Hi there.
I intend to add a play room underneath an existing deck.
I started digging out to get to the proper height.
In the process, I hit a fair bit of sandstone.
Basically the brick wall supporting the house is sitting on the sandstone and I have used a jack hammer to cut straight down into the sandstone to the required height. The whole extension will be sitting on sandstone. I intend to lay about 50mm of 10mm blue metal then lay the 100mm concrete slab. I will leave a 150mm trench around the slab to fit in an agg line that will run into the existing storm water drain. The brick veneer wall will then be built onto the slab. I can't dig the trench around the slad too deep or I will be below my storm water level. Will that be adequate drainage for my new room or should I run 75mm agg line underneath the concrete slab at 1meter center starting from the brick wall or underneath the agg line that runs around the slab and towards the storm water drain.

I can't see that much water coming up through the sandstone unless you are in a very wet area. You might put down say 50 mm of gravel between the sandstone and the concrete slab. Use coarse rounded stuff, not crushed rock or pea gravel. Surely that would be enough for any water to flow out to the perimeter drains rather than up through the cement. You might put a plastic membrane under the concrete too.

You mean 75 mm agg line or stripdrain is not necessary underneath the slab. Just the plastic and blue metal?
Being on sandstone already can the slab be less than 100mm.

If you don't get this right, it will be very difficult to redo. Concrete isn't that expensive. I would put down the gravel, a plastic membrane, and then the full 100 mm of concrete.