Well I just finished a little job extending mine. By a whopping 600mm.
It's a tiny shed, 3x4m so the extra width made a massive difference. Plus I swapped the fall of the roof to give me a lot more headroom. The narrow strip is a thin concrete base, Screwfix self levelling compound on top. I just stuck a row of fenceposts up to make a rear wall and sheeted with some spare 24mm ext plywood. Then whitewashed it with some stinky paint.
The weird brackety things are temporary () structures for long lengths of steel stock, rebar etc, above that, narrow brackets for long lengths of wood. There will be a trapdoor to the left of the picture which allows me to store and manipulate 3600mm stock. I'll be moving the horizontal bandsaw in there too, next to the linishing machine. Plenty of room left for an eight foot work table.
Biggest problem was working around all that "stock" which has nowhere else to go. Especially the 2400 lengths of steel propped up in the corner. A right pain to work around but I dared not leave it outside, even overnight so I had to work around it.
All because my neighbour decided to replace his old fence and tearing out the hedgerow on my side gave me a fair bit more room to move into.