The shading seems to be rather inconsistant, or rather, the outlines. The outlines are a darker version of the colour inside at some points, and at others, it's simply a black outline. I'd suggest you make either a black outline for everything, or a darker version of the colour outline for everything. The windows could also use some work, they look like white rectangles as opposed to actual windows. Maybe you could make them a light blue, and add some shine?
One last thing, the grass texture looks a little weird. The green is too vibrant, it contrasts with the rest of the image's darker greyer tones. Other than that, looks pretty good.
The remaining black lines are an error, they were supposed to be a darker version of the colour. I'll remove them shortly and update the image.
I'm likely going to leave the grass how it is but you are right about the windows. I just wasn't sure how to proceed with them.
Thank you for your kind comment :)