This photo gives a better sense of the planes that will be formed by the plaster board once it’s installed. I’ve asked the builder to install a small door on both sides of the room to allow access after the work is complete to the long space behind these walls. More for access reasons than for any practical need for more storage or to access trains.
I was originally looking at doing this work myself and while I feel I could match the builders quality of work I think the comparison has convinced me that getting even this far would have meant the job stretching on into the never-never. I roughly estimate that if I spent every free weekend on this job it would have taken me at least 6 months to progress this far. The danger would have been hat the work would have ground to a halt and I know from experience that getting a builder to pick up a half-finished job is very difficult.