freefall

Hi Im sure this has been written about before but I need advice. a Landlord has asked if I could follow up on repairs to a number door problems in two of his houses. These are the gaps around the doors possibly exceed the 4mm and the threshold is to large due to the way the floor rises upwards as the door opens so when closed the gap under the door is around 10mm or more. what can be done to reduce the gap under the door. without having to lift the floor and raising it causing a trip hazard in to the room. are there any brush strips that can be fitted ? what can I do regarding the door surround the doors already have intumescent door seals but the fibres might not be touching the door its self.