Forgot to say I also did floor lowering but not by underpinning. Underpinning not a sensible option (for me) as walls are 600mm thick and rubble stone so liable to keep falling into any trench dug out for underpinning. If you don't need much height gain (I've gone from 6'3" to 6'8") then you can use a reinforced 'bathtub' floor where the reinforcing forms an upstand at the edge of the room, which when concreted forms a ring beam which holds back the earth at the base of the foundations. The whole floor needs to be reinforced to prevent any problems from heave lifting the middle of the floor.
I did all the digging out myself (with a little help from friends paid with beer), put in the under floor drainage, and had a contractor put in the sub-base and reinforced concrete, with a joint venture on the membrane as I wasn't confident they'd give it the same attention to detail that I would.
The other thing you need to consider is humidity as the membrane system works best when not fully sealed at the ceiling so you still need to deal with water vapour. I'm planning to put in a heat recovery ventilator which can deliver 4 air changes/hr but with the same heat loss as 1 change/hr. If this turns out not to be sufficient I'll add a wall-mounted dehumidifier.
Total conversion cost including drainage, contractor, a new RSJ across the room, stud walls, flooring (including undertile heating), lighting etc. is estimated at =A312-15k and gives me a room which is 19' x 13'.
If you want to share my ignorance further just ask.
Fash