Building will be better than buying, almost guaranteed.
I was chatting to some "garden room" installers, they were charging £35k for a sips panel building clad in softwood about the size of a single garage, if that, a couple of windows and a bifold door.
I estimate materials would have cost about £7k and there was 2 guys there for a week !
It was fairly nice, they dry lined it and put in lights but really even they knew they were taking thep**s.
Ollie
Agreed assuming you want a decent solid shed. The biggest problem with trying to build the lower spec shed is that you can't get hold of the materials. Youd spend so much time ripping down timber into smaller bits for the frames it isnt worth it.
My shed is basically a garden room apart from a concrete uninsulated floor and if you took the cost of it and added a door it would be maybe 6k for a 4.5m x 3m room. How they can be charging such massive amounts for the proper ones is beyond me!