I don't have stacker doors, I am too far south and want an airtight seal. We are doing Binq slider which are 2400 high and around 4 mtr long - only opening one side. I can have it open to the middle or the kitchen though. And we will have a large window where the living room is as I want a cosy winter nook. But of course our living window can open for breeze too and will be about 2mtr wide at least. From what you guys are saying opening to the middle of the extension is preferable! I guess as I am the cook standing about I was thinking of myself with a pleasant breeze blowing around me as I stir my pot. ;-)

bump!!...we considered a similar option but were convinced that a separate heater in each rooms was a more economical and efficient option although new products and your local climate may make a difference to your options

Carpet on stairs is a practical safe, cost effective option. I would gate the bottom of the stairs to restrict access to the stairs, recarpet the stairs (if it is already damaged), and tile the front foyer. You can also purchase indoor doggy litter trays (like a cat litter tray), which should reduce accidents, and make cleaning up easier. Are they accidents, or do they not have another toilet option apart from holding on until somebody gets home? If you do decide to tile the stairs you can purchase terracotta or natural stone coping tiles (used for pool surrounds) which has a curved front lip and avoids the need for metal edge trim. They are non-slip and practical, but will be more expensive than new carpet, and expensive to install. https://pool-coping-tiles.com.au/products/?keyword=pool%20capping&gclid=CjwKCAjwsdfZBRAkEiwAh2z65lzb388qnXait7iZiWmpIH_jvieG_4VnQlbk3JqShgSAc_jOz4G-5xoCEKwQAvD_BwE Best of luck, Dr Retro of Dr Retro House Calls

Sliding doors allow for more robust insect/security screens