You want to demolish old and rebuild ?

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.

If you are not adding any new wall or structure on empty land, then not necessary. For eg, if just to change the kitchen window, then is not require.

I cant be sure now, but mbsa could have those standard ready plans for inter unit. Buy the plan and ask the contractor to follow. My guess it could cost few hundreds to maybe slightly above a thousand for everything.

A new drawing will cost about 2k+.all in.