As it stands, that looks to me like a 23-30k plus VAT extension, depending on where it is and excluding groundworks etc, minus what you do yourself.
Useful whatprice calculators here:
http://www.whatprice.co.uk/prices/
I could say detailed things like get the cloakroom door into the hall not not the kitchen, but I think you need to go back to your concept stage and decide what you actually need from your extension.
This gives you a dining area separate from the galley kitchen and a garage, to add to one reception room and three nice bedrooms. That sounds like the obvious thing to have done 30 years ago.
I'd suggest that for a family with teens now you actually want something more like:
1 - Social multiuse kitchen. Simply you could take out part of the wall and make the existing run of cupboards a peninsula.
2 - At least one extra reception room, or space, to be somewhere for your teens to be away and out of sight from parents, ideally where they can also leave things out. Do you want making-out or game-playing teenagers draped over your lounge sofa in a live demonstration of the nature documentary about bonobos you are watching? You probably could benefit from a separate alternative to bedrooms.
3 - Garage build to be a suitable games room / teen space for a few years / ganny flat for the future?
I'd also suggest that a larger one storey extension is more important than a 2 storey, as your current house is over balanced to bedrooms vs receptions (imo) - though you could do a 2 storey with teen-room / workroom / future studio upstairs.
I would be looking at enlarging the kitchen into the "link" area, coming forward more, and wrapping something around the back (could be a conservatory or sun room for morning/evening use as it gets E/W sun there), with that wall removed so it is no longer a galley.
But work from a list of the uses you will actually want, and then use that to build a use / budget table to help your thinking.
From the plan it feels (assuming to scale) that the house has a huge amount (20-25%) of floor space dedicated (wasted?) to circulation, but I cannot see easy ways to recover it (eg extend lounge to be up against the staircase) without knowing where the structural walls are.
Sorry for the candour, but I am about to go offline for a couple of weeks to chase kangaroos.
Ferdinand