to the other room. Packing will work fine but how much depends on whether the boards run at right angles to the existing floor or parallel to the existing floor.

That was a fast response!
I was thinking of packing under the bearers so once the room is done it will be too heavy to lift. The floorboards run parallel to the existing.

Put a small piece of Cyprus on the new joists up against the existing flooring, if it sits 10 mm below the existing floor ask the chippie how the hell he thinks it's "Gonna be alright, later on".

Get it right now, while you can. It only gets harder and harder the further along the project goes.