If the existing extension was approved previously, then not necesssry. Otherwise, need to submit plan and pay a fine for the old extension without approval.

Not necessary to demolish the entire wall. The contractor will know what to do. This small job, few days can kaotim unless you are also doing other things.

Unless neighbour complain or if you are unlucky mbsa pass through your area, they will check on your house. If is at back lane area, normally not obvious unless you have a roro bin infront of the house.