We've been thinking about this. Suspect it would be more like £60-70k though. My parents have just had their loft remodelled, not even converted, just the rooms changed from 2 bedrooms to one big bedroom with en suite, and it was £50k. Prices are insane here (Scotland).
The thing that's stopping us is that it would put our house over 4 floors and have 6 toilets 
seems mental. But the layout at the moment isn't working for 3 kids.
The jump to the next level property for us would be a purchase price of approx £900k - £1m so the stamp duty would be between £66 and 78k... Are we better spending that on a loft or on tax...?