Garage floor.

I've dug out the trench and floor foundations for my garage extension (well a man with digger did). The intention is to convert the existing garage into a double one. I'm a little unsure about laying the floor slab.
Before the floor slab can be layed I obviously need to put in the trench foundations and brick up the groundwork walls to the DPC level. The existing side of the garage has to come down so that I can connect the old floor slab to the new one (with steel rods). The garage has a single skin wall.
I should fill in the main slab area with concrete then have screed over that laid at a slope (I'll check what's going on first with the existing floor). Here are my questions..
1) Between the screed and the slab below will be a plastic membrane. What do I do with this membrane at the edges of the slab? Do I trim it at the surface, run it into the wall DPC and what about the garage door and where the new floor and old floor meet?
2) what depth should the screed be (not specified on the architects drawings - slab thickness is 200mm thick - it's not clear to me whether this includes the screed or not)?
3) It would be really good if I could build the outer shell before laying the floor - particularly because we have a 3M hole in our garden wall and a garage full of stuff (don't ask). If I caould build the wall before the floor, at least I wouldn't have a problem over security and having to deal with relocating/exposing the garage contents over a longer period.
Any thoughts?
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