I have made a little more progress...
Is it working? I am trying to keep the colours, shadows and saturation levels consistent... but as I zoom in and out I think I might have made some buildings too dark.
I want to add trees, fields, paving and such later on, but as I have not made the buildings photorealistic, photo textures won't blend.
I tried adding some trees (top left) but I am not sure if they suit. Ideas?