Inconsistent lightsource. The stairs/stonework and general highlights on the edge suggest that the lightsource roughly south east from the objects position, but the shading on the house itself (by having the south east side of the house darker then the south-west side) indicates the opposite. Same with the chimney.
It also looks like you used neutral gray on the stonework, (grey that lacks any saturation or tint what-so-ever) which is generally a bad idea as it doesn't really exist in nature. Tint your grays slightly (about a saturation of about 5-10) and it should look better. (I'd suggest following general hue-shift doctrine for that; tint the darker ones bluer and the lighter ones yellower)
