DPC clearance from finished ground levels

[MarkR]

Need to pick your brains on this one. The DPC on my daughters new build is either level with the finished external ground levels or is only just proud. I have read Buiding Regs stating there should be 150mm clearance. Is this variable in any circumstances? The builders are now telling us that the house has a double DPC; one membrane on top of the other in the same mortar joint and this allows the clearance to be reduced to 75mm. This sounds like a load of rubbish to me and I can’t find anything on line to substantiate it. Any thoughts?

[New Home Expert]

There are lots of posts on here about DPC levels.
Years after the mobility standards Part M were introduced in the Building Regulations (the cause of this defect) housebuilders are still not ensuring the dpc is at least 150mm (2 brick courses) above finished ground level.

Unfortunately, the Building Regulations do state "should be" and not "must be". The NHBC standards are similar.
Worse, unless the home is affected by this (damp on walls inside and/or out) housebuilders can and do, dodge doing anything about it.

Dpc "one on top of another in the same mortar joint" serves no purpose at all. 
I expect they could be talking about a cavity tray which would help stop damp above ground level on the inside walls. They may also be talking about another separate dpc above the one level with the ground which is OK.
But with what you have said it is more likely they don't know what they are doing and are now trying to pull the wool over your eyes.

Have the home professionally inspected and snagged.
Who is this housebuilder?