I’m all for a bit of diy, but what’s your plan here?
How will you make it habitable? What spec insulation for the floors and walls, how are you going to do an inner skin for the walls?
I assume you’re replacing the roof? Are you going for a cold roof or a warm one? Do you know what depth of insulation you’ll need? How about the vapour barrier, etc?
Are you any good a brick laying if you’re extending it?
While it may seem counterintuitive to demolish what’s there, a well built timber frame room of better proportions maybe on par with working with what you’ve got there already.