I have.

It is not difficult.

I drew the plans, applied for planning permission, sorted out the party wall agreement and built it.

The only things I didn't couldn't do were;

Electrical testing to suit part P of the building regs.
The gas pipework and connection to the hob.

Ha !

Ignor the assualt rifle next to the shovel, you don't need one to build.

Mine is a big'un measuring 8m x 4.5m



I spent more time in the ground digging and sorting out the drainage than anything else.