I'm doing mine at the mo too - from a single to a double. I had to run it by building control as it'll be over 30m2. You should be able to stick a flat roof on the new bit which will give you the extra head room so long as you don't go above the current apex height (assuming that's within regs). Personally, if there are no cracks or subsidence I would do the top bit out of breeze block and render - it'll be quicker and cheaper than brick and no more heavy than what's there now. I'd also stick a steel between the piers in the downstairs bit to support the floor although you'll have to knock the top block off and pad it with brick. I'm jealous of your dungeon by the way, that would make a cracking machine shop! Have a look at the building control website - all the info is there and they're not bad if you call them for advice.