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Lofts as an after thought can be expensive and difficult to install
So I've included a simple lightweight steel (Cold Formed Section) staircase that the average DIYer should find relatively simple & cost effective to knock up with a basic tool set
If you played with a mecanno set as a child 
then this is for you
https://dovedaler.com/2012/08/16/workbench-news/
A few points to consider
- Purlin spans are for illustration purposes only
- All floor loft spans to be checked by engineer
- Stairs suitable for 1.50 KPa Live loads
- Planning and Building applications are applicable check with your council
- etc,etc
@BibbyKat If you can get someone to take some photos and then upload them to this topic I'd be happy to take a quick look 
and I may well have some other solutions to share that are more suited to your requirements in my Bim (Building Information Modelling) Engineering & Construction Handbook . Remember Safety Always First (SAF) when climbing into roof spaces.
Following on in The Loft series is the floor .
The Structural Floor will need to be designed and certified by an Engineer
Points to consider when determining the type of flooring system
- Roof Access by means of ladders and scaffolding
- Material handling within the roof space
- Safe working Platforms within the roof space
- Condtion of the ceiling beneath the loft
- Limiting the amount of work to be done within the roofspace to erection only where possible
- etc,etc
Because of the above factors The floor has been designed to sit on walls above the ceiling with sufficent clearance to the ceiling joists & beam members
The floor can also be prefabricated on the gound and re-assembled in the roof
For support beams spanning short distances I've use Timber LVL Beams
For Beams spanning larger distances I've used bolted Prefab Steel Truss Beams
These beams will not be seen from within the loft once the walls to the loft are installed
The exploded view (down to up) is in sequence to the way it will be installed
Hope you find it useful
Other posts in this loft series:
Following on in The Loft Series after the stuctural floor are the walls
these are best constructed within the roofspace where the floor provides a safe large working platform for fabrication
The walls are designed to enclose the loft and to support the rafters & roof.
Cut collar ties that previously passed through the loft will need to be trimmed & attached to these walls. Brace and fix the wall framing to the existing structure ready for lining.
Other posts in this loft series:
DIY Placement of a Loft within a Roof Space
Hope you find these links useful
Also please check out Bunnings DIY advice Here
G'day @BIM_Engineer Chris, is there any way that you can post higher definition diagrams of your plans, more specifically, the exploded detail images? Maybe it's just me, but I can't make much out of it in the current format, & enlarging a "View Image", just makes it blurry? 
Ceiling & a optional window completes the Loft Series
At this point you might want to consider having a licenced Electrician
install power points and lights.
After fixing Ceiling battens to the rafters place ceiling insulation
from within the loft space covering insulation with plasterboard ceilings
The wall insulation can be place from within the roof space
The approximate quantities and data are provided for material pricing and handling.
Hope you found the series benefical and informative.
Fitout, trim, paint and decorate accordingly and please share your loft photos
Other posts in this loft series:
DIY Placement of a Loft within a Roof Space
Hope you find these links useful
Also please check out Bunnings DIY advice Here
