joining old DPM to new DPC

[mxhx2023]

hello. Could I get your thoughts? Two years ago, a contractor completed phase 1 of my renovation, creating a passage to an annex. This resulted with a 7cm step down. I now plan to raise the floor level but noticed efflorescence on the concrete at the patio doors and want to remedy that first. Builders removed Aco drains during construction, so I will reinstate those but due to ground level outside I don’t think that’ll be enough.

Can I use a Bituthene layer to close the gap between the new DPC and floor, replacing CLS with engineering brick? The annex is 55m2, and I'm hesitant about doing a whole new DPM under the screed as there are no issues anywhere else. Someone mentioned an exterior impermeable concrete plinth barrier as an alternative along the length of the doors.

 

In many ways this is a bit like when people brick up garage doors. I just can't figure out how you stop moisture seeping through those lower bricks.

Diagram attached along with photos before (white doors), during the construction and how it is now (black doors).

Thank you.