Literally building one in my garden right now (no not at 4am but you know what I mean!). I used pallets all screwed together with 200mm coach screws.
Once I had my ground level it was like lego, slotting pallets together meant it went down onto the blocks in about half an hour and as they're all screwed together it's solid as f**k. I've also got some chip board flooring to put down and I can batter nice flooring down on top.
The frame is (mostly) out of pallets too, its 5m x 2.4m and I just got a hand off a pal to make the roof trusses (with a 200mm overhang). I used 2x6" boards for these and have one side about 75mm higher for a run off. Using 15mm OSB for the roof and lining the inside with 9mm OSB to hide the manky pallets. Im not bothering to insulate as im sticking a wood burner in it, leaving the cavity between the pallets to run electrical cables in and it'll all be hidden.
Don't skimp on the roofing felt either if possible - I think i was about £80 for two rolls of heavy duty 36kg stuff. Works out at 20m².
The higher side is basically 4 hardwood french doors to let light in, the 2 central ones being actual doors. All built with treated CLS timber and screwed together basically.
Maybe overkill but id run DPC damp proofing over the slabs and grass just to help make a better moisture barrier too if you can get your hands on any - the weed fabric is permeable.
Also, watch the height for planning permission. The rule is it any part of the building thats within a meter of a boundary can't be higher than 2.5m. If its further than a meter then I think you're good up to 3m for the eves or something and 4m in total. Mine is going to work out about 2.5m I think.
I am also working without a plan, plans are overrated!