new concrete subfloor level lower than existing house

Not without seeing it But it might need tanking.sorry can't give a disitive ANSWER

We have just done an extension similar to yours the new kitchen extension was 500mm lower than finished floor height in the house the structural engineers requirements were that we installed 700mm wide footing with double mesh and L-shaped rebar sticking out of the then lay the blocks with the rebar going through the middle of the retaining wall, then we had tank between the wall and the underneath the house

Hi, for the sake of 30 mm you could definitely make something work whether it be a dpm (damp proof membrane) or some form of damp but you should be able to find a way around it other than tanking which can be very expensive.

Usually if you were to have a FFL below your current Threshold level, you would sink the footings deeper and the outer leaf would be built in Class 1 Engineering Brick as this would act as a DPC in it's own right. The cavity would employ a drainage gully channelled to a soak away at least 5 metres from the footings and the internal walls would be concrete blocks with an integral waterproofer in the cement mortar as with the engineering brickwork. The internal blockwork would then be either tanked with Vandex bb75, or mesh membrane. You would also have cavity trays above DPC, and French Drainage butted up to brickwork outside to reduce splashback. Thereafter internal substrate would be painted with a fixative primer and breathable paint. Only an optimistic cavalier would defer from this methodology. All this is inexpensive provided you plan this in your pre construction planning.