I am very envious of the space and its location. I have to go to the end of the garden to get to my 10' x 12' shed which I cannot use yet as my son has his MG Midget that is being restored parked in there!
When I start my next layout, I will be using mainly 6mm plywood for construction. I plan to use open framing to reduce the weight and get as much out of the sheets as I can. That method will also allow access from underneath to otherwise inaccessible areas. I will make the edges of the framing from two pieces (one 76mm wide, the other 70mm wide) screwed and glued to form a 12mm piece of 10-ply with a rebate to accept the base / trackbed support or fix extensions from the bracing (strips of 10mm ply, 70mm wide this time) to support the open higher level bases. I intend to have three levels in the plan. That way I can have one level above another on one side with a further level at half height on the opposite side.
At this stage, my plan is fictitious GWR, probably including a station based on Penzance and a through station like Bath Spa. As a lad, I grew up on the outskirts of Bath so got to see plenty of Castles, Kings and the like. Bath also was the terminus for the S&DJR which was a fascinating line in the early 60's until closure (and mainly single track) but the rolling stock from my previous modelling is GWR because almost nothing was available then for a fan of the S&D. Bath Green Park is too big a project for me to replicate in my space or on my budget. I also want to get something that I can run fairly quickly and be expanded to the full plan later because I have two grandsons who I hope will take an interest.
Good luck with yours,
Tony