How do I hide white line between monocouche meeting of the old house and new extension?

There is an angle bead that you can buy and install to divide the old from new or to finish nicely the monocouche . I could also recommend someone that can do a really good job of monocouche .

U would fit a stop bead vertical to separate the old an new

Hi. Because the new extension have new foundations, the ground below them takes time to settle compared to already settled existing foundations. Due to this movement, movement ties are used when new walls connect to original walls. The white line you are referring to is probably exterior mastic sealant which divides the new and old walls and the render. Unfortunately, the builder must have left a big gap between old and new walls than required; hence wider 'white line'. It is supposed to be 1-2cms wide. But yes it is required and you cannot get rid of it in a viable way unless you re-render the whole house in coming years. When the time comes for re-rendering, continuous mesh to be used between new and existing before rendering; which will get rid of the white line. Hope it helps. Regards. A