I agree that you ground floor flats you should be able to escape through a window if there in a kitchen fire between your bedroom and the front door. There are however guidelines about window apertures for this purpose and it may be that your current windows may have too small an aperture or it might be too high to easily get out of. Personally I would be requesting a site visit from building control since their advice three years ago led to you being in this situation. I would be looking for them to explicitly give you your options to resolve this and it might be that there are several possible solutions.

You need to have a fire protected route. There are a few ways to do that. It is possible to create this as a more open plan look but it will be more expensive than just keeping the stair enclosed and having a door from the hall to the living/dining rooms. So it depends on your budget and how valuable it is to you to have that feel more open and spacious. Your options are: 1. enclose the staircase with adequate fire protecting walls and doors 2. open up and use a mechanical system to provide fire protection 3. enclose the staircase as in option 1 but use fire rated glass to the stair to give an open look. However fire rated glass is really expensive! If you want to go down a more complex fire protection route then I would recommend that you speak with a fire engineer who could design and spec the system for you. You might be able to use a fire blanket or a sprinkler system. I believe that the costs of sprinkler systems has come down a lot recently, and they do seem to be becoming more used in private residential, though still fairly unusual. I've not seen fire blankets used in houses, but I used to include them a lot in commercial and community building projects.

The rules for fire safety say you can’t worsen the route of fire escape. Assuming your replacement wall and door are equivalent you should be confident that the building regs people will agree - but ultimately that’s their prerogative following an inspection. As far as putting a waste pipe on the side of the house that is likely ok as well- again check with the building regs people, ensure you own the drain you intend to go in to, check you are not in a conservation area as this might change the rules, and check there are no restrictive covenant in the house that prevent it.

Sorry for the delay Victoria. The Building Inspector actually came back with more information yesterday, and has asked for me to send the window with a few details to them and he will confirm. But it very much seems like 800mm is the one as you suggested so thank you. They sent me a PDF the same as this link which explains in more detail a few scenarios for window heights and guarding. I will message on this thread again if he comes back with any reason why 800mm is not ok. He did mention in his email that parts K and Parts B sometimes conflict, so it can be very scenario dependant. Cheers