That's like saying you can walk into a garage and buy a nice new shiny car...
Or you can walk around several scrap dealers get the parts together take them back with you and build the car yourself.
£3000 isn't a realistic price for a job that's been described whether he's building it by himself of not.
What would you say would be an accurate material cost with rough breakdown?
eg, Slab / foundations
Walls including mortar
Lintels
Doors (patio door and garage door)
Roofing Trusses
Roofing material (tiles or something like this https://www.roofingsuperstore.co.uk...HhXgai--SFK8LiRRaMz6wkmHTYucRRKIaAkMwEALw_wcB)
Any other materials required to get to a watertight state