height of Monolithic slab

We find ourselves facing the difficult task to put water tanks in the ground without the house slab in place and I'm hoping somebody could give me some advice on the following.

In order not to place the tanks too high or low in relation to the house we have to know how high the threshold of the laundry door will sit. The first tank will be placed at a distance of 4.5 meters from that point and the top of the lid should sit 100mm higher than the surrounding soil.

The house pad is cut/fill to 14.78AHD with a max crossfall over the house area of 0.45 m. We have a good idea where the cut and where the fill will be so if somebody would know how much the slab will add to that we might be able to figure out how deep the exavator has to go. The tanks are 2.8 mtr high, so normally he has to go 2.6 mtr deep to allow 100mm gravel at the base. By adding the extra height of the slab and threshold we would be able to get the distance between the house and edge of tank level
or could we get this info from the surveyers without asking them to come on site again? They already are charging us $1200 for replacing a couple of missing PEGS so I don't want them around to much.

Your drawings should have a Finished Floor Level (FFL) you should also have levels near a title peg or other fixed point on you block. If you know these a few stakes, stringlines, and a stringline level you should be able to work it all out.

Great TDL, thanks. We don't have the drawings yet but I'll give the Sales Representative a call today and he might know the FFL. I know where the DUMPY sits but I'm not sure about the hight anymore. We were flooded and I had to dig the little thing out. Hopefully it didn't move.

cheers