What should be under my Bifold Door?

[vaderag]

OP, do you have a solid floor - is the threshold frame partially sitting on the solid floor?
i ask because the plastic air vent seems to be too high for a suspended floor?
and, unless there's a cavity tray, what is the weep drain draining?
can you post a photo of inside showing the sill at floor level?

i cant see a DPC under the wood packer?
the sand and cement plinth might be bridging any DPC in the walls?
can you mark on the pic where your FFL is?

ffwiw: there are better ways of building the steps.
typically, the top riser should have been the same rise , say 7 -1/2", as the lower risers.

Some of the above went over my head but answering what I can.

Yes, solid floor behind, but the frame is not sitting on the floor itself as the floor stopped prior to the internal cavity. What is shown is the door on the front brickwork of the cavity

The wood packer is sat on the external wall of the cavity (not sure what an FFL is)

 

[crank39]

Finished floor level

 

[vaderag]

Finished floor level

Finished floor level (now that the wood flooring inside has gone down) is just under the frame of the door - basically level with the top of the weather sill

 

[bobasd]

get the installer back.

 

[catlad]

If you cant get them back to finish the job then you need a length of dpc half a bucket of cement and a 20mm finger trowel.