Concrete pad for attached garden room advice please

[LGWIZZ]

Hi All, Hope you are well, we have recently purchased a property and currently putting some plans in place. One thing we will be doing is putting a addroom on the back (a lean to garden room attached to the house. However i have a dilemma with the air bricks of the original house. In usual extensions we would obviously build foundations brick up and run a air vent extension across. However in this situation we just require a 100mm Concrete pad on top of compacted type 1. I plan to have the concrete pad slightly above ground level but how could i extend the air bricks? Would i be able to extend under the concrete base and fix the vent to the front face of the pad? The other option i have thought of is to put in aco drain at the back of the bad and build the addroom on top as it sits on 4 x 2 treated timber batten. but would this be enough air flow under? Thankyou in advance.