Any builder should lay a slab level unless asked to do otherwise. 'We've always done it that way' is the typical get-out-of-jail catchphrase for them. I've lost plenty of panes of glass from greenhouses due to the frame flexing in strong winds so it's not worth the risk in my opinion. You could suggest to the builder that he runs a course of blockwork around and he'll have to disc-cut the blocks to make it level. You'll end up with a step over at the door but more height inside. Make sure he puts a drain hole in the lower corner (and that he knows to make it level and square this time).
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
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