On Sunday, 1 October 2017 12:06:31 UTC+1, GB wrote:

Are you going to increase the size of the rear extension, compared to
the conservatory? If so, a good solution would be a brick pillar at each
side, with strong foundations, specifically designed to take the weight
of that beam plus all it will support. Tack that onto the end of the
existing side wall.

The current plan is for the footprint is expected to stay the same.

I'd be just as concerned, if not more so, to check the house
foundations. At the moment, the weight of the two storeys is spread over
the entire back wall, but if you knock through it will be concentrated.

The current back wall has a window and a pair of french doors (two windows upstairs) but, yes, it will be changing significantly - not least the fact that the opening will be 5m wide so not at all insignificant.

Do you need a structural engineer to advise you?

Absolutely; I have every intention of getting one for the knock-through beam calculations and so I will be sure to get their views on other pertinent aspects too. I will also be employing an architectural technician for the drawings. At the moment I'm just trying to pre-empt some of the inevitable open questions that will arise such as how deep are the existing foundations..