Initially I’m planning on having it outdoors at ground level using offcuts of wooden decking as a base. To expand where I plan though I’ll probably have to raise it to the level of the patios. I will move the large gravel aside to lay the decking then use a small gravel as ballast.
Mixing scales on people is not a big issue as long as the scale differences are not too big. We do all come in various sizes after all.
If you decided to go for a German line option there are many good prototypes done by Piko for DB if you are happy to accept the scale compromise.
