What I did in the end...I had bespoke cupboard made. The tall cupboard on the left you can walk into and behind the tv. And the others are normal size apart from the top which are shallow and the tall cupboard in the right is actually full height and really shallow for my colouramas (tall background paper rolls) Sorry not the next photos on the grey day but you can get the idea

The diameter will only apply if your putting in a spiral, which I'm guessing your not. As said above, building regs say 900mm for accessible stairs, This is for the allowance of a stairlift should one be required in future. But since yours is a conversion, if you have step up to your property or doorways that aren't wide enough for wheelchair access, your house won't have to meet requirements for accessible stairs and you have some give or take, but your stairs can't be narrower than what's already in place. For example, if your stairs are 850mm just now, you have to be 850mm or over for the new stairs. If it was me I would contact somebody who sells stairs and ask them as they will deal with this every day so will be more knowledgable in this. I'm guessing a standard stair of 900 will be less expensive than a stair that's been made to fit the space. Cost will depend on if it's load baring walls that have to be moved. But I wouldn't underestimate the use of an architect/ technologist as they may see issues that we haven't picked up on, have a better idea for the use of space and how to comply with building regs, It can work out cheaper for these reasons too as sometimes we overcomplicate things we don't fully understand. They will also be able to give you a cost for the work and tell you if it's going to be worth doing it, remember the consultation won't cost you anything but will allow you to get a better understanding of what amount of work is going to be required. Where abouts in Scotland are you? I'm in Glasgow and know a few technologists in the west/ east of Scotland if you would want their contact details. You can also look on CIAT (chartered institute of architectural technology) website for technologists/ technicians in your area or RIAS ( royal incorporation of architects in Scotland) for an architect. If you have a builder in mind who will be carrying out the work then they may also have an in house design team who can help you out.

Wow, thank you everyone for your extremely helpful advice and ideas. I had not appreciated the safety issue with the current layout as strongly, but you have swayed me. (Thanks Jonathan). First, as a few of you have asked, I’ve attached a photo with the bathroom dimensions. Hopefully this is legible! I like the shelf/utility cupboard idea, however I’m concerned that this would take away too much space for toilet/sink/shower area. Jules Mc, I think the toilet reconfiguration behind the pony wall is a brilliant idea. If we are unable to play around with the current levels, I think this is an ideal solution. Jonathan, thank you so much for these options. Unfortunately we have a wall protruding in front of the entrance to the bathroom, so I don’t think that there is room for a door to open outwards, or for stairs to be built outside. Man About the House, this is absolutely incredible! It maximises the space while still allowing for the fun/eccentric aspect of the elevated floor plan to remain. I would never have thought of this arrangement. Would raising the floors for most of the bathroom be a major added cost? I’m a bit nervous that the diagonal wall (currently next to toilet where staircase is underneath) would protrude too much into the walkway of the shower. But maybe there would be enough space, and could build a vertical cabinet or shelf to cover up this slope? Again, thank you all for your comment and your time, I’m seeing this project in a whole new light now!

If you’re short on storage then cupboards underneath. Not a job for the amateur though. Plenty of images online. When you say over do you mean replacing spindles? A photo would be better.