I think the option of having the tv under the stairs.. Is the best idea.. Maybe have a tv, media unit made to measure if possible .. Draws or cupboards underneath for storage. Small shelves at the sides for little ink-naks. If needed.. A smaller maybe glass oblong table in front of the window. The table will still allow light through.. Making sure passable through to the stairs.. And the sofa on the back wall.. Lovely rug in the centre with coffee table or chest for extra storage if needed... Maybe some open floating shelves off set above the sofa. Use an accent colour with accessories.. From the rug or cushions.. Don't go overboard with large piece of furniture... Simple statements....

Could you add a faux 'log' store adjoining it, or a faux cupboard, to centre the whole thing in the space without moving the fireplace? If you added a shallow wall-frame to the right of the fireplace (same height, and extending to the right to create exact symmetry of the outside edge of the fireplace) you could some log slices in it to simulate a log store. Or just add a custom cupboard front to the right, so it looks like a part of the fireplace unit...

If you're leaning more towards a pitched roof, perhaps you could extend your glazing right up into the gable to maximize the light coming in. I've attached a couple of images of projects where we have done just that. The first was necessary due to it being a listed building so we suggested an additional tiled roof would be more in keeping than, for example, a flat roof with a roof lantern. The second was on a lovely Victorian terrace in Coggeshall and again, we suggested echoing the original architecture with a pitched roof and glazed gable. If you would prefer an orangery style extension, despite some previous comments it is definitely possible to have a space that is a nice even temperature all year round, doesn't collect lots of flies and doesn't leak. We have built these for 30 years. All of our roof lanterns are available with solar controlled glass and motorised vents, plus the combination of masonry and glass (as opposed to a traditional conservatory which is mostly glass) provides a a more consistent room temperature anyway. We offer a 10 year guarantee on our joinery, glass and paint coatings (parts and labour) - that's how confident we are that it won't leak. We also use Accoya wood in our products which is incredibly durable, stable, moisture resistant, fungus proof, inedible to insects and thus incredibly low maintenance. It's definitely worth googling Accoya if you're considering going for a structure that incorporates timber joinery.

Fire escape from upper floor ( as this would be a type 3 space ) I can go out onto the roof over the office as that is single storey or from the landing via a new escape type window. Which means I don't need a kitchen door. Closing off the old side door entirely could be a workable idea having read through a few posts here. Putting a partition and door to form a small hallway outside the office is actually a neat idea and effectively completely separates a home based business from the living space as well as offering an 'air lock' to keep weather out.