We did the loft 2 years ago when the Mrs was up the duff. Its fairly easy to live through TBH as all the work is done from the outside appart from when they run the plumbing and elec down to the main supply.
Extending out the back is far far more disruptive to day to day life as we will loose the kitchen and they will mess up the garden. The costs seem about right, out loft was 35K and the back extension of the dining room and kitchen into one massive room is going to be 30K. This is much cheaper than moving to the equivalent house where I live, probably in the order of 50K.