My extension was built with an underfloor cavity

If the cold is coming from the floor it must not of been insulated properly.

Presuming that the floor is timber it is highly unlikely that it has been insulated. You would need to take up floor boards and insulate between the joists using a 4 inch or 6 inch think insulation board

Remove flooring and insulate between joists using kingspan or the equivalent 100mm foil back foam insulation. Ensure the kingspan is towards the bottom half of the joist allowing air to flow freely under the finish floor level. Use fix n fill foam to hold them in place and close up any gaps from cutting. Relay Flooring... Kind Regards Craig

The floor is either a block and beam floor (most likely) or a timber joist floor (not very common and not the correct way to build it). If it's block and beam then you would need to take the screed up and fit new solid insulation like kingspan or celotex. As much as you can get in but then lay a 22mm chipboard floor straight on top of the insulation.(known as a floating floor) . This will help keep the cold out. If the floor is timber joists then do as previous builders have suggested and fit insulation between the joist to the full depth of the joist. You'd be better off battening the underside of the joists in the opposite direction to the joists which will hold the insulation in place. You need to maintain the air gap below the floor. Then foil tape all the joins and refit flooring. Hope this helps. Dan BPS (MK) Ltd

The radial tree roots that extend outwards that are of primary concern here; these can influence soil conditions well beyond the circumference of the tree's leaf canopy. ... Conversely, healthy trees take large amounts of water out of the soil, often forcing clay soils to shrink. But Building Control Officers should have seen this if you notified them? keep an eye on any wall movement. Your “cold floor” needs to be looked under. If there’s no insulation at all, then you need to lift ALL the floor boarding up and cut & fit 100mm ridged insulation in between all the joists. Leave a 50mm air gap above the insulation and spike nails below to stop it falling out. You could also fit floor heating? Wayne Scott Building Yorkshire

Insulation should be secured between the joist , this should have been done during the build ! , a building inspector would have /should have requested this ? .