Next project... scribbles and aliens!

So, onto the next brief... to create artwork for a book called 'Sebastian's sink city,' where a young boy discovers an imaginary world under the kitchen sink. The idea is that squirty bottles become buildings, the pipes could become a crazy transport system etc etc!

 I have had lots of ideas floating around and I am just turning them into scribbles... question is, do I start with the buildings and homes then invent the characters who live in them or the other way around?? 

I was originally thinking mice, but since moved onto the idea of little aliens, and I will make them super colourful, after all its a pretty dark and gloomy place under the kitchen sink! I couldn't resist getting the pastels out today....